Add API to explain why a shard is or isn't assigned

This adds a new `/_cluster/allocation/explain` API that explains why a
shard can or cannot be allocated to nodes in the cluster. Additionally,
it will show where the master *desires* to put the shard, according to
the `ShardsAllocator`.

It looks like this:

```
GET /_cluster/allocation/explain?pretty
{
  "index": "only-foo",
  "shard": 0,
  "primary": false
}
```

Though, you can optionally send an empty body, which means "explain the
allocation for the first unassigned shard you find".

The output when a shard is unassigned looks like this:

```
{
  "shard" : {
    "index" : "only-foo",
    "index_uuid" : "KnW0-zELRs6PK84l0r38ZA",
    "id" : 0,
    "primary" : false
  },
  "assigned" : false,
  "unassigned_info" : {
    "reason" : "INDEX_CREATED",
    "at" : "2016-03-22T20:04:23.620Z"
  },
  "nodes" : {
    "V-Spi0AyRZ6ZvKbaI3691w" : {
      "node_name" : "Susan Storm",
      "node_attributes" : {
        "bar" : "baz"
      },
      "final_decision" : "NO",
      "weight" : 0.06666675,
      "decisions" : [ {
        "decider" : "filter",
        "decision" : "NO",
        "explanation" : "node does not match index include filters [foo:\"bar\"]"
      } ]
    },
    "Qc6VL8c5RWaw1qXZ0Rg57g" : {
      "node_name" : "Slipstream",
      "node_attributes" : {
        "bar" : "baz",
        "foo" : "bar"
      },
      "final_decision" : "NO",
      "weight" : -1.3833332,
      "decisions" : [ {
        "decider" : "same_shard",
        "decision" : "NO",
        "explanation" : "the shard cannot be allocated on the same node id [Qc6VL8c5RWaw1qXZ0Rg57g] on which it already exists"
      } ]
    },
    "PzdyMZGXQdGhqTJHF_hGgA" : {
      "node_name" : "The Symbiote",
      "node_attributes" : { },
      "final_decision" : "NO",
      "weight" : 2.3166666,
      "decisions" : [ {
        "decider" : "filter",
        "decision" : "NO",
        "explanation" : "node does not match index include filters [foo:\"bar\"]"
      } ]
    }
  }
}
```

And when the shard *is* assigned, the output looks like:

```
{
  "shard" : {
    "index" : "only-foo",
    "index_uuid" : "KnW0-zELRs6PK84l0r38ZA",
    "id" : 0,
    "primary" : true
  },
  "assigned" : true,
  "assigned_node_id" : "Qc6VL8c5RWaw1qXZ0Rg57g",
  "nodes" : {
    "V-Spi0AyRZ6ZvKbaI3691w" : {
      "node_name" : "Susan Storm",
      "node_attributes" : {
        "bar" : "baz"
      },
      "final_decision" : "NO",
      "weight" : 1.4499999,
      "decisions" : [ {
        "decider" : "filter",
        "decision" : "NO",
        "explanation" : "node does not match index include filters [foo:\"bar\"]"
      } ]
    },
    "Qc6VL8c5RWaw1qXZ0Rg57g" : {
      "node_name" : "Slipstream",
      "node_attributes" : {
        "bar" : "baz",
        "foo" : "bar"
      },
      "final_decision" : "CURRENTLY_ASSIGNED",
      "weight" : 0.0,
      "decisions" : [ {
        "decider" : "same_shard",
        "decision" : "NO",
        "explanation" : "the shard cannot be allocated on the same node id [Qc6VL8c5RWaw1qXZ0Rg57g] on which it already exists"
      } ]
    },
    "PzdyMZGXQdGhqTJHF_hGgA" : {
      "node_name" : "The Symbiote",
      "node_attributes" : { },
      "final_decision" : "NO",
      "weight" : 3.6999998,
      "decisions" : [ {
        "decider" : "filter",
        "decision" : "NO",
        "explanation" : "node does not match index include filters [foo:\"bar\"]"
      } ]
    }
  }
}
```

Only "NO" decisions are returned by default, but all decisions can be
shown by specifying the `?include_yes_decisions=true` parameter in the
request.

Resolves #14593

buildSrc/src/main/resources/checkstyle_suppressions.xml

@@ -326,17 +326,6 @@
   <suppress files="core[/\\]src[/\\]main[/\\]java[/\\]org[/\\]elasticsearch[/\\]cluster[/\\]routing[/\\]allocation[/\\]command[/\\]CancelAllocationCommand.java" checks="LineLength" />
   <suppress files="core[/\\]src[/\\]main[/\\]java[/\\]org[/\\]elasticsearch[/\\]cluster[/\\]routing[/\\]allocation[/\\]command[/\\]MoveAllocationCommand.java" checks="LineLength" />
   <suppress files="core[/\\]src[/\\]main[/\\]java[/\\]org[/\\]elasticsearch[/\\]cluster[/\\]routing[/\\]allocation[/\\]decider[/\\]AllocationDeciders.java" checks="LineLength" />
-  <suppress files="core[/\\]src[/\\]main[/\\]java[/\\]org[/\\]elasticsearch[/\\]cluster[/\\]routing[/\\]allocation[/\\]decider[/\\]AwarenessAllocationDecider.java" checks="LineLength" />
-  <suppress files="core[/\\]src[/\\]main[/\\]java[/\\]org[/\\]elasticsearch[/\\]cluster[/\\]routing[/\\]allocation[/\\]decider[/\\]ClusterRebalanceAllocationDecider.java" checks="LineLength" />
-  <suppress files="core[/\\]src[/\\]main[/\\]java[/\\]org[/\\]elasticsearch[/\\]cluster[/\\]routing[/\\]allocation[/\\]decider[/\\]ConcurrentRebalanceAllocationDecider.java" checks="LineLength" />
-  <suppress files="core[/\\]src[/\\]main[/\\]java[/\\]org[/\\]elasticsearch[/\\]cluster[/\\]routing[/\\]allocation[/\\]decider[/\\]DiskThresholdDecider.java" checks="LineLength" />
-  <suppress files="core[/\\]src[/\\]main[/\\]java[/\\]org[/\\]elasticsearch[/\\]cluster[/\\]routing[/\\]allocation[/\\]decider[/\\]EnableAllocationDecider.java" checks="LineLength" />
-  <suppress files="core[/\\]src[/\\]main[/\\]java[/\\]org[/\\]elasticsearch[/\\]cluster[/\\]routing[/\\]allocation[/\\]decider[/\\]FilterAllocationDecider.java" checks="LineLength" />
-  <suppress files="core[/\\]src[/\\]main[/\\]java[/\\]org[/\\]elasticsearch[/\\]cluster[/\\]routing[/\\]allocation[/\\]decider[/\\]NodeVersionAllocationDecider.java" checks="LineLength" />
-  <suppress files="core[/\\]src[/\\]main[/\\]java[/\\]org[/\\]elasticsearch[/\\]cluster[/\\]routing[/\\]allocation[/\\]decider[/\\]SameShardAllocationDecider.java" checks="LineLength" />
-  <suppress files="core[/\\]src[/\\]main[/\\]java[/\\]org[/\\]elasticsearch[/\\]cluster[/\\]routing[/\\]allocation[/\\]decider[/\\]ShardsLimitAllocationDecider.java" checks="LineLength" />
-  <suppress files="core[/\\]src[/\\]main[/\\]java[/\\]org[/\\]elasticsearch[/\\]cluster[/\\]routing[/\\]allocation[/\\]decider[/\\]SnapshotInProgressAllocationDecider.java" checks="LineLength" />
-  <suppress files="core[/\\]src[/\\]main[/\\]java[/\\]org[/\\]elasticsearch[/\\]cluster[/\\]routing[/\\]allocation[/\\]decider[/\\]ThrottlingAllocationDecider.java" checks="LineLength" />
   <suppress files="core[/\\]src[/\\]main[/\\]java[/\\]org[/\\]elasticsearch[/\\]cluster[/\\]service[/\\]InternalClusterService.java" checks="LineLength" />
   <suppress files="core[/\\]src[/\\]main[/\\]java[/\\]org[/\\]elasticsearch[/\\]common[/\\]Base64.java" checks="LineLength" />
   <suppress files="core[/\\]src[/\\]main[/\\]java[/\\]org[/\\]elasticsearch[/\\]common[/\\]Booleans.java" checks="LineLength" />

core/src/main/java/org/elasticsearch/action/ActionModule.java

@@ -19,6 +19,8 @@
 
 package org.elasticsearch.action;
 
+import org.elasticsearch.action.admin.cluster.allocation.ClusterAllocationExplainAction;
+import org.elasticsearch.action.admin.cluster.allocation.TransportClusterAllocationExplainAction;
 import org.elasticsearch.action.admin.cluster.health.ClusterHealthAction;
 import org.elasticsearch.action.admin.cluster.health.TransportClusterHealthAction;
 import org.elasticsearch.action.admin.cluster.node.hotthreads.NodesHotThreadsAction;
@@ -263,6 +265,7 @@ public class ActionModule extends AbstractModule {
         registerAction(ListTasksAction.INSTANCE, TransportListTasksAction.class);
         registerAction(CancelTasksAction.INSTANCE, TransportCancelTasksAction.class);
 
+        registerAction(ClusterAllocationExplainAction.INSTANCE, TransportClusterAllocationExplainAction.class);
         registerAction(ClusterStatsAction.INSTANCE, TransportClusterStatsAction.class);
         registerAction(ClusterStateAction.INSTANCE, TransportClusterStateAction.class);
         registerAction(ClusterHealthAction.INSTANCE, TransportClusterHealthAction.class);

core/src/main/java/org/elasticsearch/action/admin/cluster/allocation/ClusterAllocationExplainAction.java

@@ -0,0 +1,48 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.elasticsearch.action.admin.cluster.allocation;
+
+import org.elasticsearch.action.Action;
+import org.elasticsearch.client.ElasticsearchClient;
+
+/**
+ * Action for explaining shard allocation for a shard in the cluster
+ */
+public class ClusterAllocationExplainAction extends Action<ClusterAllocationExplainRequest,
+                                                               ClusterAllocationExplainResponse,
+                                                               ClusterAllocationExplainRequestBuilder> {
+
+    public static final ClusterAllocationExplainAction INSTANCE = new ClusterAllocationExplainAction();
+    public static final String NAME = "cluster:monitor/allocation/explain";
+
+    private ClusterAllocationExplainAction() {
+        super(NAME);
+    }
+
+    @Override
+    public ClusterAllocationExplainResponse newResponse() {
+        return new ClusterAllocationExplainResponse();
+    }
+
+    @Override
+    public ClusterAllocationExplainRequestBuilder newRequestBuilder(ElasticsearchClient client) {
+        return new ClusterAllocationExplainRequestBuilder(client, this);
+    }
+}

core/src/main/java/org/elasticsearch/action/admin/cluster/allocation/ClusterAllocationExplainRequest.java

@@ -0,0 +1,195 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.elasticsearch.action.admin.cluster.allocation;
+
+import org.elasticsearch.ElasticsearchParseException;
+import org.elasticsearch.action.ActionRequestValidationException;
+import org.elasticsearch.action.support.master.MasterNodeRequest;
+import org.elasticsearch.common.Nullable;
+import org.elasticsearch.common.Strings;
+import org.elasticsearch.common.io.stream.StreamInput;
+import org.elasticsearch.common.io.stream.StreamOutput;
+import org.elasticsearch.common.xcontent.XContentParser;
+
+import java.io.IOException;
+import java.util.Objects;
+
+import static org.elasticsearch.action.ValidateActions.addValidationError;
+
+/**
+ * A request to explain the allocation of a shard in the cluster
+ */
+public class ClusterAllocationExplainRequest extends MasterNodeRequest<ClusterAllocationExplainRequest> {
+
+    private String index;
+    private Integer shard;
+    private Boolean primary;
+    private boolean includeYesDecisions = false;
+
+    /** Explain the first unassigned shard */
+    public ClusterAllocationExplainRequest() {
+        this.index = null;
+        this.shard = null;
+        this.primary = null;
+    }
+
+    /**
+     * Create a new allocation explain request. If {@code primary} is false, the first unassigned replica
+     * will be picked for explanation. If no replicas are unassigned, the first assigned replica will
+     * be explained.
+     */
+    public ClusterAllocationExplainRequest(String index, int shard, boolean primary) {
+        this.index = index;
+        this.shard = shard;
+        this.primary = primary;
+    }
+
+    @Override
+    public ActionRequestValidationException validate() {
+        ActionRequestValidationException validationException = null;
+        if (this.useAnyUnassignedShard() == false) {
+            if (this.index == null) {
+                validationException = addValidationError("index must be specified", validationException);
+            }
+            if (this.shard == null) {
+                validationException = addValidationError("shard must be specified", validationException);
+            }
+            if (this.primary == null) {
+                validationException = addValidationError("primary must be specified", validationException);
+            }
+        }
+        return validationException;
+    }
+
+    /**
+     * Returns {@code true} iff the first unassigned shard is to be used
+     */
+    public boolean useAnyUnassignedShard() {
+        return this.index == null && this.shard == null && this.primary == null;
+    }
+
+    public ClusterAllocationExplainRequest setIndex(String index) {
+        this.index = index;
+        return this;
+    }
+
+    @Nullable
+    public String getIndex() {
+        return this.index;
+    }
+
+    public ClusterAllocationExplainRequest setShard(Integer shard) {
+        this.shard = shard;
+        return this;
+    }
+
+    @Nullable
+    public int getShard() {
+        return this.shard;
+    }
+
+    public ClusterAllocationExplainRequest setPrimary(Boolean primary) {
+        this.primary = primary;
+        return this;
+    }
+
+    @Nullable
+    public boolean isPrimary() {
+        return this.primary;
+    }
+
+    public void includeYesDecisions(boolean includeYesDecisions) {
+        this.includeYesDecisions = includeYesDecisions;
+    }
+
+    /** Returns true if all decisions should be included. Otherwise only "NO" and "THROTTLE" decisions are returned */
+    public boolean includeYesDecisions() {
+        return this.includeYesDecisions;
+    }
+
+    @Override
+    public String toString() {
+        StringBuilder sb = new StringBuilder("ClusterAllocationExplainRequest[");
+        if (this.useAnyUnassignedShard()) {
+            sb.append("useAnyUnassignedShard=true");
+        } else {
+            sb.append("index=").append(index);
+            sb.append(",shard=").append(shard);
+            sb.append(",primary?=").append(primary);
+        }
+        sb.append(",includeYesDecisions?=").append(includeYesDecisions);
+        return sb.toString();
+    }
+
+    public static ClusterAllocationExplainRequest parse(XContentParser parser) throws IOException {
+        String currentFieldName = null;
+        String index = null;
+        Integer shard = null;
+        Boolean primary = null;
+        XContentParser.Token token;
+        while ((token = parser.nextToken()) != XContentParser.Token.END_OBJECT) {
+            if (token == XContentParser.Token.FIELD_NAME) {
+                currentFieldName = parser.currentName();
+            } else if (token.isValue()) {
+                if ("index".equals(currentFieldName)) {
+                    index = parser.text();
+                } else if ("shard".equals(currentFieldName)) {
+                    shard = parser.intValue();
+                } else if ("primary".equals(currentFieldName)) {
+                    primary = parser.booleanValue();
+                } else {
+                    throw new ElasticsearchParseException("unexpected field [" + currentFieldName + "] in allocation explain request");
+                }
+
+            } else if (token == XContentParser.Token.START_OBJECT) {
+                // the object was started
+                continue;
+            } else {
+                throw new ElasticsearchParseException("unexpected token [" + token + "] in allocation explain request");
+            }
+        }
+
+        if (index == null && shard == null && primary == null) {
+            // If it was an empty body, use the "any unassigned shard" request
+            return new ClusterAllocationExplainRequest();
+        } else if (index == null || shard == null || primary == null) {
+            throw new ElasticsearchParseException("'index', 'shard', and 'primary' must be specified in allocation explain request");
+        }
+        return new ClusterAllocationExplainRequest(index, shard, primary);
+    }
+
+    @Override
+    public void readFrom(StreamInput in) throws IOException {
+        super.readFrom(in);
+        this.index = in.readOptionalString();
+        this.shard = in.readOptionalVInt();
+        this.primary = in.readOptionalBoolean();
+        this.includeYesDecisions = in.readBoolean();
+    }
+
+    @Override
+    public void writeTo(StreamOutput out) throws IOException {
+        super.writeTo(out);
+        out.writeOptionalString(index);
+        out.writeOptionalVInt(shard);
+        out.writeOptionalBoolean(primary);
+        out.writeBoolean(includeYesDecisions);
+    }
+}

core/src/main/java/org/elasticsearch/action/admin/cluster/allocation/ClusterAllocationExplainRequestBuilder.java

