[[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"
  },
  "allocation_delay_ms" : 0,                      <3>
  "remaining_delay_ms" : 0,                       <4>
  "nodes" : {
    "V-Spi0AyRZ6ZvKbaI3691w" : {
      "node_name" : "node1",
      "node_attributes" : {                       <5>
        "bar" : "baz"
      },
      "store" : {
        "shard_copy" : "NONE"                     <6>
      },
      "final_decision" : "NO",                    <7>
      "final_explanation" : "the shard cannot be assigned because one or more allocation decider returns a 'NO' decision",
      "weight" : 0.06666675,                      <8>
      "decisions" : [ {                           <9>
        "decider" : "filter",
        "decision" : "NO",
        "explanation" : "node does not match index include filters [foo:\"bar\"]"
      } ]
    },
    "Qc6VL8c5RWaw1qXZ0Rg57g" : {
      "node_name" : "node2",
      "node_attributes" : {
        "bar" : "baz",
        "foo" : "bar"
      },
      "store" : {
        "shard_copy" : "AVAILABLE"
      },
      "final_decision" : "NO",
      "final_explanation" : "the shard cannot be assigned because one or more allocation decider returns a 'NO' decision",
      "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" : { },
      "store" : {
        "shard_copy" : "NONE"
      },
      "final_decision" : "NO",
      "final_explanation" : "the shard cannot be assigned because one or more allocation decider returns a 'NO' decision",
      "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> Configured delay before the shard can be allocated
<4> Remaining delay before the shard can be allocated
<5> User-added attributes the node has
<6> The shard copy information for this node and error (if applicable)
<7> Final decision and explanation of whether the shard can be allocated to this node
<8> Weight for how much the allocator would like to allocate the shard to this node
<9> List of node decisions factoring into final decision about the shard

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>
  "allocation_delay_ms" : 0,
  "remaining_delay_ms" : 0,
  "nodes" : {
    "V-Spi0AyRZ6ZvKbaI3691w" : {
      "node_name" : "Susan Storm",
      "node_attributes" : {
        "bar" : "baz"
      },
      "store" : {
        "shard_copy" : "NONE"
      },
      "final_decision" : "NO",
      "final_explanation" : "the shard cannot be assigned because one or more allocation decider returns a 'NO' decision",
      "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"
      },
      "store" : {
        "shard_copy" : "AVAILABLE"
      },
      "final_decision" : "ALREADY_ASSIGNED",        <2>
      "final_explanation" : "the shard is already assigned to this node",
      "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" : { },
      "store" : {
        "shard_copy" : "NONE"
      },
      "final_decision" : "NO",
      "final_explanation" : "the shard cannot be assigned because one or more allocation decider returns a 'NO' decision",
      "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 "ALREADY_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'
--------------------------------------------------