diff --git a/src/main/java/org/elasticsearch/indices/SyncedFlushService.java b/src/main/java/org/elasticsearch/indices/SyncedFlushService.java
index 79b33b9e0de..2974b55ae72 100644
--- a/src/main/java/org/elasticsearch/indices/SyncedFlushService.java
+++ b/src/main/java/org/elasticsearch/indices/SyncedFlushService.java
@@ -18,6 +18,7 @@
  */
 package org.elasticsearch.indices;
 
+import com.google.common.collect.ImmutableMap;
 import org.elasticsearch.ElasticsearchException;
 import org.elasticsearch.action.ActionListener;
 import org.elasticsearch.action.admin.indices.flush.FlushRequest;
@@ -57,7 +58,6 @@ import java.util.concurrent.atomic.AtomicInteger;
 
 public class SyncedFlushService extends AbstractComponent {
 
-    // nocommmit: check these are ok
     public static final String PRE_SYNCED_FLUSH_ACTION_NAME = "internal:indices/flush/synced/pre";
     public static final String SYNCED_FLUSH_ACTION_NAME = "internal:indices/flush/synced/sync";
     public static final String IN_FLIGHT_OPS_ACTION_NAME = "internal:indices/flush/synced/in_flight";
@@ -91,7 +91,7 @@ public class SyncedFlushService extends AbstractComponent {
             public void onShardInactive(final IndexShard indexShard) {
                 // we only want to call sync flush once, so only trigger it when we are on a primary
                 if (indexShard.routingEntry().primary()) {
-                    attemptSyncedFlush(indexShard.shardId(),  new ActionListener<SyncedFlushResult>() {
+                    attemptSyncedFlush(indexShard.shardId(), new ActionListener<SyncedFlushResult>() {
                         @Override
                         public void onResponse(SyncedFlushResult syncedFlushResult) {
                             logger.debug("{} sync flush on inactive shard returned successfully for sync_id: {}", syncedFlushResult.getShardId(), syncedFlushResult.syncId());
@@ -368,6 +368,9 @@ public class SyncedFlushService extends AbstractComponent {
         return new InFlightOpsResponse(opCount);
     }
 
+    /**
+     * Result for all copies of a shard
+     */
     public static class SyncedFlushResult extends TransportResponse {
         private String failureReason;
         private Map<ShardRouting, SyncedFlushResponse> shardResponses;
@@ -375,7 +378,6 @@ public class SyncedFlushService extends AbstractComponent {
         private ShardId shardId;
 
         public SyncedFlushResult() {
-
         }
 
         public ShardId getShardId() {
@@ -385,29 +387,34 @@ public class SyncedFlushService extends AbstractComponent {
         /**
          * failure constructor
          */
-
         public SyncedFlushResult(ShardId shardId, String failureReason) {
             this.syncId = null;
             this.failureReason = failureReason;
-            this.shardResponses = new HashMap<>();
+            this.shardResponses = ImmutableMap.of();
             this.shardId = shardId;
         }
 
         /**
          * success constructor
          */
-
         public SyncedFlushResult(ShardId shardId, String syncId, Map<ShardRouting, SyncedFlushResponse> shardResponses) {
             this.failureReason = null;
-            this.shardResponses = shardResponses;
+            ImmutableMap.Builder<ShardRouting, SyncedFlushResponse> builder = ImmutableMap.builder();
+            this.shardResponses = builder.putAll(shardResponses).build();
             this.syncId = syncId;
             this.shardId = shardId;
         }
 
+        /**
+         * @return true if one or more shard copies was successful, false if all failed before step three of synced flush
+         */
         public boolean success() {
             return syncId != null;
         }
 
+        /**
+         * @return the reason for the failure if synced flush failed before step three of synced flush
+         */
         public String failureReason() {
             return failureReason;
         }
@@ -417,12 +424,15 @@ public class SyncedFlushService extends AbstractComponent {
         }
 
         /**
-         * total number of shards for which a sync attempt was made
+         * @return total number of shards for which a sync attempt was made
          */
         public int totalShards() {
             return shardResponses.size();
         }
 
+        /**
+         * @return total number of successful shards
+         */
         public int successfulShards() {
             int i = 0;
             for (SyncedFlushResponse result : shardResponses.values()) {
@@ -433,6 +443,10 @@ public class SyncedFlushService extends AbstractComponent {
             return i;
         }
 
+        /**
+         * @return Individual responses for each shard copy with a detailed failure message if the copy failed to perform the synced flush.
+         * Empty if synced flush failed before step three.
+         */
         public Map<ShardRouting, SyncedFlushResponse> shardResponses() {
             return shardResponses;
         }
@@ -456,13 +470,14 @@ public class SyncedFlushService extends AbstractComponent {
             failureReason = in.readOptionalString();
             syncId = in.readOptionalString();
             int size = in.readVInt();
-            shardResponses = new HashMap<>();
+            ImmutableMap.Builder<ShardRouting, SyncedFlushResponse> builder = ImmutableMap.builder();
             for (int i = 0; i < size; i++) {
                 ImmutableShardRouting shardRouting = ImmutableShardRouting.readShardRoutingEntry(in);
                 SyncedFlushResponse syncedFlushRsponse = new SyncedFlushResponse();
                 syncedFlushRsponse.readFrom(in);
-                shardResponses.put(shardRouting, syncedFlushRsponse);
+                builder.put(shardRouting, syncedFlushRsponse);
             }
+            shardResponses = builder.build();
             shardId = ShardId.readShardId(in);
         }
 
@@ -506,6 +521,9 @@ public class SyncedFlushService extends AbstractComponent {
         }
     }
 
+    /**
+     * Response for first step of synced flush (flush) for one shard copy
+     */
     final static class PreSyncedFlushResponse extends TransportResponse {
 
         private byte[] commitId;
@@ -586,6 +604,9 @@ public class SyncedFlushService extends AbstractComponent {
         }
     }
 
+    /**
+     * Response for third step of synced flush (writing the sync id) for one shard copy
+     */
     public static final class SyncedFlushResponse extends TransportResponse {
 
         /**
@@ -666,6 +687,9 @@ public class SyncedFlushService extends AbstractComponent {
         }
     }
 
+    /**
+     * Response for second step of synced flush (check operations in flight)
+     */
     static final class InFlightOpsResponse extends TransportResponse {
 
         int opCount;