@@ -0,0 +1,66 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.elasticsearch.action.admin.cluster.allocation;
+
+import org.elasticsearch.action.support.IndicesOptions;
+import org.elasticsearch.action.support.master.MasterNodeOperationRequestBuilder;
+import org.elasticsearch.client.ElasticsearchClient;
+
+/**
+ * Builder for requests to explain the allocation of a shard in the cluster
+ */
+public class ClusterAllocationExplainRequestBuilder
+        extends MasterNodeOperationRequestBuilder<ClusterAllocationExplainRequest,
+                                                          ClusterAllocationExplainResponse,
+                                                          ClusterAllocationExplainRequestBuilder> {
+
+    public ClusterAllocationExplainRequestBuilder(ElasticsearchClient client, ClusterAllocationExplainAction action) {
+        super(client, action, new ClusterAllocationExplainRequest());
+    }
+
+    /** The index name to use when finding the shard to explain */
+    public ClusterAllocationExplainRequestBuilder setIndex(String index) {
+        request.setIndex(index);
+        return this;
+    }
+
+    /** The shard number to use when finding the shard to explain */
+    public ClusterAllocationExplainRequestBuilder setShard(int shard) {
+        request.setShard(shard);
+        return this;
+    }
+
+    /** Whether the primary or replica should be explained */
+    public ClusterAllocationExplainRequestBuilder setPrimary(boolean primary) {
+        request.setPrimary(primary);
+        return this;
+    }
+
+    /**
+     * Signal that the first unassigned shard should be used
+     */
+    public ClusterAllocationExplainRequestBuilder useAnyUnassignedShard() {
+        request.setIndex(null);
+        request.setShard(null);
+        request.setPrimary(null);
+        return this;
+    }
+
+}

core/src/main/java/org/elasticsearch/action/admin/cluster/allocation/ClusterAllocationExplainResponse.java

@@ -0,0 +1,61 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.elasticsearch.action.admin.cluster.allocation;
+
+import org.elasticsearch.action.ActionResponse;
+import org.elasticsearch.cluster.ClusterName;
+import org.elasticsearch.common.io.stream.StreamInput;
+import org.elasticsearch.common.io.stream.StreamOutput;
+
+import java.io.IOException;
+
+/**
+ * Explanation response for a shard in the cluster
+ */
+public class ClusterAllocationExplainResponse extends ActionResponse {
+
+    private ClusterAllocationExplanation cae;
+
+    public ClusterAllocationExplainResponse() {
+    }
+
+    public ClusterAllocationExplainResponse(ClusterAllocationExplanation cae) {
+        this.cae = cae;
+    }
+
+    /**
+     * Return the explanation for shard allocation in the cluster
+     */
+    public ClusterAllocationExplanation getExplanation() {
+        return this.cae;
+    }
+
+    @Override
+    public void readFrom(StreamInput in) throws IOException {
+        super.readFrom(in);
+        this.cae = new ClusterAllocationExplanation(in);
+    }
+
+    @Override
+    public void writeTo(StreamOutput out) throws IOException {
+        super.writeTo(out);
+        cae.writeTo(out);
+    }
+}

core/src/main/java/org/elasticsearch/action/admin/cluster/allocation/ClusterAllocationExplanation.java

@@ -0,0 +1,200 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.elasticsearch.action.admin.cluster.allocation;
+
+import com.carrotsearch.hppc.cursors.ObjectObjectCursor;
+import org.elasticsearch.cluster.node.DiscoveryNode;
+import org.elasticsearch.cluster.routing.UnassignedInfo;
+import org.elasticsearch.cluster.routing.allocation.decider.Decision;
+import org.elasticsearch.common.Nullable;
+import org.elasticsearch.common.io.stream.StreamInput;
+import org.elasticsearch.common.io.stream.StreamOutput;
+import org.elasticsearch.common.io.stream.Writeable;
+import org.elasticsearch.common.xcontent.ToXContent;
+import org.elasticsearch.common.xcontent.ToXContent.Params;
+import org.elasticsearch.common.xcontent.XContentBuilder;
+import org.elasticsearch.index.shard.ShardId;
+
+import java.io.IOException;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * A {@code ClusterAllocationExplanation} is an explanation of why a shard may or may not be allocated to nodes. It also includes weights
+ * for where the shard is likely to be assigned. It is an immutable class
+ */
+public final class ClusterAllocationExplanation implements ToXContent, Writeable<ClusterAllocationExplanation> {
+
+    private final ShardId shard;
+    private final boolean primary;
+    private final String assignedNodeId;
+    private final Map<DiscoveryNode, Decision> nodeToDecision;
+    private final Map<DiscoveryNode, Float> nodeWeights;
+    private final UnassignedInfo unassignedInfo;
+
+    public ClusterAllocationExplanation(StreamInput in) throws IOException {
+        this.shard = ShardId.readShardId(in);
+        this.primary = in.readBoolean();
+        this.assignedNodeId = in.readOptionalString();
+        this.unassignedInfo = in.readOptionalWriteable(UnassignedInfo::new);
+
+        Map<DiscoveryNode, Decision> ntd = null;
+        int size = in.readVInt();
+        ntd = new HashMap<>(size);
+        for (int i = 0; i < size; i++) {
+            DiscoveryNode dn = DiscoveryNode.readNode(in);
+            Decision decision = Decision.readFrom(in);
+            ntd.put(dn, decision);
+        }
+        this.nodeToDecision = ntd;
+
+        Map<DiscoveryNode, Float> ntw = null;
+        size = in.readVInt();
+        ntw = new HashMap<>(size);
+        for (int i = 0; i < size; i++) {
+            DiscoveryNode dn = DiscoveryNode.readNode(in);
+            float weight = in.readFloat();
+            ntw.put(dn, weight);
+        }
+        this.nodeWeights = ntw;
+    }
+
+    public ClusterAllocationExplanation(ShardId shard, boolean primary, @Nullable String assignedNodeId,
+                                        UnassignedInfo unassignedInfo, Map<DiscoveryNode, Decision> nodeToDecision,
+                                        Map<DiscoveryNode, Float> nodeWeights) {
+        this.shard = shard;
+        this.primary = primary;
+        this.assignedNodeId = assignedNodeId;
+        this.unassignedInfo = unassignedInfo;
+        this.nodeToDecision = nodeToDecision == null ? Collections.emptyMap() : nodeToDecision;
+        this.nodeWeights = nodeWeights == null ? Collections.emptyMap() : nodeWeights;
+    }
+
+    public ShardId getShard() {
+        return this.shard;
+    }
+
+    public boolean isPrimary() {
+        return this.primary;
+    }
+
+    /** Return turn if the shard is assigned to a node */
+    public boolean isAssigned() {
+        return this.assignedNodeId != null;
+    }
+
+    /** Return the assigned node id or null if not assigned */
+    @Nullable
+    public String getAssignedNodeId() {
+        return this.assignedNodeId;
+    }
+
+    /** Return the unassigned info for the shard or null if the shard is assigned */
+    @Nullable
+    public UnassignedInfo getUnassignedInfo() {
+        return this.unassignedInfo;
+    }
+
+    /** Return a map of node to decision for shard allocation */
+    public Map<DiscoveryNode, Decision> getNodeDecisions() {
+        return this.nodeToDecision;
+    }
+
+    /**
+     * Return a map of node to balancer "weight" for allocation. Higher weights mean the balancer wants to allocated the shard to that node
+     * more
+     */
+    public Map<DiscoveryNode, Float> getNodeWeights() {
+        return this.nodeWeights;
+    }
+
+    public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
+        builder.startObject(); {
+            builder.startObject("shard"); {
+                builder.field("index", shard.getIndexName());
+                builder.field("index_uuid", shard.getIndex().getUUID());
+                builder.field("id", shard.getId());
+                builder.field("primary", primary);
+            }
+            builder.endObject(); // end shard
+            builder.field("assigned", this.assignedNodeId != null);
+            // If assigned, show the node id of the node it's assigned to
+            if (assignedNodeId != null) {
+                builder.field("assigned_node_id", this.assignedNodeId);
+            }
+            // If we have unassigned info, show that
+            if (unassignedInfo != null) {
+                unassignedInfo.toXContent(builder, params);
+            }
+            builder.startObject("nodes");
+            for (Map.Entry<DiscoveryNode, Float> entry : nodeWeights.entrySet()) {
+                DiscoveryNode node = entry.getKey();
+                builder.startObject(node.getId()); {
+                    builder.field("node_name", node.getName());
+                    builder.startObject("node_attributes"); {
+                        for (ObjectObjectCursor<String, String> attrKV : node.attributes()) {
+                            builder.field(attrKV.key, attrKV.value);
+                        }
+                    }
+                    builder.endObject(); // end attributes
+                    Decision d = nodeToDecision.get(node);
+                    if (node.getId().equals(assignedNodeId)) {
+                        builder.field("final_decision", "CURRENTLY_ASSIGNED");
+                    } else {
+                        builder.field("final_decision", d.type().toString());
+                    }
+                    builder.field("weight", entry.getValue());
+                    d.toXContent(builder, params);
+                }
+                builder.endObject(); // end node <uuid>
+            }
+            builder.endObject(); // end nodes
+        }
+        builder.endObject(); // end wrapping object
+        return builder;
+    }
+
+    @Override
+    public ClusterAllocationExplanation readFrom(StreamInput in) throws IOException {
+        return new ClusterAllocationExplanation(in);
+    }
+
+    @Override
+    public void writeTo(StreamOutput out) throws IOException {
+        this.getShard().writeTo(out);
+        out.writeBoolean(this.isPrimary());
+        out.writeOptionalString(this.getAssignedNodeId());
+        out.writeOptionalWriteable(this.getUnassignedInfo());
+
+        Map<DiscoveryNode, Decision> ntd = this.getNodeDecisions();
+        out.writeVInt(ntd.size());
+        for (Map.Entry<DiscoveryNode, Decision> entry : ntd.entrySet()) {
+            entry.getKey().writeTo(out);
+            Decision.writeTo(entry.getValue(), out);
+        }
+        Map<DiscoveryNode, Float> ntw = this.getNodeWeights();
+        out.writeVInt(ntw.size());
+        for (Map.Entry<DiscoveryNode, Float> entry : ntw.entrySet()) {
+            entry.getKey().writeTo(out);
+            out.writeFloat(entry.getValue());
+        }
+    }
+}

core/src/main/java/org/elasticsearch/action/admin/cluster/allocation/TransportClusterAllocationExplainAction.java

@@ -0,0 +1,194 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.elasticsearch.action.admin.cluster.allocation;
+
+import com.carrotsearch.hppc.cursors.ObjectObjectCursor;
+import org.elasticsearch.ElasticsearchException;
+import org.elasticsearch.action.ActionListener;
+import org.elasticsearch.action.support.ActionFilters;
+import org.elasticsearch.action.support.master.TransportMasterNodeAction;
+import org.elasticsearch.cluster.ClusterInfoService;
+import org.elasticsearch.cluster.ClusterName;
+import org.elasticsearch.cluster.ClusterState;
+import org.elasticsearch.cluster.block.ClusterBlockException;
+import org.elasticsearch.cluster.block.ClusterBlockLevel;
+import org.elasticsearch.cluster.metadata.IndexMetaData;
+import org.elasticsearch.cluster.metadata.IndexNameExpressionResolver;
+import org.elasticsearch.cluster.metadata.MetaData;
+import org.elasticsearch.cluster.metadata.MetaData.Custom;
+import org.elasticsearch.cluster.node.DiscoveryNode;
+import org.elasticsearch.cluster.routing.RoutingNode;
+import org.elasticsearch.cluster.routing.RoutingNodes;
+import org.elasticsearch.cluster.routing.RoutingNodes.RoutingNodesIterator;
+import org.elasticsearch.cluster.routing.RoutingTable;
+import org.elasticsearch.cluster.routing.ShardRouting;
+import org.elasticsearch.cluster.routing.UnassignedInfo;
+import org.elasticsearch.cluster.routing.allocation.AllocationService;
+import org.elasticsearch.cluster.routing.allocation.RoutingAllocation;
+import org.elasticsearch.cluster.routing.allocation.RoutingExplanations;
+import org.elasticsearch.cluster.routing.allocation.allocator.ShardsAllocator;
+import org.elasticsearch.cluster.routing.allocation.decider.AllocationDeciders;
+import org.elasticsearch.cluster.routing.allocation.decider.Decision;
+import org.elasticsearch.cluster.service.ClusterService;
+import org.elasticsearch.common.inject.Inject;
+import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.threadpool.ThreadPool;
+import org.elasticsearch.transport.TransportService;
+
+import java.util.HashMap;
+import java.util.Iterator;
+import java.util.List;
+import java.util.Map;
+
+/**
+ * The {@code TransportClusterAllocationExplainAction} is responsible for actually executing the explanation of a shard's allocation on the
+ * master node in the cluster.
+ */
+public class TransportClusterAllocationExplainAction
+        extends TransportMasterNodeAction<ClusterAllocationExplainRequest, ClusterAllocationExplainResponse> {
+
+    private final AllocationService allocationService;
+    private final ClusterInfoService clusterInfoService;
+    private final AllocationDeciders allocationDeciders;
+    private final ShardsAllocator shardAllocator;
+
+    @Inject
+    public TransportClusterAllocationExplainAction(Settings settings, TransportService transportService, ClusterService clusterService,
+                                                   ThreadPool threadPool, ActionFilters actionFilters,
+                                                   IndexNameExpressionResolver indexNameExpressionResolver,
+                                                   AllocationService allocationService, ClusterInfoService clusterInfoService,
+                                                   AllocationDeciders allocationDeciders, ShardsAllocator shardAllocator) {
+        super(settings, ClusterAllocationExplainAction.NAME, transportService, clusterService, threadPool, actionFilters,
+                indexNameExpressionResolver, ClusterAllocationExplainRequest::new);
+        this.allocationService = allocationService;
+        this.clusterInfoService = clusterInfoService;
+        this.allocationDeciders = allocationDeciders;
+        this.shardAllocator = shardAllocator;
+    }
+
+    @Override
+    protected String executor() {
+        return ThreadPool.Names.MANAGEMENT;
+    }
+
+    @Override
+    protected ClusterBlockException checkBlock(ClusterAllocationExplainRequest request, ClusterState state) {
+        return state.blocks().globalBlockedException(ClusterBlockLevel.METADATA_READ);
+    }
+
+    @Override
+    protected ClusterAllocationExplainResponse newResponse() {
+        return new ClusterAllocationExplainResponse();
+    }
+
+    /**
+     * Return the decisions for the given {@code ShardRouting} on the given {@code RoutingNode}. If {@code includeYesDecisions} is not true,
+     * only non-YES (NO and THROTTLE) decisions are returned.
+     */
+    public static Decision tryShardOnNode(ShardRouting shard, RoutingNode node, RoutingAllocation allocation, boolean includeYesDecisions) {
+        Decision d = allocation.deciders().canAllocate(shard, node, allocation);
+        if (includeYesDecisions) {
+            return d;
+        } else {
+            Decision.Multi nonYesDecisions = new Decision.Multi();
+            List<Decision> decisions = d.getDecisions();
+            for (Decision decision : decisions) {
+                if (decision.type() != Decision.Type.YES) {
+                    nonYesDecisions.add(decision);
+                }
+            }
+            return nonYesDecisions;
+        }
+    }
+
+    /**
+     * For the given {@code ShardRouting}, return the explanation of the allocation for that shard on all nodes. If {@code
+     * includeYesDecisions} is true, returns all decisions, otherwise returns only 'NO' and 'THROTTLE' decisions.
+     */
+    public static ClusterAllocationExplanation explainShard(ShardRouting shard, RoutingAllocation allocation, RoutingNodes routingNodes,
+                                                            boolean includeYesDecisions, ShardsAllocator shardAllocator) {
+        // don't short circuit deciders, we want a full explanation
+        allocation.debugDecision(true);
+        // get the existing unassigned info if available
+        UnassignedInfo ui = shard.unassignedInfo();
+
+        RoutingNodesIterator iter = routingNodes.nodes();
+        Map<DiscoveryNode, Decision> nodeToDecision = new HashMap<>();
+        while (iter.hasNext()) {
+            RoutingNode node = iter.next();
+            DiscoveryNode discoNode = node.node();
+            if (discoNode.isDataNode()) {
+                Decision d = tryShardOnNode(shard, node, allocation, includeYesDecisions);
+                nodeToDecision.put(discoNode, d);
+            }
+        }
+        return new ClusterAllocationExplanation(shard.shardId(), shard.primary(), shard.currentNodeId(), ui, nodeToDecision,
+                shardAllocator.weighShard(allocation, shard));
+    }
+
+    @Override
+    protected void masterOperation(final ClusterAllocationExplainRequest request, final ClusterState state,
+                                   final ActionListener<ClusterAllocationExplainResponse> listener) {
+        final RoutingNodes routingNodes = state.getRoutingNodes();
+        final RoutingAllocation allocation = new RoutingAllocation(allocationDeciders, routingNodes, state.nodes(),
+                clusterInfoService.getClusterInfo(), System.nanoTime());
+
+        ShardRouting shardRouting = null;
+        if (request.useAnyUnassignedShard()) {
+            // If we can use any shard, just pick the first unassigned one (if there are any)
+            RoutingNodes.UnassignedShards.UnassignedIterator ui = routingNodes.unassigned().iterator();
+            if (ui.hasNext()) {
+                shardRouting = ui.next();
+            }
+        } else {
+            String index = request.getIndex();
+            int shard = request.getShard();
+            if (request.isPrimary()) {
+                // If we're looking for the primary shard, there's only one copy, so pick it directly
+                shardRouting = allocation.routingTable().shardRoutingTable(index, shard).primaryShard();
+            } else {
+                // If looking for a replica, go through all the replica shards
+                List<ShardRouting> replicaShardRoutings = allocation.routingTable().shardRoutingTable(index, shard).replicaShards();
+                if (replicaShardRoutings.size() > 0) {
+                    // Pick the first replica at the very least
+                    shardRouting = replicaShardRoutings.get(0);
+                    // In case there are multiple replicas where some are assigned and some aren't,
+                    // try to find one that is unassigned at least
+                    for (ShardRouting replica : replicaShardRoutings) {
+                        if (replica.unassigned()) {
+                            shardRouting = replica;
+                            break;
+                        }
+                    }
+                }
+            }
+        }
+
+        if (shardRouting == null) {
+            listener.onFailure(new ElasticsearchException("unable to find any shards to explain [{}] in the routing table", request));
+            return;
+        }
+        logger.debug("explaining the allocation for [{}], found shard [{}]", request, shardRouting);
+
+        ClusterAllocationExplanation cae = explainShard(shardRouting, allocation, routingNodes,
+                request.includeYesDecisions(), shardAllocator);
+        listener.onResponse(new ClusterAllocationExplainResponse(cae));
+    }
+}

core/src/main/java/org/elasticsearch/client/ClusterAdminClient.java

@@ -21,6 +21,9 @@ package org.elasticsearch.client;
 
 import org.elasticsearch.action.ActionFuture;
 import org.elasticsearch.action.ActionListener;
+import org.elasticsearch.action.admin.cluster.allocation.ClusterAllocationExplainRequest;
+import org.elasticsearch.action.admin.cluster.allocation.ClusterAllocationExplainRequestBuilder;
+import org.elasticsearch.action.admin.cluster.allocation.ClusterAllocationExplainResponse;
 import org.elasticsearch.action.admin.cluster.health.ClusterHealthRequest;
 import org.elasticsearch.action.admin.cluster.health.ClusterHealthRequestBuilder;
 import org.elasticsearch.action.admin.cluster.health.ClusterHealthResponse;
@@ -572,4 +575,19 @@ public interface ClusterAdminClient extends ElasticsearchClient {
      * Simulates an ingest pipeline
      */
     SimulatePipelineRequestBuilder prepareSimulatePipeline(BytesReference source);
+
+    /**
+     * Explain the allocation of a shard
+     */
+    void allocationExplain(ClusterAllocationExplainRequest request, ActionListener<ClusterAllocationExplainResponse> listener);
+
+    /**
+     * Explain the allocation of a shard
+     */
+    ActionFuture<ClusterAllocationExplainResponse> allocationExplain(ClusterAllocationExplainRequest request);
+
+    /**
+     * Explain the allocation of a shard
+     */
+    ClusterAllocationExplainRequestBuilder prepareAllocationExplain();
 }

core/src/main/java/org/elasticsearch/client/support/AbstractClient.java

@@ -25,6 +25,10 @@ import org.elasticsearch.action.ActionListener;
 import org.elasticsearch.action.ActionRequest;
 import org.elasticsearch.action.ActionRequestBuilder;
 import org.elasticsearch.action.ActionResponse;
+import org.elasticsearch.action.admin.cluster.allocation.ClusterAllocationExplainAction;
+import org.elasticsearch.action.admin.cluster.allocation.ClusterAllocationExplainRequest;
+import org.elasticsearch.action.admin.cluster.allocation.ClusterAllocationExplainRequestBuilder;
+import org.elasticsearch.action.admin.cluster.allocation.ClusterAllocationExplainResponse;
 import org.elasticsearch.action.admin.cluster.health.ClusterHealthAction;
 import org.elasticsearch.action.admin.cluster.health.ClusterHealthRequest;
 import org.elasticsearch.action.admin.cluster.health.ClusterHealthRequestBuilder;
@@ -1245,6 +1249,21 @@ public abstract class AbstractClient extends AbstractComponent implements Client
         public SimulatePipelineRequestBuilder prepareSimulatePipeline(BytesReference source) {
             return new SimulatePipelineRequestBuilder(this, SimulatePipelineAction.INSTANCE, source);
         }
+
+        @Override
+        public void allocationExplain(ClusterAllocationExplainRequest request, ActionListener<ClusterAllocationExplainResponse> listener) {
+            execute(ClusterAllocationExplainAction.INSTANCE, request, listener);
+        }
+
+        @Override
+        public ActionFuture<ClusterAllocationExplainResponse> allocationExplain(ClusterAllocationExplainRequest request) {
+            return execute(ClusterAllocationExplainAction.INSTANCE, request);
+        }
+
+        @Override
+        public ClusterAllocationExplainRequestBuilder prepareAllocationExplain() {
+            return new ClusterAllocationExplainRequestBuilder(this, ClusterAllocationExplainAction.INSTANCE);
+        }
     }
 
     static class IndicesAdmin implements IndicesAdminClient {

core/src/main/java/org/elasticsearch/cluster/routing/UnassignedInfo.java

@@ -139,7 +139,7 @@ public class UnassignedInfo implements ToXContent, Writeable<UnassignedInfo> {
         assert !(message == null && failure != null) : "provide a message if a failure exception is provided";
     }
 
-    UnassignedInfo(StreamInput in) throws IOException {
+    public UnassignedInfo(StreamInput in) throws IOException {
         this.reason = Reason.values()[(int) in.readByte()];
         this.unassignedTimeMillis = in.readLong();
         // As System.nanoTime() cannot be compared across different JVMs, reset it to now.

core/src/main/java/org/elasticsearch/cluster/routing/allocation/allocator/BalancedShardsAllocator.java

@@ -23,6 +23,7 @@ import org.apache.lucene.util.ArrayUtil;
 import org.apache.lucene.util.IntroSorter;
 import org.elasticsearch.cluster.metadata.IndexMetaData;
 import org.elasticsearch.cluster.metadata.MetaData;
+import org.elasticsearch.cluster.node.DiscoveryNode;
 import org.elasticsearch.cluster.routing.RoutingNode;
 import org.elasticsearch.cluster.routing.RoutingNodes;
 import org.elasticsearch.cluster.routing.ShardRouting;
@@ -100,6 +101,12 @@ public class BalancedShardsAllocator extends AbstractComponent implements Shards
         this.threshold = threshold;
     }
 
+    @Override
+    public Map<DiscoveryNode, Float> weighShard(RoutingAllocation allocation, ShardRouting shard) {
+        final Balancer balancer = new Balancer(logger, allocation, weightFunction, threshold);
+        return balancer.weighShard(shard);
+    }
+
     @Override
     public boolean allocate(RoutingAllocation allocation) {
         if (allocation.routingNodes().size() == 0) {
@@ -298,6 +305,29 @@ public class BalancedShardsAllocator extends AbstractComponent implements Shards
             return balanceByWeights();
         }
 
+        public Map<DiscoveryNode, Float> weighShard(ShardRouting shard) {
+            final NodeSorter sorter = newNodeSorter();
+            final ModelNode[] modelNodes = sorter.modelNodes;
+            final float[] weights = sorter.weights;
+
+            buildWeightOrderedIndices(sorter);
+            Map<DiscoveryNode, Float> nodes = new HashMap<>(modelNodes.length);
+            float currentNodeWeight = 0.0f;
+            for (int i = 0; i < modelNodes.length; i++) {
+                if (modelNodes[i].getNodeId().equals(shard.currentNodeId())) {
+                    // If a node was found with the shard, use that weight instead of 0.0
+                    currentNodeWeight = weights[i];
+                    break;
+                }
+            }
+
+            for (int i = 0; i < modelNodes.length; i++) {
+                final float delta = currentNodeWeight - weights[i];
+                nodes.put(modelNodes[i].getRoutingNode().node(), delta);
+            }
+            return nodes;
+        }
+
         /**
          * Balances the nodes on the cluster model according to the weight
          * function. The configured threshold is the minimum delta between the

core/src/main/java/org/elasticsearch/cluster/routing/allocation/allocator/ShardsAllocator.java

@@ -19,8 +19,11 @@
 
 package org.elasticsearch.cluster.routing.allocation.allocator;
 
+import org.elasticsearch.cluster.node.DiscoveryNode;
+import org.elasticsearch.cluster.routing.ShardRouting;
 import org.elasticsearch.cluster.routing.allocation.RoutingAllocation;
 
+import java.util.Map;
 /**
  * <p>
  * A {@link ShardsAllocator} is the main entry point for shard allocation on nodes in the cluster.
@@ -40,4 +43,15 @@ public interface ShardsAllocator {
      * @return <code>true</code> if the allocation has changed, otherwise <code>false</code>
      */
     boolean allocate(RoutingAllocation allocation);
+
+    /**
+     * Returns a map of node to a float "weight" of where the allocator would like to place the shard.
+     * Higher weights signify greater desire to place the shard on that node.
+     * Does not modify the allocation at all.
+     *
+     * @param allocation current node allocation
+     * @param shard shard to weigh
+     * @return map of nodes to float weights
+     */
+    Map<DiscoveryNode, Float> weighShard(RoutingAllocation allocation, ShardRouting shard);
 }

core/src/main/java/org/elasticsearch/cluster/routing/allocation/decider/AwarenessAllocationDecider.java

@@ -110,7 +110,8 @@ public class AwarenessAllocationDecider extends AllocationDecider {
         this.awarenessAttributes = CLUSTER_ROUTING_ALLOCATION_AWARENESS_ATTRIBUTE_SETTING.get(settings);
         clusterSettings.addSettingsUpdateConsumer(CLUSTER_ROUTING_ALLOCATION_AWARENESS_ATTRIBUTE_SETTING, this::setAwarenessAttributes);
         setForcedAwarenessAttributes(CLUSTER_ROUTING_ALLOCATION_AWARENESS_FORCE_GROUP_SETTING.get(settings));
-        clusterSettings.addSettingsUpdateConsumer(CLUSTER_ROUTING_ALLOCATION_AWARENESS_FORCE_GROUP_SETTING, this::setForcedAwarenessAttributes);
+        clusterSettings.addSettingsUpdateConsumer(CLUSTER_ROUTING_ALLOCATION_AWARENESS_FORCE_GROUP_SETTING,
+                this::setForcedAwarenessAttributes);
     }
 
     private void setForcedAwarenessAttributes(Settings forceSettings) {
@@ -150,7 +151,7 @@ public class AwarenessAllocationDecider extends AllocationDecider {
 
     private Decision underCapacity(ShardRouting shardRouting, RoutingNode node, RoutingAllocation allocation, boolean moveToNode) {
         if (awarenessAttributes.length == 0) {
-            return allocation.decision(Decision.YES, NAME, "no allocation awareness enabled");
+            return allocation.decision(Decision.YES, NAME, "allocation awareness is not enabled");
         }
 
         IndexMetaData indexMetaData = allocation.metaData().getIndexSafe(shardRouting.index());
@@ -158,7 +159,7 @@ public class AwarenessAllocationDecider extends AllocationDecider {
         for (String awarenessAttribute : awarenessAttributes) {
             // the node the shard exists on must be associated with an awareness attribute
             if (!node.node().attributes().containsKey(awarenessAttribute)) {
-                return allocation.decision(Decision.NO, NAME, "node does not contain awareness attribute: [%s]", awarenessAttribute);
+                return allocation.decision(Decision.NO, NAME, "node does not contain the awareness attribute: [%s]", awarenessAttribute);
             }
 
             // build attr_value -> nodes map
@@ -180,7 +181,8 @@ public class AwarenessAllocationDecider extends AllocationDecider {
                     String nodeId = shardRouting.relocating() ? shardRouting.relocatingNodeId() : shardRouting.currentNodeId();
                     if (!node.nodeId().equals(nodeId)) {
                         // we work on different nodes, move counts around
-                        shardPerAttribute.putOrAdd(allocation.routingNodes().node(nodeId).node().attributes().get(awarenessAttribute), 0, -1);
+                        shardPerAttribute.putOrAdd(allocation.routingNodes().node(nodeId).node().attributes().get(awarenessAttribute),
+                                0, -1);
                         shardPerAttribute.addTo(node.node().attributes().get(awarenessAttribute), 1);
                     }
                 } else {
@@ -215,8 +217,15 @@ public class AwarenessAllocationDecider extends AllocationDecider {
             // if we are above with leftover, then we know we are not good, even with mod
             if (currentNodeCount > (requiredCountPerAttribute + leftoverPerAttribute)) {
                 return allocation.decision(Decision.NO, NAME,
-                        "too many shards on node for attribute: [%s], required per attribute: [%d], node count: [%d], leftover: [%d]",
-                        awarenessAttribute, requiredCountPerAttribute, currentNodeCount, leftoverPerAttribute);
+                        "there are too many shards on the node for attribute [%s], there are [%d] total shards for the index " +
+                        " and [%d] total attributes values, expected the node count [%d] to be lower or equal to the required " +
+                        "number of shards per attribute [%d] plus leftover [%d]",
+                        awarenessAttribute,
+                        shardCount,
+                        numberOfAttributes,
+                        currentNodeCount,
+                        requiredCountPerAttribute,
+                        leftoverPerAttribute);
             }
             // all is well, we are below or same as average
             if (currentNodeCount <= requiredCountPerAttribute) {
@@ -224,6 +233,6 @@ public class AwarenessAllocationDecider extends AllocationDecider {
             }
         }
 
-        return allocation.decision(Decision.YES, NAME, "node meets awareness requirements");
+        return allocation.decision(Decision.YES, NAME, "node meets all awareness attribute requirements");
     }
 }

core/src/main/java/org/elasticsearch/cluster/routing/allocation/decider/ClusterRebalanceAllocationDecider.java

@@ -78,7 +78,8 @@ public class ClusterRebalanceAllocationDecider extends AllocationDecider {
             } else if ("indices_all_active".equalsIgnoreCase(typeString) || "indicesAllActive".equalsIgnoreCase(typeString)) {
                 return ClusterRebalanceType.INDICES_ALL_ACTIVE;
             }
-            throw new IllegalArgumentException("Illegal value for " + CLUSTER_ROUTING_ALLOCATION_ALLOW_REBALANCE_SETTING + ": " + typeString);
+            throw new IllegalArgumentException("Illegal value for " +
+                            CLUSTER_ROUTING_ALLOCATION_ALLOW_REBALANCE_SETTING + ": " + typeString);
         }
     }
 
@@ -90,10 +91,13 @@ public class ClusterRebalanceAllocationDecider extends AllocationDecider {
         try {
             type = CLUSTER_ROUTING_ALLOCATION_ALLOW_REBALANCE_SETTING.get(settings);
         } catch (IllegalStateException e) {
-            logger.warn("[{}] has a wrong value {}, defaulting to 'indices_all_active'", CLUSTER_ROUTING_ALLOCATION_ALLOW_REBALANCE_SETTING, CLUSTER_ROUTING_ALLOCATION_ALLOW_REBALANCE_SETTING.getRaw(settings));
+            logger.warn("[{}] has a wrong value {}, defaulting to 'indices_all_active'",
+                    CLUSTER_ROUTING_ALLOCATION_ALLOW_REBALANCE_SETTING,
+                    CLUSTER_ROUTING_ALLOCATION_ALLOW_REBALANCE_SETTING.getRaw(settings));
             type = ClusterRebalanceType.INDICES_ALL_ACTIVE;
         }
-        logger.debug("using [{}] with [{}]", CLUSTER_ROUTING_ALLOCATION_ALLOW_REBALANCE_SETTING.getKey(), type.toString().toLowerCase(Locale.ROOT));
+        logger.debug("using [{}] with [{}]", CLUSTER_ROUTING_ALLOCATION_ALLOW_REBALANCE_SETTING.getKey(),
+                type.toString().toLowerCase(Locale.ROOT));
 
         clusterSettings.addSettingsUpdateConsumer(CLUSTER_ROUTING_ALLOCATION_ALLOW_REBALANCE_SETTING, this::setType);
     }
@@ -112,11 +116,13 @@ public class ClusterRebalanceAllocationDecider extends AllocationDecider {
         if (type == ClusterRebalanceType.INDICES_PRIMARIES_ACTIVE) {
             // check if there are unassigned primaries.
             if ( allocation.routingNodes().hasUnassignedPrimaries() ) {
-                return allocation.decision(Decision.NO, NAME, "cluster has unassigned primary shards");
+                return allocation.decision(Decision.NO, NAME,
+                        "the cluster has unassigned primary shards and rebalance type is set to [%s]", type);
             }
             // check if there are initializing primaries that don't have a relocatingNodeId entry.
             if ( allocation.routingNodes().hasInactivePrimaries() ) {
-                return allocation.decision(Decision.NO, NAME, "cluster has inactive primary shards");
+                return allocation.decision(Decision.NO, NAME,
+                        "the cluster has inactive primary shards and rebalance type is set to [%s]", type);
             }
 
             return allocation.decision(Decision.YES, NAME, "all primary shards are active");
@@ -124,15 +130,17 @@ public class ClusterRebalanceAllocationDecider extends AllocationDecider {
         if (type == ClusterRebalanceType.INDICES_ALL_ACTIVE) {
             // check if there are unassigned shards.
             if (allocation.routingNodes().hasUnassignedShards() ) {
-                return allocation.decision(Decision.NO, NAME, "cluster has unassigned shards");
+                return allocation.decision(Decision.NO, NAME,
+                        "the cluster has unassigned shards and rebalance type is set to [%s]", type);
             }
             // in case all indices are assigned, are there initializing shards which
             // are not relocating?
             if ( allocation.routingNodes().hasInactiveShards() ) {
-                return allocation.decision(Decision.NO, NAME, "cluster has inactive shards");
+                return allocation.decision(Decision.NO, NAME,
+                        "the cluster has inactive shards and rebalance type is set to [%s]", type);
             }
         }
         // type == Type.ALWAYS
-        return allocation.decision(Decision.YES, NAME, "all shards are active");
+        return allocation.decision(Decision.YES, NAME, "all shards are active, rebalance type is [%s]", type);
     }
 }

core/src/main/java/org/elasticsearch/cluster/routing/allocation/decider/ConcurrentRebalanceAllocationDecider.java

@@ -53,7 +53,8 @@ public class ConcurrentRebalanceAllocationDecider extends AllocationDecider {
         super(settings);
         this.clusterConcurrentRebalance = CLUSTER_ROUTING_ALLOCATION_CLUSTER_CONCURRENT_REBALANCE_SETTING.get(settings);
         logger.debug("using [cluster_concurrent_rebalance] with [{}]", clusterConcurrentRebalance);
-        clusterSettings.addSettingsUpdateConsumer(CLUSTER_ROUTING_ALLOCATION_CLUSTER_CONCURRENT_REBALANCE_SETTING, this::setClusterConcurrentRebalance);
+        clusterSettings.addSettingsUpdateConsumer(CLUSTER_ROUTING_ALLOCATION_CLUSTER_CONCURRENT_REBALANCE_SETTING,
+                this::setClusterConcurrentRebalance);
     }
 
     private void setClusterConcurrentRebalance(int concurrentRebalance) {
@@ -63,12 +64,16 @@ public class ConcurrentRebalanceAllocationDecider extends AllocationDecider {
     @Override
     public Decision canRebalance(ShardRouting shardRouting, RoutingAllocation allocation) {
         if (clusterConcurrentRebalance == -1) {
-            return allocation.decision(Decision.YES, NAME, "all concurrent rebalances are allowed");
+            return allocation.decision(Decision.YES, NAME, "unlimited concurrent rebalances are allowed");
         }
-        if (allocation.routingNodes().getRelocatingShardCount() >= clusterConcurrentRebalance) {
-            return allocation.decision(Decision.NO, NAME, "too many concurrent rebalances [%d], limit: [%d]",
-                    allocation.routingNodes().getRelocatingShardCount(), clusterConcurrentRebalance);
+        int relocatingShards = allocation.routingNodes().getRelocatingShardCount();
+        if (relocatingShards >= clusterConcurrentRebalance) {
+            return allocation.decision(Decision.NO, NAME,
+                    "too many shards are concurrently rebalancing [%d], limit: [%d]",
+                    relocatingShards, clusterConcurrentRebalance);
         }
-        return allocation.decision(Decision.YES, NAME, "below threshold [%d] for concurrent rebalances", clusterConcurrentRebalance);
+        return allocation.decision(Decision.YES, NAME,
+                "below threshold [%d] for concurrent rebalances, current rebalance shard count [%d]",
+                clusterConcurrentRebalance, relocatingShards);
     }
 }

core/src/main/java/org/elasticsearch/cluster/routing/allocation/decider/Decision.java

@@ -26,6 +26,7 @@ import org.elasticsearch.common.xcontent.XContentBuilder;
 
 import java.io.IOException;
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.List;
 import java.util.Locale;
 
@@ -145,6 +146,11 @@ public abstract class Decision implements ToXContent {
 
     public abstract String label();
 
+    /**
+     * Return the list of all decisions that make up this decision
+     */
+    public abstract List<Decision> getDecisions();
+
     /**
      * Simple class representing a single decision
      */
@@ -191,6 +197,11 @@ public abstract class Decision implements ToXContent {
             return this.label;
         }
 
+        @Override
+        public List<Decision> getDecisions() {
+            return Collections.singletonList(this);
+        }
+
         /**
          * Returns the explanation string, fully formatted. Only formats the string once
          */
@@ -202,11 +213,35 @@ public abstract class Decision implements ToXContent {
         }
 
         @Override
-        public String toString() {
-            if (explanation == null) {
-                return type + "()";
+        public boolean equals(Object object) {
+            if (this == object) {
+                return true;
             }
-            return type + "(" + getExplanation() + ")";
+
+            if (object == null || getClass() != object.getClass()) {
+                return false;
+            }
+
+            Decision.Single s = (Decision.Single) object;
+            return this.type == s.type &&
+                    this.label.equals(s.label) &&
+                    this.getExplanation().equals(s.getExplanation());
+        }
+
+        @Override
+        public int hashCode() {
+            int result = this.type.hashCode();
+            result = 31 * result + this.label.hashCode();
+            result = 31 * result + this.getExplanation().hashCode();
+            return result;
+        }
+
+        @Override
+        public String toString() {
+            if (explanationString != null || explanation != null) {
+                return type + "(" + getExplanation() + ")";
+            }
+            return type + "()";
         }
 
         @Override
@@ -258,6 +293,31 @@ public abstract class Decision implements ToXContent {
             return null;
         }
 
+        @Override
+        public List<Decision> getDecisions() {
+            return Collections.unmodifiableList(this.decisions);
+        }
+
+        @Override
+        public boolean equals(final Object object) {
+            if (this == object) {
+                return true;
+            }
+
+            if (object == null || getClass() != object.getClass()) {
+                return false;
+            }
+
+            final Decision.Multi m = (Decision.Multi) object;
+
+            return this.decisions.equals(m.decisions);
+        }
+
+        @Override
+        public int hashCode() {
+            return 31 * decisions.hashCode();
+        }
+
         @Override
         public String toString() {
             StringBuilder sb = new StringBuilder();

core/src/main/java/org/elasticsearch/cluster/routing/allocation/decider/DiskThresholdDecider.java

@@ -164,7 +164,8 @@ public class DiskThresholdDecider extends AllocationDecider {
                             reroute = true;
                             explanation = "high disk watermark exceeded on one or more nodes";
                         } else {
-                            logger.debug("high disk watermark exceeded on {} but an automatic reroute has occurred in the last [{}], skipping reroute",
+                            logger.debug("high disk watermark exceeded on {} but an automatic reroute has occurred " +
+                                            "in the last [{}], skipping reroute",
                                     node, DiskThresholdDecider.this.rerouteInterval);
                         }
                         nodeHasPassedWatermark.add(node);
@@ -183,7 +184,8 @@ public class DiskThresholdDecider extends AllocationDecider {
                                 explanation = "one or more nodes has gone under the high or low watermark";
                                 nodeHasPassedWatermark.remove(node);
                             } else {
-                                logger.debug("{} has gone below a disk threshold, but an automatic reroute has occurred in the last [{}], skipping reroute",
+                                logger.debug("{} has gone below a disk threshold, but an automatic reroute has occurred " +
+                                                "in the last [{}], skipping reroute",
                                         node, DiskThresholdDecider.this.rerouteInterval);
                             }
                         }
@@ -238,13 +240,15 @@ public class DiskThresholdDecider extends AllocationDecider {
     private void setLowWatermark(String lowWatermark) {
         // Watermark is expressed in terms of used data, but we need "free" data watermark
         this.freeDiskThresholdLow = 100.0 - thresholdPercentageFromWatermark(lowWatermark);
-        this.freeBytesThresholdLow = thresholdBytesFromWatermark(lowWatermark, CLUSTER_ROUTING_ALLOCATION_LOW_DISK_WATERMARK_SETTING.getKey());
+        this.freeBytesThresholdLow = thresholdBytesFromWatermark(lowWatermark,
+                CLUSTER_ROUTING_ALLOCATION_LOW_DISK_WATERMARK_SETTING.getKey());
     }
 
     private void setHighWatermark(String highWatermark) {
         // Watermark is expressed in terms of used data, but we need "free" data watermark
         this.freeDiskThresholdHigh = 100.0 - thresholdPercentageFromWatermark(highWatermark);
-        this.freeBytesThresholdHigh = thresholdBytesFromWatermark(highWatermark, CLUSTER_ROUTING_ALLOCATION_LOW_DISK_WATERMARK_SETTING.getKey());
+        this.freeBytesThresholdHigh = thresholdBytesFromWatermark(highWatermark,
+                CLUSTER_ROUTING_ALLOCATION_LOW_DISK_WATERMARK_SETTING.getKey());
     }
 
     // For Testing
@@ -299,7 +303,8 @@ public class DiskThresholdDecider extends AllocationDecider {
      * If subtractShardsMovingAway is set then the size of shards moving away is subtracted from the total size
      * of all shards
      */
-    public static long sizeOfRelocatingShards(RoutingNode node, ClusterInfo clusterInfo, boolean subtractShardsMovingAway, String dataPath) {
+    public static long sizeOfRelocatingShards(RoutingNode node, ClusterInfo clusterInfo,
+                                              boolean subtractShardsMovingAway, String dataPath) {
         long totalSize = 0;
         for (ShardRouting routing : node.shardsWithState(ShardRoutingState.RELOCATING, ShardRoutingState.INITIALIZING)) {
             String actualPath = clusterInfo.getDataPath(routing);
@@ -353,7 +358,8 @@ public class DiskThresholdDecider extends AllocationDecider {
                     logger.debug("less than the required {} free bytes threshold ({} bytes free) on node {}, preventing allocation",
                             freeBytesThresholdLow, freeBytes, node.nodeId());
                 }
-                return allocation.decision(Decision.NO, NAME, "less than required [%s] free on node, free: [%s]",
+                return allocation.decision(Decision.NO, NAME,
+                        "the node is above the low watermark and has less than required [%s] free, free: [%s]",
                         freeBytesThresholdLow, new ByteSizeValue(freeBytes));
             } else if (freeBytes > freeBytesThresholdHigh.bytes()) {
                 // Allow the shard to be allocated because it is primary that
@@ -363,7 +369,8 @@ public class DiskThresholdDecider extends AllocationDecider {
                                     "but allowing allocation because primary has never been allocated",
                             freeBytesThresholdLow, freeBytes, node.nodeId());
                 }
-                return allocation.decision(Decision.YES, NAME, "primary has never been allocated before");
+                return allocation.decision(Decision.YES, NAME,
+                        "the node is above the low watermark, but this primary shard has never been allocated before");
             } else {
                 // Even though the primary has never been allocated, the node is
                 // above the high watermark, so don't allow allocating the shard
@@ -372,7 +379,9 @@ public class DiskThresholdDecider extends AllocationDecider {
                                     "preventing allocation even though primary has never been allocated",
                             freeBytesThresholdHigh, freeBytes, node.nodeId());
                 }
-                return allocation.decision(Decision.NO, NAME, "less than required [%s] free on node, free: [%s]",
+                return allocation.decision(Decision.NO, NAME,
+                        "the node is above the high watermark even though this shard has never been allocated " +
+                                "and has less than required [%s] free on node, free: [%s]",
                         freeBytesThresholdHigh, new ByteSizeValue(freeBytes));
             }
         }
@@ -386,7 +395,8 @@ public class DiskThresholdDecider extends AllocationDecider {
                             Strings.format1Decimals(usedDiskThresholdLow, "%"),
                             Strings.format1Decimals(usedDiskPercentage, "%"), node.nodeId());
                 }
-                return allocation.decision(Decision.NO, NAME, "more than allowed [%s%%] used disk on node, free: [%s%%]",
+                return allocation.decision(Decision.NO, NAME,
+                        "the node is above the low watermark and has more than allowed [%s%%] used disk, free: [%s%%]",
                         usedDiskThresholdLow, freeDiskPercentage);
             } else if (freeDiskPercentage > freeDiskThresholdHigh) {
                 // Allow the shard to be allocated because it is primary that
@@ -397,7 +407,8 @@ public class DiskThresholdDecider extends AllocationDecider {
                             Strings.format1Decimals(usedDiskThresholdLow, "%"),
                             Strings.format1Decimals(usedDiskPercentage, "%"), node.nodeId());
                 }
-                return allocation.decision(Decision.YES, NAME, "primary has never been allocated before");
+                return allocation.decision(Decision.YES, NAME,
+                        "the node is above the low watermark, but this primary shard has never been allocated before");
             } else {
                 // Even though the primary has never been allocated, the node is
                 // above the high watermark, so don't allow allocating the shard
@@ -407,7 +418,9 @@ public class DiskThresholdDecider extends AllocationDecider {
                             Strings.format1Decimals(freeDiskThresholdHigh, "%"),
                             Strings.format1Decimals(freeDiskPercentage, "%"), node.nodeId());
                 }
-                return allocation.decision(Decision.NO, NAME, "more than allowed [%s%%] used disk on node, free: [%s%%]",
+                return allocation.decision(Decision.NO, NAME,
+                        "the node is above the high watermark even though this shard has never been allocated " +
+                                "and has more than allowed [%s%%] used disk, free: [%s%%]",
                         usedDiskThresholdHigh, freeDiskPercentage);
             }
         }
@@ -417,19 +430,29 @@ public class DiskThresholdDecider extends AllocationDecider {
         double freeSpaceAfterShard = freeDiskPercentageAfterShardAssigned(usage, shardSize);
         long freeBytesAfterShard = freeBytes - shardSize;
         if (freeBytesAfterShard < freeBytesThresholdHigh.bytes()) {
-            logger.warn("after allocating, node [{}] would have less than the required {} free bytes threshold ({} bytes free), preventing allocation",
+            logger.warn("after allocating, node [{}] would have less than the required " +
+                    "{} free bytes threshold ({} bytes free), preventing allocation",
                     node.nodeId(), freeBytesThresholdHigh, freeBytesAfterShard);
-            return allocation.decision(Decision.NO, NAME, "after allocation less than required [%s] free on node, free: [%s]",
+            return allocation.decision(Decision.NO, NAME,
+                    "after allocating the shard to this node, it would be above the high watermark " +
+                            "and have less than required [%s] free, free: [%s]",
                     freeBytesThresholdLow, new ByteSizeValue(freeBytesAfterShard));
         }
         if (freeSpaceAfterShard < freeDiskThresholdHigh) {
-            logger.warn("after allocating, node [{}] would have more than the allowed {} free disk threshold ({} free), preventing allocation",
+            logger.warn("after allocating, node [{}] would have more than the allowed " +
+                            "{} free disk threshold ({} free), preventing allocation",
                     node.nodeId(), Strings.format1Decimals(freeDiskThresholdHigh, "%"), Strings.format1Decimals(freeSpaceAfterShard, "%"));
-            return allocation.decision(Decision.NO, NAME, "after allocation more than allowed [%s%%] used disk on node, free: [%s%%]",
+            return allocation.decision(Decision.NO, NAME,
+                    "after allocating the shard to this node, it would be above the high watermark " +
+                            "and have more than allowed [%s%%] used disk, free: [%s%%]",
                     usedDiskThresholdLow, freeSpaceAfterShard);
         }
 
-        return allocation.decision(Decision.YES, NAME, "enough disk for shard on node, free: [%s]", new ByteSizeValue(freeBytes));
+        return allocation.decision(Decision.YES, NAME,
+                "enough disk for shard on node, free: [%s], shard size: [%s], free after allocating shard: [%s]",
+                new ByteSizeValue(freeBytes),
+                new ByteSizeValue(shardSize),
+                new ByteSizeValue(freeBytesAfterShard));
     }
 
     @Override
@@ -453,14 +476,17 @@ public class DiskThresholdDecider extends AllocationDecider {
             logger.trace("node [{}] has {}% free disk ({} bytes)", node.nodeId(), freeDiskPercentage, freeBytes);
         }
         if (dataPath == null || usage.getPath().equals(dataPath) == false) {
-            return allocation.decision(Decision.YES, NAME, "shard is not allocated on the most utilized disk");
+            return allocation.decision(Decision.YES, NAME,
+                    "this shard is not allocated on the most utilized disk and can remain");
         }
         if (freeBytes < freeBytesThresholdHigh.bytes()) {
             if (logger.isDebugEnabled()) {
                 logger.debug("less than the required {} free bytes threshold ({} bytes free) on node {}, shard cannot remain",
                         freeBytesThresholdHigh, freeBytes, node.nodeId());
             }
-            return allocation.decision(Decision.NO, NAME, "after allocation less than required [%s] free on node, free: [%s]",
+            return allocation.decision(Decision.NO, NAME,
+                    "after allocating this shard this node would be above the high watermark " +
+                            "and there would be less than required [%s] free on node, free: [%s]",
                     freeBytesThresholdHigh, new ByteSizeValue(freeBytes));
         }
         if (freeDiskPercentage < freeDiskThresholdHigh) {
@@ -468,11 +494,14 @@ public class DiskThresholdDecider extends AllocationDecider {
                 logger.debug("less than the required {}% free disk threshold ({}% free) on node {}, shard cannot remain",
                         freeDiskThresholdHigh, freeDiskPercentage, node.nodeId());
             }
-            return allocation.decision(Decision.NO, NAME, "after allocation less than required [%s%%] free disk on node, free: [%s%%]",
+            return allocation.decision(Decision.NO, NAME,
+                    "after allocating this shard this node would be above the high watermark " +
+                            "and there would be less than required [%s%%] free disk on node, free: [%s%%]",
                     freeDiskThresholdHigh, freeDiskPercentage);
         }
 
-        return allocation.decision(Decision.YES, NAME, "enough disk for shard to remain on node, free: [%s]", new ByteSizeValue(freeBytes));
+        return allocation.decision(Decision.YES, NAME,
+                "there is enough disk on this node for the shard to remain, free: [%s]", new ByteSizeValue(freeBytes));
     }
 
     private DiskUsage getDiskUsage(RoutingNode node, RoutingAllocation allocation, ImmutableOpenMap<String, DiskUsage> usages) {
@@ -543,7 +572,8 @@ public class DiskThresholdDecider extends AllocationDecider {
         try {
             return RatioValue.parseRatioValue(watermark).getAsPercent();
         } catch (ElasticsearchParseException ex) {
-            // NOTE: this is not end-user leniency, since up above we check that it's a valid byte or percentage, and then store the two cases separately
+            // NOTE: this is not end-user leniency, since up above we check that it's a valid byte or percentage, and then store the two
+            // cases separately
             return 100.0;
         }
     }
@@ -556,7 +586,8 @@ public class DiskThresholdDecider extends AllocationDecider {
         try {
             return ByteSizeValue.parseBytesSizeValue(watermark, settingName);
         } catch (ElasticsearchParseException ex) {
-            // NOTE: this is not end-user leniency, since up above we check that it's a valid byte or percentage, and then store the two cases separately
+            // NOTE: this is not end-user leniency, since up above we check that it's a valid byte or percentage, and then store the two
+            // cases separately
             return ByteSizeValue.parseBytesSizeValue("0b", settingName);
         }
     }
@@ -583,7 +614,7 @@ public class DiskThresholdDecider extends AllocationDecider {
     private Decision earlyTerminate(RoutingAllocation allocation, ImmutableOpenMap<String, DiskUsage> usages) {
         // Always allow allocation if the decider is disabled
         if (!enabled) {
-            return allocation.decision(Decision.YES, NAME, "disk threshold decider disabled");
+            return allocation.decision(Decision.YES, NAME, "the disk threshold decider is disabled");
         }
 
         // Allow allocation regardless if only a single data node is available
@@ -591,7 +622,7 @@ public class DiskThresholdDecider extends AllocationDecider {
             if (logger.isTraceEnabled()) {
                 logger.trace("only a single data node is present, allowing allocation");
             }
-            return allocation.decision(Decision.YES, NAME, "only a single data node is present");
+            return allocation.decision(Decision.YES, NAME, "there is only a single data node present");
         }
 
         // Fail open there is no info available
@@ -600,7 +631,7 @@ public class DiskThresholdDecider extends AllocationDecider {
             if (logger.isTraceEnabled()) {
                 logger.trace("cluster info unavailable for disk threshold decider, allowing allocation.");
             }
-            return allocation.decision(Decision.YES, NAME, "cluster info unavailable");
+            return allocation.decision(Decision.YES, NAME, "the cluster info is unavailable");
         }
 
         // Fail open if there are no disk usages available
@@ -608,7 +639,7 @@ public class DiskThresholdDecider extends AllocationDecider {
             if (logger.isTraceEnabled()) {
                 logger.trace("unable to determine disk usages for disk-aware allocation, allowing allocation");
             }
-            return allocation.decision(Decision.YES, NAME, "disk usages unavailable");
+            return allocation.decision(Decision.YES, NAME, "disk usages are unavailable");
         }
         return null;
     }

core/src/main/java/org/elasticsearch/cluster/routing/allocation/decider/EnableAllocationDecider.java

@@ -32,8 +32,9 @@ import org.elasticsearch.common.settings.Settings;
 import java.util.Locale;
 
 /**
- * This allocation decider allows shard allocations / rebalancing via the cluster wide settings {@link #CLUSTER_ROUTING_ALLOCATION_ENABLE_SETTING} /
- * {@link #CLUSTER_ROUTING_REBALANCE_ENABLE_SETTING} and the per index setting {@link #INDEX_ROUTING_ALLOCATION_ENABLE_SETTING} / {@link #INDEX_ROUTING_REBALANCE_ENABLE_SETTING}.
+ * This allocation decider allows shard allocations / rebalancing via the cluster wide settings
+ * {@link #CLUSTER_ROUTING_ALLOCATION_ENABLE_SETTING} / {@link #CLUSTER_ROUTING_REBALANCE_ENABLE_SETTING} and the per index setting
+ * {@link #INDEX_ROUTING_ALLOCATION_ENABLE_SETTING} / {@link #INDEX_ROUTING_REBALANCE_ENABLE_SETTING}.
  * The per index settings overrides the cluster wide setting.
  *
  * <p>
@@ -98,7 +99,7 @@ public class EnableAllocationDecider extends AllocationDecider {
     @Override
     public Decision canAllocate(ShardRouting shardRouting, RoutingNode node, RoutingAllocation allocation) {
         if (allocation.ignoreDisable()) {
-            return allocation.decision(Decision.YES, NAME, "allocation disabling is ignored");
+            return allocation.decision(Decision.YES, NAME, "allocation is explicitly ignoring any disabling of allocation");
         }
 
         final IndexMetaData indexMetaData = allocation.metaData().getIndexSafe(shardRouting.index());
@@ -133,7 +134,7 @@ public class EnableAllocationDecider extends AllocationDecider {
     @Override
     public Decision canRebalance(ShardRouting shardRouting, RoutingAllocation allocation) {
         if (allocation.ignoreDisable()) {
-            return allocation.decision(Decision.YES, NAME, "rebalance disabling is ignored");
+            return allocation.decision(Decision.YES, NAME, "allocation is explicitly ignoring any disabling of relocation");
         }
 
         Settings indexSettings = allocation.routingNodes().metaData().getIndexSafe(shardRouting.index()).getSettings();
@@ -167,7 +168,8 @@ public class EnableAllocationDecider extends AllocationDecider {
 
     /**
      * Allocation values or rather their string representation to be used used with
-     * {@link EnableAllocationDecider#CLUSTER_ROUTING_ALLOCATION_ENABLE_SETTING} / {@link EnableAllocationDecider#INDEX_ROUTING_ALLOCATION_ENABLE_SETTING}
+     * {@link EnableAllocationDecider#CLUSTER_ROUTING_ALLOCATION_ENABLE_SETTING} /
+     * {@link EnableAllocationDecider#INDEX_ROUTING_ALLOCATION_ENABLE_SETTING}
      * via cluster / index settings.
      */
     public enum Allocation {
@@ -193,7 +195,8 @@ public class EnableAllocationDecider extends AllocationDecider {
 
     /**
      * Rebalance values or rather their string representation to be used used with
-     * {@link EnableAllocationDecider#CLUSTER_ROUTING_REBALANCE_ENABLE_SETTING} / {@link EnableAllocationDecider#INDEX_ROUTING_REBALANCE_ENABLE_SETTING}
+     * {@link EnableAllocationDecider#CLUSTER_ROUTING_REBALANCE_ENABLE_SETTING} /
+     * {@link EnableAllocationDecider#INDEX_ROUTING_REBALANCE_ENABLE_SETTING}
      * via cluster / index settings.
      */
     public enum Rebalance {

core/src/main/java/org/elasticsearch/cluster/routing/allocation/decider/FilterAllocationDecider.java

@@ -50,11 +50,14 @@ import static org.elasticsearch.cluster.node.DiscoveryNodeFilters.OpType.OR;
  * would disallow the allocation. Filters are applied in the following order:
  * <ol>
  * <li><tt>required</tt> - filters required allocations.
- * If any <tt>required</tt> filters are set the allocation is denied if the index is <b>not</b> in the set of <tt>required</tt> to allocate on the filtered node</li>
+ * If any <tt>required</tt> filters are set the allocation is denied if the index is <b>not</b> in the set of <tt>required</tt> to allocate
+ * on the filtered node</li>
  * <li><tt>include</tt> - filters "allowed" allocations.
- * If any <tt>include</tt> filters are set the allocation is denied if the index is <b>not</b> in the set of <tt>include</tt> filters for the filtered node</li>
+ * If any <tt>include</tt> filters are set the allocation is denied if the index is <b>not</b> in the set of <tt>include</tt> filters for
+ * the filtered node</li>
  * <li><tt>exclude</tt> - filters "prohibited" allocations.
- * If any <tt>exclude</tt> filters are set the allocation is denied if the index is in the set of <tt>exclude</tt> filters for the filtered node</li>
+ * If any <tt>exclude</tt> filters are set the allocation is denied if the index is in the set of <tt>exclude</tt> filters for the
+ * filtered node</li>
  * </ol>
  */
 public class FilterAllocationDecider extends AllocationDecider {

core/src/main/java/org/elasticsearch/cluster/routing/allocation/decider/NodeVersionAllocationDecider.java

@@ -52,7 +52,7 @@ public class NodeVersionAllocationDecider extends AllocationDecider {
                     return isVersionCompatible(shardRouting.restoreSource(), node, allocation);
                 } else {
                     // fresh primary, we can allocate wherever
-                    return allocation.decision(Decision.YES, NAME, "primary shard can be allocated anywhere");
+                    return allocation.decision(Decision.YES, NAME, "the primary shard is new and can be allocated anywhere");
                 }
             } else {
                 // relocating primary, only migrate to newer host
@@ -70,16 +70,17 @@ public class NodeVersionAllocationDecider extends AllocationDecider {
         }
     }
 
-    private Decision isVersionCompatible(final RoutingNodes routingNodes, final String sourceNodeId, final RoutingNode target, RoutingAllocation allocation) {
+    private Decision isVersionCompatible(final RoutingNodes routingNodes, final String sourceNodeId, final RoutingNode target,
+                                         RoutingAllocation allocation) {
         final RoutingNode source = routingNodes.node(sourceNodeId);
         if (target.node().version().onOrAfter(source.node().version())) {
             /* we can allocate if we can recover from a node that is younger or on the same version
              * if the primary is already running on a newer version that won't work due to possible
              * differences in the lucene index format etc.*/
-            return allocation.decision(Decision.YES, NAME, "target node version [%s] is same or newer than source node version [%s]",
+            return allocation.decision(Decision.YES, NAME, "target node version [%s] is the same or newer than source node version [%s]",
                     target.node().version(), source.node().version());
         } else {
-            return allocation.decision(Decision.NO, NAME, "target node version [%s] is older than source node version [%s]",
+            return allocation.decision(Decision.NO, NAME, "target node version [%s] is older than the source node version [%s]",
                     target.node().version(), source.node().version());
         }
     }
@@ -87,10 +88,10 @@ public class NodeVersionAllocationDecider extends AllocationDecider {
     private Decision isVersionCompatible(RestoreSource restoreSource, final RoutingNode target, RoutingAllocation allocation) {
         if (target.node().version().onOrAfter(restoreSource.version())) {
             /* we can allocate if we can restore from a snapshot that is older or on the same version */
-            return allocation.decision(Decision.YES, NAME, "target node version [%s] is same or newer than snapshot version [%s]",
+            return allocation.decision(Decision.YES, NAME, "target node version [%s] is the same or newer than snapshot version [%s]",
                 target.node().version(), restoreSource.version());
         } else {
-            return allocation.decision(Decision.NO, NAME, "target node version [%s] is older than snapshot version [%s]",
+            return allocation.decision(Decision.NO, NAME, "target node version [%s] is older than the snapshot version [%s]",
                 target.node().version(), restoreSource.version());
         }
     }

core/src/main/java/org/elasticsearch/cluster/routing/allocation/decider/RebalanceOnlyWhenActiveAllocationDecider.java

@@ -41,8 +41,8 @@ public class RebalanceOnlyWhenActiveAllocationDecider extends AllocationDecider
         // its ok to check for active here, since in relocation, a shard is split into two in routing
         // nodes, once relocating, and one initializing
         if (!allocation.routingNodes().allReplicasActive(shardRouting)) {
-            return allocation.decision(Decision.NO, NAME, "not all replicas are active in cluster");
+            return allocation.decision(Decision.NO, NAME, "rebalancing can not occur if not all replicas are active in the cluster");
         }
-        return allocation.decision(Decision.YES, NAME, "all replicas are active in cluster");
+        return allocation.decision(Decision.YES, NAME, "all replicas are active in the cluster, rebalancing can occur");
     }
 }

core/src/main/java/org/elasticsearch/cluster/routing/allocation/decider/ReplicaAfterPrimaryActiveAllocationDecider.java

@@ -45,12 +45,12 @@ public class ReplicaAfterPrimaryActiveAllocationDecider extends AllocationDecide
     @Override
     public Decision canAllocate(ShardRouting shardRouting, RoutingAllocation allocation) {
         if (shardRouting.primary()) {
-            return allocation.decision(Decision.YES, NAME, "shard is primary");
+            return allocation.decision(Decision.YES, NAME, "shard is primary and can be allocated");
         }
         ShardRouting primary = allocation.routingNodes().activePrimary(shardRouting);
         if (primary == null) {
-            return allocation.decision(Decision.NO, NAME, "primary shard is not yet active");
+            return allocation.decision(Decision.NO, NAME, "primary shard for this replica is not yet active");
         }
-        return allocation.decision(Decision.YES, NAME, "primary is already active");
+        return allocation.decision(Decision.YES, NAME, "primary shard for this replica is already active");
     }
 }

core/src/main/java/org/elasticsearch/cluster/routing/allocation/decider/SameShardAllocationDecider.java

@@ -61,7 +61,8 @@ public class SameShardAllocationDecider extends AllocationDecider {
         Iterable<ShardRouting> assignedShards = allocation.routingNodes().assignedShards(shardRouting);
         for (ShardRouting assignedShard : assignedShards) {
             if (node.nodeId().equals(assignedShard.currentNodeId())) {
-                return allocation.decision(Decision.NO, NAME, "shard cannot be allocated on same node [%s] it already exists on", node.nodeId());
+                return allocation.decision(Decision.NO, NAME,
+                        "the shard cannot be allocated on the same node id [%s] on which it already exists", node.nodeId());
             }
         }
         if (sameHost) {
@@ -85,7 +86,7 @@ public class SameShardAllocationDecider extends AllocationDecider {
                         for (ShardRouting assignedShard : assignedShards) {
                             if (checkNode.nodeId().equals(assignedShard.currentNodeId())) {
                                 return allocation.decision(Decision.NO, NAME,
-                                        "shard cannot be allocated on same host [%s] it already exists on", node.nodeId());
+                                        "shard cannot be allocated on the same host [%s] on which it already exists", node.nodeId());
                             }
                         }
                     }

core/src/main/java/org/elasticsearch/cluster/routing/allocation/decider/ShardsLimitAllocationDecider.java

@@ -93,7 +93,7 @@ public class ShardsLimitAllocationDecider extends AllocationDecider {
         final int clusterShardLimit = this.clusterShardLimit;
 
         if (indexShardLimit <= 0 && clusterShardLimit <= 0) {
-            return allocation.decision(Decision.YES, NAME, "total shard limit disabled: [index: %d, cluster: %d] <= 0",
+            return allocation.decision(Decision.YES, NAME, "total shard limits are disabled: [index: %d, cluster: %d] <= 0",
                     indexShardLimit, clusterShardLimit);
         }
 
@@ -110,14 +110,16 @@ public class ShardsLimitAllocationDecider extends AllocationDecider {
             }
         }
         if (clusterShardLimit > 0 && nodeShardCount >= clusterShardLimit) {
-            return allocation.decision(Decision.NO, NAME, "too many shards for this node [%d], limit: [%d]",
+            return allocation.decision(Decision.NO, NAME, "too many shards for this node [%d], cluster-level limit per node: [%d]",
                     nodeShardCount, clusterShardLimit);
         }
         if (indexShardLimit > 0 && indexShardCount >= indexShardLimit) {
-            return allocation.decision(Decision.NO, NAME, "too many shards for this index [%s] on node [%d], limit: [%d]",
+            return allocation.decision(Decision.NO, NAME,
+                    "too many shards for this index [%s] on node [%d], index-level limit per node: [%d]",
                     shardRouting.index(), indexShardCount, indexShardLimit);
         }
-        return allocation.decision(Decision.YES, NAME, "shard count under index limit [%d] and node limit [%d] of total shards per node",
+        return allocation.decision(Decision.YES, NAME,
+                "the shard count is under index limit [%d] and cluster level node limit [%d] of total shards per node",
                 indexShardLimit, clusterShardLimit);
     }
 
@@ -130,7 +132,7 @@ public class ShardsLimitAllocationDecider extends AllocationDecider {
         final int clusterShardLimit = this.clusterShardLimit;
 
         if (indexShardLimit <= 0 && clusterShardLimit <= 0) {
-            return allocation.decision(Decision.YES, NAME, "total shard limit disabled: [index: %d, cluster: %d] <= 0",
+            return allocation.decision(Decision.YES, NAME, "total shard limits are disabled: [index: %d, cluster: %d] <= 0",
                     indexShardLimit, clusterShardLimit);
         }
 
@@ -149,14 +151,16 @@ public class ShardsLimitAllocationDecider extends AllocationDecider {
         // Subtle difference between the `canAllocate` and `canRemain` is that
         // this checks > while canAllocate checks >=
         if (clusterShardLimit > 0 && nodeShardCount > clusterShardLimit) {
-            return allocation.decision(Decision.NO, NAME, "too many shards for this node [%d], limit: [%d]",
+            return allocation.decision(Decision.NO, NAME, "too many shards for this node [%d], cluster-level limit per node: [%d]",
                     nodeShardCount, clusterShardLimit);
         }
         if (indexShardLimit > 0 && indexShardCount > indexShardLimit) {
-            return allocation.decision(Decision.NO, NAME, "too many shards for this index [%s] on node [%d], limit: [%d]",
+            return allocation.decision(Decision.NO, NAME,
+                    "too many shards for this index [%s] on node [%d], index-level limit per node: [%d]",
                     shardRouting.index(), indexShardCount, indexShardLimit);
         }
-        return allocation.decision(Decision.YES, NAME, "shard count under index limit [%d] and node limit [%d] of total shards per node",
+        return allocation.decision(Decision.YES, NAME,
+                "the shard count is under index limit [%d] and cluster level node limit [%d] of total shards per node",
                 indexShardLimit, clusterShardLimit);
     }
 
@@ -168,7 +172,7 @@ public class ShardsLimitAllocationDecider extends AllocationDecider {
         final int clusterShardLimit = this.clusterShardLimit;
 
         if (clusterShardLimit <= 0) {
-            return allocation.decision(Decision.YES, NAME, "total shard limit disabled: [cluster: %d] <= 0",
+            return allocation.decision(Decision.YES, NAME, "total shard limits are disabled: [cluster: %d] <= 0",
                     clusterShardLimit);
         }
 
@@ -181,10 +185,10 @@ public class ShardsLimitAllocationDecider extends AllocationDecider {
             nodeShardCount++;
         }
         if (clusterShardLimit >= 0 && nodeShardCount >= clusterShardLimit) {
-            return allocation.decision(Decision.NO, NAME, "too many shards for this node [%d], limit: [%d]",
+            return allocation.decision(Decision.NO, NAME, "too many shards for this node [%d], cluster-level limit per node: [%d]",
                     nodeShardCount, clusterShardLimit);
         }
-        return allocation.decision(Decision.YES, NAME, "shard count under node limit [%d] of total shards per node",
+        return allocation.decision(Decision.YES, NAME, "the shard count is under node limit [%d] of total shards per node",
                 clusterShardLimit);
     }
 }

core/src/main/java/org/elasticsearch/cluster/routing/allocation/decider/SnapshotInProgressAllocationDecider.java

@@ -54,7 +54,8 @@ public class SnapshotInProgressAllocationDecider extends AllocationDecider {
     }
 
     /**
-     * Creates a new {@link org.elasticsearch.cluster.routing.allocation.decider.SnapshotInProgressAllocationDecider} instance from given settings
+     * Creates a new {@link org.elasticsearch.cluster.routing.allocation.decider.SnapshotInProgressAllocationDecider} instance from
+     * given settings
      *
      * @param settings {@link org.elasticsearch.common.settings.Settings} to use
      */
@@ -66,7 +67,8 @@ public class SnapshotInProgressAllocationDecider extends AllocationDecider {
     public SnapshotInProgressAllocationDecider(Settings settings, ClusterSettings clusterSettings) {
         super(settings);
         enableRelocation = CLUSTER_ROUTING_ALLOCATION_SNAPSHOT_RELOCATION_ENABLED_SETTING.get(settings);
-        clusterSettings.addSettingsUpdateConsumer(CLUSTER_ROUTING_ALLOCATION_SNAPSHOT_RELOCATION_ENABLED_SETTING, this::setEnableRelocation);
+        clusterSettings.addSettingsUpdateConsumer(CLUSTER_ROUTING_ALLOCATION_SNAPSHOT_RELOCATION_ENABLED_SETTING,
+                this::setEnableRelocation);
     }
 
     private void setEnableRelocation(boolean enableRelocation) {
@@ -104,14 +106,18 @@ public class SnapshotInProgressAllocationDecider extends AllocationDecider {
 
             for (SnapshotsInProgress.Entry snapshot : snapshotsInProgress.entries()) {
                 SnapshotsInProgress.ShardSnapshotStatus shardSnapshotStatus = snapshot.shards().get(shardRouting.shardId());
-                if (shardSnapshotStatus != null && !shardSnapshotStatus.state().completed() && shardSnapshotStatus.nodeId() != null && shardSnapshotStatus.nodeId().equals(shardRouting.currentNodeId())) {
-                    logger.trace("Preventing snapshotted shard [{}] to be moved from node [{}]", shardRouting.shardId(), shardSnapshotStatus.nodeId());
+                if (shardSnapshotStatus != null && !shardSnapshotStatus.state().completed() && shardSnapshotStatus.nodeId() != null &&
+                        shardSnapshotStatus.nodeId().equals(shardRouting.currentNodeId())) {
+                    if (logger.isTraceEnabled()) {
+                        logger.trace("Preventing snapshotted shard [{}] to be moved from node [{}]",
+                                shardRouting.shardId(), shardSnapshotStatus.nodeId());
+                    }
                     return allocation.decision(Decision.NO, NAME, "snapshot for shard [%s] is currently running on node [%s]",
                             shardRouting.shardId(), shardSnapshotStatus.nodeId());
                 }
             }
         }
-        return allocation.decision(Decision.YES, NAME, "shard not primary or relocation disabled");
+        return allocation.decision(Decision.YES, NAME, "the shard is not primary or relocation is disabled");
     }
 
 }

core/src/main/java/org/elasticsearch/cluster/routing/allocation/decider/ThrottlingAllocationDecider.java

@@ -84,11 +84,16 @@ public class ThrottlingAllocationDecider extends AllocationDecider {
         concurrentIncomingRecoveries = CLUSTER_ROUTING_ALLOCATION_NODE_CONCURRENT_INCOMING_RECOVERIES_SETTING.get(settings);
         concurrentOutgoingRecoveries = CLUSTER_ROUTING_ALLOCATION_NODE_CONCURRENT_OUTGOING_RECOVERIES_SETTING.get(settings);
 
-        clusterSettings.addSettingsUpdateConsumer(CLUSTER_ROUTING_ALLOCATION_NODE_INITIAL_PRIMARIES_RECOVERIES_SETTING, this::setPrimariesInitialRecoveries);
-        clusterSettings.addSettingsUpdateConsumer(CLUSTER_ROUTING_ALLOCATION_NODE_CONCURRENT_INCOMING_RECOVERIES_SETTING, this::setConcurrentIncomingRecoverries);
-        clusterSettings.addSettingsUpdateConsumer(CLUSTER_ROUTING_ALLOCATION_NODE_CONCURRENT_OUTGOING_RECOVERIES_SETTING, this::setConcurrentOutgoingRecoverries);
+        clusterSettings.addSettingsUpdateConsumer(CLUSTER_ROUTING_ALLOCATION_NODE_INITIAL_PRIMARIES_RECOVERIES_SETTING,
+                this::setPrimariesInitialRecoveries);
+        clusterSettings.addSettingsUpdateConsumer(CLUSTER_ROUTING_ALLOCATION_NODE_CONCURRENT_INCOMING_RECOVERIES_SETTING,
+                this::setConcurrentIncomingRecoverries);
+        clusterSettings.addSettingsUpdateConsumer(CLUSTER_ROUTING_ALLOCATION_NODE_CONCURRENT_OUTGOING_RECOVERIES_SETTING,
+                this::setConcurrentOutgoingRecoverries);
 
-        logger.debug("using node_concurrent_outgoing_recoveries [{}], node_concurrent_incoming_recoveries [{}], node_initial_primaries_recoveries [{}]", concurrentOutgoingRecoveries, concurrentIncomingRecoveries, primariesInitialRecoveries);
+        logger.debug("using node_concurrent_outgoing_recoveries [{}], node_concurrent_incoming_recoveries [{}], " +
+                        "node_initial_primaries_recoveries [{}]",
+                concurrentOutgoingRecoveries, concurrentIncomingRecoveries, primariesInitialRecoveries);
     }
 
     private void setConcurrentIncomingRecoverries(int concurrentIncomingRecoveries) {
@@ -118,7 +123,7 @@ public class ThrottlingAllocationDecider extends AllocationDecider {
                     }
                 }
                 if (primariesInRecovery >= primariesInitialRecoveries) {
-                    return allocation.decision(Decision.THROTTLE, NAME, "too many primaries currently recovering [%d], limit: [%d]",
+                    return allocation.decision(Decision.THROTTLE, NAME, "too many primaries are currently recovering [%d], limit: [%d]",
                             primariesInRecovery, primariesInitialRecoveries);
                 } else {
                     return allocation.decision(Decision.YES, NAME, "below primary recovery limit of [%d]", primariesInitialRecoveries);
@@ -137,13 +142,17 @@ public class ThrottlingAllocationDecider extends AllocationDecider {
         int currentOutRecoveries = allocation.routingNodes().getOutgoingRecoveries(node.nodeId());
         int currentInRecoveries = allocation.routingNodes().getIncomingRecoveries(node.nodeId());
         if (currentOutRecoveries >= concurrentOutgoingRecoveries) {
-            return allocation.decision(Decision.THROTTLE, NAME, "too many outgoing shards currently recovering [%d], limit: [%d]",
+            return allocation.decision(Decision.THROTTLE, NAME, "too many outgoing shards are currently recovering [%d], limit: [%d]",
                 currentOutRecoveries, concurrentOutgoingRecoveries);
         } else if (currentInRecoveries >= concurrentIncomingRecoveries) {
-            return allocation.decision(Decision.THROTTLE, NAME, "too many incoming shards currently recovering [%d], limit: [%d]",
+            return allocation.decision(Decision.THROTTLE, NAME, "too many incoming shards are currently recovering [%d], limit: [%d]",
                 currentInRecoveries, concurrentIncomingRecoveries);
         }  else {
-            return allocation.decision(Decision.YES, NAME, "below shard recovery limit of outgoing: [%d] incoming: [%d]", concurrentOutgoingRecoveries, concurrentIncomingRecoveries);
+            return allocation.decision(Decision.YES, NAME, "below shard recovery limit of outgoing: [%d < %d] incoming: [%d < %d]",
+                    currentOutRecoveries,
+                    concurrentOutgoingRecoveries,
+                    currentInRecoveries,
+                    concurrentIncomingRecoveries);
         }
     }
 }

core/src/main/java/org/elasticsearch/common/network/NetworkModule.java

@@ -38,6 +38,7 @@ import org.elasticsearch.http.HttpServerTransport;
 import org.elasticsearch.http.netty.NettyHttpServerTransport;
 import org.elasticsearch.rest.RestController;
 import org.elasticsearch.rest.RestHandler;
+import org.elasticsearch.rest.action.admin.cluster.allocation.RestClusterAllocationExplainAction;
 import org.elasticsearch.rest.action.admin.cluster.health.RestClusterHealthAction;
 import org.elasticsearch.rest.action.admin.cluster.node.hotthreads.RestNodesHotThreadsAction;
 import org.elasticsearch.rest.action.admin.cluster.node.info.RestNodesInfoAction;
@@ -171,6 +172,7 @@ public class NetworkModule extends AbstractModule {
         RestNodesInfoAction.class,
         RestNodesStatsAction.class,
         RestNodesHotThreadsAction.class,
+        RestClusterAllocationExplainAction.class,
         RestClusterStatsAction.class,
         RestClusterStateAction.class,
         RestClusterHealthAction.class,

core/src/main/java/org/elasticsearch/rest/action/admin/cluster/allocation/RestClusterAllocationExplainAction.java

@@ -0,0 +1,89 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.elasticsearch.rest.action.admin.cluster.allocation;
+
+import org.elasticsearch.ExceptionsHelper;
+import org.elasticsearch.action.admin.cluster.allocation.ClusterAllocationExplainRequest;
+import org.elasticsearch.action.admin.cluster.allocation.ClusterAllocationExplainResponse;
+import org.elasticsearch.client.Client;
+import org.elasticsearch.client.Requests;
+import org.elasticsearch.common.bytes.BytesReference;
+import org.elasticsearch.common.inject.Inject;
+import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.common.xcontent.ToXContent;
+import org.elasticsearch.common.xcontent.XContentBuilder;
+import org.elasticsearch.common.xcontent.XContentFactory;
+import org.elasticsearch.common.xcontent.XContentParser;
+import org.elasticsearch.rest.BaseRestHandler;
+import org.elasticsearch.rest.BytesRestResponse;
+import org.elasticsearch.rest.RestChannel;
+import org.elasticsearch.rest.RestController;
+import org.elasticsearch.rest.RestRequest;
+import org.elasticsearch.rest.RestResponse;
+import org.elasticsearch.rest.RestStatus;
+import org.elasticsearch.rest.action.support.RestActions;
+import org.elasticsearch.rest.action.support.RestBuilderListener;
+
+import java.io.IOException;
+
+/**
+ * Class handling cluster allocation explanation at the REST level
+ */
+public class RestClusterAllocationExplainAction extends BaseRestHandler {
+
+    @Inject
+    public RestClusterAllocationExplainAction(Settings settings, RestController controller, Client client) {
+        super(settings, client);
+        controller.registerHandler(RestRequest.Method.GET, "/_cluster/allocation/explain", this);
+        controller.registerHandler(RestRequest.Method.POST, "/_cluster/allocation/explain", this);
+    }
+
+    @Override
+    public void handleRequest(final RestRequest request, final RestChannel channel, final Client client) {
+        ClusterAllocationExplainRequest req;
+        if (RestActions.hasBodyContent(request) == false) {
+            // Empty request signals "explain the first unassigned shard you find"
+            req = new ClusterAllocationExplainRequest();
+        } else {
+            BytesReference content = RestActions.getRestContent(request);
+            try (XContentParser parser = XContentFactory.xContent(content).createParser(content)) {
+                req = ClusterAllocationExplainRequest.parse(parser);
+            } catch (IOException e) {
+                logger.debug("failed to parse allocation explain request", e);
+                channel.sendResponse(new BytesRestResponse(ExceptionsHelper.status(e)));
+                return;
+            }
+        }
+
+        try {
+            req.includeYesDecisions(request.paramAsBoolean("include_yes_decisions", false));
+            client.admin().cluster().allocationExplain(req, new RestBuilderListener<ClusterAllocationExplainResponse>(channel) {
+                @Override
+                public RestResponse buildResponse(ClusterAllocationExplainResponse response, XContentBuilder builder) throws Exception {
+                    response.getExplanation().toXContent(builder, ToXContent.EMPTY_PARAMS);
+                    return new BytesRestResponse(RestStatus.OK, builder);
+                }
+            });
+        } catch (Exception e) {
+            logger.error("failed to explain allocation", e);
+            channel.sendResponse(new BytesRestResponse(ExceptionsHelper.status(e)));
+        }
+    }
+}

core/src/test/java/org/elasticsearch/action/admin/cluster/allocation/ClusterAllocationExplainTests.java

@@ -0,0 +1,86 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.elasticsearch.action.admin.cluster.allocation;
+
+import org.elasticsearch.client.Requests;
+import org.elasticsearch.cluster.routing.allocation.decider.Decision;
+import org.elasticsearch.test.ESSingleNodeTestCase;
+
+
+/**
+ * Tests for the cluster allocation explanation
+ */
+public final class ClusterAllocationExplainTests extends ESSingleNodeTestCase {
+
+    public void testShardExplain() throws Exception {
+        client().admin().indices().prepareCreate("test")
+                .setSettings("index.number_of_shards", 1, "index.number_of_replicas", 1).get();
+        client().admin().cluster().health(Requests.clusterHealthRequest("test").waitForYellowStatus()).get();
+        ClusterAllocationExplainResponse resp = client().admin().cluster().prepareAllocationExplain()
+                .setIndex("test").setShard(0).setPrimary(false).get();
+
+        ClusterAllocationExplanation cae = resp.getExplanation();
+        assertNotNull("should always have an explanation", cae);
+        assertEquals("test", cae.getShard().getIndexName());
+        assertEquals(0, cae.getShard().getId());
+        assertEquals(false, cae.isPrimary());
+        assertNull(cae.getAssignedNodeId());
+        assertNotNull(cae.getUnassignedInfo());
+        Decision d = cae.getNodeDecisions().values().iterator().next();
+        assertNotNull("should have a decision", d);
+        assertEquals(Decision.Type.NO, d.type());
+        assertTrue(d.toString(), d.toString().contains("NO(the shard cannot be allocated on the same node id"));
+        assertTrue(d instanceof Decision.Multi);
+        Decision.Multi md = (Decision.Multi) d;
+        Decision ssd = md.getDecisions().get(0);
+        assertEquals(Decision.Type.NO, ssd.type());
+        assertTrue(ssd.toString(), ssd.toString().contains("NO(the shard cannot be allocated on the same node id"));
+        Float weight = cae.getNodeWeights().values().iterator().next();
+        assertNotNull("should have a weight", weight);
+
+        resp = client().admin().cluster().prepareAllocationExplain().setIndex("test").setShard(0).setPrimary(true).get();
+
+        cae = resp.getExplanation();
+        assertNotNull("should always have an explanation", cae);
+        assertEquals("test", cae.getShard().getIndexName());
+        assertEquals(0, cae.getShard().getId());
+        assertEquals(true, cae.isPrimary());
+        assertNotNull("shard should have assigned node id", cae.getAssignedNodeId());
+        assertNull("assigned shard should not have unassigned info", cae.getUnassignedInfo());
+        d = cae.getNodeDecisions().values().iterator().next();
+        assertNotNull("should have a decision", d);
+        assertEquals(Decision.Type.NO, d.type());
+        assertTrue(d.toString(), d.toString().contains("NO(the shard cannot be allocated on the same node id"));
+        assertTrue(d instanceof Decision.Multi);
+        md = (Decision.Multi) d;
+        ssd = md.getDecisions().get(0);
+        assertEquals(Decision.Type.NO, ssd.type());
+        assertTrue(ssd.toString(), ssd.toString().contains("NO(the shard cannot be allocated on the same node id"));
+        weight = cae.getNodeWeights().values().iterator().next();
+        assertNotNull("should have a weight", weight);
+
+        resp = client().admin().cluster().prepareAllocationExplain().useAnyUnassignedShard().get();
+        cae = resp.getExplanation();
+        assertNotNull("should always have an explanation", cae);
+        assertEquals("test", cae.getShard().getIndexName());
+        assertEquals(0, cae.getShard().getId());
+        assertEquals(false, cae.isPrimary());
+    }
+}

core/src/test/java/org/elasticsearch/action/admin/cluster/allocation/ClusterAllocationExplanationTests.java

@@ -0,0 +1,91 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.elasticsearch.action.admin.cluster.allocation;
+
+import org.elasticsearch.Version;
+import org.elasticsearch.cluster.node.DiscoveryNode;
+import org.elasticsearch.cluster.routing.RoutingNode;
+import org.elasticsearch.cluster.routing.ShardRouting;
+import org.elasticsearch.cluster.routing.allocation.RoutingAllocation;
+import org.elasticsearch.cluster.routing.allocation.decider.AllocationDecider;
+import org.elasticsearch.cluster.routing.allocation.decider.Decision;
+import org.elasticsearch.common.io.stream.BytesStreamOutput;
+import org.elasticsearch.common.io.stream.StreamInput;
+import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.common.transport.DummyTransportAddress;
+import org.elasticsearch.common.xcontent.ToXContent;
+import org.elasticsearch.common.xcontent.XContentBuilder;
+import org.elasticsearch.index.Index;
+import org.elasticsearch.index.shard.ShardId;
+import org.elasticsearch.test.ESTestCase;
+
+import java.util.HashMap;
+import java.util.Map;
+
+import static org.elasticsearch.common.xcontent.XContentFactory.jsonBuilder;
+
+/**
+ * Tests for the cluster allocation explanation
+ */
+public final class ClusterAllocationExplanationTests extends ESTestCase {
+
+    public void testDecisionEquality() {
+        Decision.Multi d = new Decision.Multi();
+        Decision.Multi d2 = new Decision.Multi();
+        d.add(Decision.single(Decision.Type.NO, "no label", "because I said no"));
+        d.add(Decision.single(Decision.Type.YES, "yes label", "yes please"));
+        d.add(Decision.single(Decision.Type.THROTTLE, "throttle label", "wait a sec"));
+        d2.add(Decision.single(Decision.Type.NO, "no label", "because I said no"));
+        d2.add(Decision.single(Decision.Type.YES, "yes label", "yes please"));
+        d2.add(Decision.single(Decision.Type.THROTTLE, "throttle label", "wait a sec"));
+        assertEquals(d, d2);
+    }
+
+    public void testExplanationSerialization() throws Exception {
+        ShardId shard = new ShardId("test", "uuid", 0);
+        Map<DiscoveryNode, Decision> nodeToDecisions = new HashMap<>();
+        Map<DiscoveryNode, Float> nodeToWeight = new HashMap<>();
+        for (int i = randomIntBetween(2, 5); i > 0; i--) {
+            DiscoveryNode dn = new DiscoveryNode("node-" + i, DummyTransportAddress.INSTANCE, Version.CURRENT);
+            Decision.Multi d = new Decision.Multi();
+            d.add(Decision.single(Decision.Type.NO, "no label", "because I said no"));
+            d.add(Decision.single(Decision.Type.YES, "yes label", "yes please"));
+            d.add(Decision.single(Decision.Type.THROTTLE, "throttle label", "wait a sec"));
+            nodeToDecisions.put(dn, d);
+            nodeToWeight.put(dn, randomFloat());
+        }
+        
+        ClusterAllocationExplanation cae = new ClusterAllocationExplanation(shard, true, "assignedNode", null,
+                nodeToDecisions, nodeToWeight);
+        BytesStreamOutput out = new BytesStreamOutput();
+        cae.writeTo(out);
+        StreamInput in = StreamInput.wrap(out.bytes());
+        ClusterAllocationExplanation cae2 = new ClusterAllocationExplanation(in);
+        assertEquals(shard, cae2.getShard());
+        assertTrue(cae2.isPrimary());
+        assertTrue(cae2.isAssigned());
+        assertEquals("assignedNode", cae2.getAssignedNodeId());
+        assertNull(cae2.getUnassignedInfo());
+        for (Map.Entry<DiscoveryNode, Decision> entry : cae2.getNodeDecisions().entrySet()) {
+            assertEquals(nodeToDecisions.get(entry.getKey()), entry.getValue());
+        }
+        assertEquals(nodeToWeight, cae2.getNodeWeights());
+    }
+}

core/src/test/java/org/elasticsearch/cluster/ClusterModuleTests.java

@@ -22,6 +22,7 @@ package org.elasticsearch.cluster;
 import org.elasticsearch.action.admin.indices.create.CreateIndexClusterStateUpdateRequest;
 import org.elasticsearch.cluster.metadata.IndexTemplateFilter;
 import org.elasticsearch.cluster.metadata.IndexTemplateMetaData;
+import org.elasticsearch.cluster.node.DiscoveryNode;
 import org.elasticsearch.cluster.routing.RoutingNode;
 import org.elasticsearch.cluster.routing.ShardRouting;
 import org.elasticsearch.cluster.routing.allocation.FailedRerouteAllocation;
@@ -39,6 +40,8 @@ import org.elasticsearch.common.settings.Setting.Property;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.settings.SettingsModule;
 
+import java.util.HashMap;
+import java.util.Map;
 public class ClusterModuleTests extends ModuleTestCase {
 
     public static class FakeAllocationDecider extends AllocationDecider {
@@ -52,6 +55,11 @@ public class ClusterModuleTests extends ModuleTestCase {
         public boolean allocate(RoutingAllocation allocation) {
             return false;
         }
+
+        @Override
+        public Map<DiscoveryNode, Float> weighShard(RoutingAllocation allocation, ShardRouting shard) {
+            return new HashMap<>();
+        }
     }
 
     static class FakeIndexTemplateFilter implements IndexTemplateFilter {

core/src/test/java/org/elasticsearch/cluster/routing/allocation/BalanceConfigurationTests.java

@@ -33,6 +33,7 @@ import org.elasticsearch.cluster.routing.RoutingNodes;
 import org.elasticsearch.cluster.routing.RoutingTable;
 import org.elasticsearch.cluster.routing.ShardRouting;
 import org.elasticsearch.cluster.routing.ShardRoutingState;
+import org.elasticsearch.cluster.routing.allocation.RoutingAllocation;
 import org.elasticsearch.cluster.routing.allocation.allocator.BalancedShardsAllocator;
 import org.elasticsearch.cluster.routing.allocation.allocator.ShardsAllocator;
 import org.elasticsearch.cluster.routing.allocation.decider.ClusterRebalanceAllocationDecider;
@@ -44,6 +45,8 @@ import org.elasticsearch.test.ESAllocationTestCase;
 import org.elasticsearch.test.gateway.NoopGatewayAllocator;
 import org.hamcrest.Matchers;
 
+import java.util.HashMap;
+import java.util.Map;
 import static org.elasticsearch.cluster.routing.ShardRoutingState.INITIALIZING;
 import static org.elasticsearch.cluster.routing.ShardRoutingState.STARTED;
 import static org.elasticsearch.common.settings.Settings.settingsBuilder;
@@ -313,6 +316,9 @@ public class BalanceConfigurationTests extends ESAllocationTestCase {
                 new ClusterSettings(Settings.Builder.EMPTY_SETTINGS, ClusterSettings.BUILT_IN_CLUSTER_SETTINGS), getRandom()),
                 NoopGatewayAllocator.INSTANCE, new ShardsAllocator() {
 
+                    public Map<DiscoveryNode, Float> weighShard(RoutingAllocation allocation, ShardRouting shard) {
+                        return new HashMap<DiscoveryNode, Float>();
+                    }
             /*
              *  // this allocator tries to rebuild this scenario where a rebalance is
              *  // triggered solely by the primary overload on node [1] where a shard

core/src/test/java/org/elasticsearch/cluster/routing/allocation/decider/DiskThresholdDeciderTests.java

@@ -59,6 +59,7 @@ import static org.elasticsearch.cluster.routing.ShardRoutingState.RELOCATING;
 import static org.elasticsearch.cluster.routing.ShardRoutingState.STARTED;
 import static org.elasticsearch.cluster.routing.ShardRoutingState.UNASSIGNED;
 import static org.elasticsearch.common.settings.Settings.settingsBuilder;
+import static org.hamcrest.Matchers.containsString;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.is;
 import static org.hamcrest.Matchers.nullValue;
@@ -794,7 +795,8 @@ public class DiskThresholdDeciderTests extends ESAllocationTestCase {
             fail("should not have been able to reroute the shard");
         } catch (IllegalArgumentException e) {
             assertThat("can't allocated because there isn't enough room: " + e.getMessage(),
-                    e.getMessage().contains("more than allowed [70.0%] used disk on node, free: [26.0%]"), equalTo(true));
+                    e.getMessage(),
+                    containsString("the node is above the low watermark and has more than allowed [70.0%] used disk, free: [26.0%]"));
         }
 
     }

docs/reference/cluster.asciidoc

@@ -46,3 +46,5 @@ include::cluster/nodes-stats.asciidoc[]
 include::cluster/nodes-info.asciidoc[]
 
 include::cluster/nodes-hot-threads.asciidoc[]
+
+include::cluster/allocation-explain.asciidoc[]

docs/reference/cluster/allocation-explain.asciidoc

@@ -0,0 +1,159 @@
+[[cluster-allocation-explain]]
+== Cluster Allocation Explain API
+
+The cluster allocation explanation API is designed to assist in answering the
+question "why is this shard unassigned?". To explain the allocation (on
+unassigned state) of a shard, issue a request like:
+
+[source,js]
+--------------------------------------------------
+$ curl -XGET 'http://localhost:9200/_cluster/allocation/explain' -d'{
+  "index": "myindex",
+  "shard": 0,
+  "primary": false
+}'
+--------------------------------------------------
+
+Specify the `index` and `shard` id of the shard you would like an explanation
+for, as well as the `primary` flag to indicate whether to explain a primary or
+replica shard.
+
+The response looks like:
+
+[source,js]
+--------------------------------------------------
+{
+  "shard" : {
+    "index" : "myindex",
+    "index_uuid" : "KnW0-zELRs6PK84l0r38ZA",
+    "id" : 0,
+    "primary" : false
+  },
+  "assigned" : false,                             <1>
+  "unassigned_info" : {
+    "reason" : "INDEX_CREATED",                   <2>
+    "at" : "2016-03-22T20:04:23.620Z"
+  },
+  "nodes" : {                                     <3>
+    "V-Spi0AyRZ6ZvKbaI3691w" : {
+      "node_name" : "node1",
+      "node_attributes" : {                       <4>
+        "bar" : "baz"
+      },
+      "final_decision" : "NO",                    <5>
+      "weight" : 0.06666675,                      <6>
+      "decisions" : [ {                           <7>
+        "decider" : "filter",
+        "decision" : "NO",
+        "explanation" : "node does not match index include filters [foo:\"bar\"]"
+      } ]
+    },
+    "Qc6VL8c5RWaw1qXZ0Rg57g" : {
+      "node_name" : "node2",
+      "node_attributes" : {
+        "bar" : "baz",
+        "foo" : "bar"
+      },
+      "final_decision" : "NO",
+      "weight" : -1.3833332,
+      "decisions" : [ {
+        "decider" : "same_shard",
+        "decision" : "NO",
+        "explanation" : "the shard cannot be allocated on the same node id [Qc6VL8c5RWaw1qXZ0Rg57g] on which it already exists"
+      } ]
+    },
+    "PzdyMZGXQdGhqTJHF_hGgA" : {
+      "node_name" : "node3",
+      "node_attributes" : { },
+      "final_decision" : "NO",
+      "weight" : 2.3166666,
+      "decisions" : [ {
+        "decider" : "filter",
+        "decision" : "NO",
+        "explanation" : "node does not match index include filters [foo:\"bar\"]"
+      } ]
+    }
+  }
+}
+--------------------------------------------------
+<1> Whether the shard is assigned or unassigned
+<2> Reason for the shard originally becoming unassigned
+<3> List of node decisions about the shard
+<4> User-added attributes the node has
+<5> Final decision for whether the shard is allowed to be allocated to this node
+<6> Weight for how much the allocator would like to allocate the shard to this node
+<7> List of decisions factoring into final decision
+
+For a shard that is already assigned, the output looks similar to:
+
+[source,js]
+--------------------------------------------------
+{
+  "shard" : {
+    "index" : "only-foo",
+    "index_uuid" : "KnW0-zELRs6PK84l0r38ZA",
+    "id" : 0,
+    "primary" : true
+  },
+  "assigned" : true,
+  "assigned_node_id" : "Qc6VL8c5RWaw1qXZ0Rg57g",      <1>
+  "nodes" : {
+    "V-Spi0AyRZ6ZvKbaI3691w" : {
+      "node_name" : "Susan Storm",
+      "node_attributes" : {
+        "bar" : "baz"
+      },
+      "final_decision" : "NO",
+      "weight" : 1.4499999,
+      "decisions" : [ {
+        "decider" : "filter",
+        "decision" : "NO",
+        "explanation" : "node does not match index include filters [foo:\"bar\"]"
+      } ]
+    },
+    "Qc6VL8c5RWaw1qXZ0Rg57g" : {
+      "node_name" : "Slipstream",
+      "node_attributes" : {
+        "bar" : "baz",
+        "foo" : "bar"
+      },
+      "final_decision" : "CURRENTLY_ASSIGNED",        <2>
+      "weight" : 0.0,
+      "decisions" : [ {
+        "decider" : "same_shard",
+        "decision" : "NO",
+        "explanation" : "the shard cannot be allocated on the same node id [Qc6VL8c5RWaw1qXZ0Rg57g] on which it already exists"
+      } ]
+    },
+    "PzdyMZGXQdGhqTJHF_hGgA" : {
+      "node_name" : "The Symbiote",
+      "node_attributes" : { },
+      "final_decision" : "NO",
+      "weight" : 3.6999998,
+      "decisions" : [ {
+        "decider" : "filter",
+        "decision" : "NO",
+        "explanation" : "node does not match index include filters [foo:\"bar\"]"
+      } ]
+    }
+  }
+}
+--------------------------------------------------
+<1> Node the shard is currently assigned to
+<2> The decision is "CURRENTLY_ASSIGNED" because the shard is currently assigned to this node
+
+You can also have Elasticsearch explain the allocation of the first unassigned
+shard it finds by sending an empty body, such as:
+
+[source,js]
+--------------------------------------------------
+$ curl -XGET 'http://localhost:9200/_cluster/allocation/explain'
+--------------------------------------------------
+
+And if you would like to include all decisions that were factored into the final
+decision, the `include_yes_decisions` parameter will return all decisions:
+
+[source,js]
+--------------------------------------------------
+$ curl -XGET 'http://localhost:9200/_cluster/allocation/explain?include_yes_decisions=true'
+--------------------------------------------------

rest-api-spec/src/main/resources/rest-api-spec/api/cluster.allocation_explain.json

@@ -0,0 +1,20 @@
+{
+  "cluster.allocation_explain": {
+    "documentation": "http://www.elastic.co/guide/en/elasticsearch/reference/master/cluster-allocation-explain.html",
+    "methods": ["GET", "POST"],
+    "url": {
+      "path": "/_cluster/allocation/explain",
+      "paths": ["/_cluster/allocation/explain"],
+      "parts": {},
+      "params": {
+        "include_yes_decisions": {
+          "type": "boolean",
+          "description": "Return 'YES' decisions in explanation (default: false)"
+        }
+      }
+    },
+    "body": {
+      "description": "The index, shard, and primary flag to explain. Empty means 'explain the first unassigned shard'"
+    }
+  }
+}

rest-api-spec/src/main/resources/rest-api-spec/test/cluster.allocation_explain/10_basic.yaml

@@ -0,0 +1,76 @@
+---
+"cluster shard allocation explanation test":
+  - do:
+      # there aren't any unassigned shards to explain
+      catch: /unable to find any shards to explain/
+      cluster.allocation_explain: {}
+
+  - do:
+      indices.create:
+        index: test
+
+  - do:
+      cluster.health:
+        wait_for_status: yellow
+
+  - do:
+      cluster.state:
+        metric: [ master_node ]
+
+  - set: {master_node: node_id}
+
+    # This relies on there only being a single node in the test cluster, which
+    # is currently true, but if this changes in the future this test will need
+    # to be changed
+  - do:
+      cluster.allocation_explain:
+        body: { "index": "test", "shard": 0, "primary": true }
+
+  - match: { assigned: true }
+  # - match: { assigned_node_id: $node_id }
+  - is_true: assigned_node_id
+  - match: { shard.index: "test" }
+  - match: { shard.id: 0 }
+  - match: { shard.primary: true }
+  # unfortunately can't test these because they break with multi-node backwords
+  # compat REST tests
+  # - is_true: nodes.$node_id.node_name
+  # - match: { nodes.$node_id.node_attributes.testattr: "test" }
+  # - match: { nodes.$node_id.node_attributes.portsfile: "true" }
+  # - match: { nodes.$node_id.final_decision: "CURRENTLY_ASSIGNED" }
+  # - match: { nodes.$node_id.weight: 0.0 }
+  # - match: { nodes.$node_id.decisions.0.decider: "same_shard" }
+  # - match: { nodes.$node_id.decisions.0.decision: "NO" }
+
+---
+"cluster shard allocation explanation test with empty request":
+  - do:
+      indices.create:
+        index: test
+        body: { "index.number_of_shards": 1, "index.number_of_replicas": 9 }
+
+  - do:
+      cluster.health:
+        wait_for_status: yellow
+
+  - do:
+      cluster.state:
+        metric: [ master_node ]
+
+  - set: {master_node: node_id}
+
+  - do:
+      cluster.allocation_explain: {}
+
+  - match: { assigned: false }
+  - match: { unassigned_info.reason: "INDEX_CREATED" }
+  - is_true: unassigned_info.at
+  - match: { shard.index: "test" }
+  - match: { shard.id: 0 }
+  - match: { shard.primary: false }
+  # - is_true: nodes.$node_id.node_name
+  # - match: { nodes.$node_id.node_attributes.testattr: "test" }
+  # - match: { nodes.$node_id.node_attributes.portsfile: "true" }
+  # - match: { nodes.$node_id.final_decision: "NO" }
+  # - match: { nodes.$node_id.decisions.0.decider: "same_shard" }
+  # - match: { nodes.$node_id.decisions.0.decision: "NO" }

test/framework/src/main/java/org/elasticsearch/test/rest/parser/RestTestSuiteParser.java

@@ -73,7 +73,7 @@ public class RestTestSuiteParser implements RestTestFragmentParser<RestTestSuite
         XContentParser parser = parseContext.parser();
 
         parser.nextToken();
-        assert parser.currentToken() == XContentParser.Token.START_OBJECT;
+        assert parser.currentToken() == XContentParser.Token.START_OBJECT : "expected token to be START_OBJECT but was " + parser.currentToken();
 
         RestTestSuite restTestSuite = new RestTestSuite(parseContext.getApi(), parseContext.getSuiteName());