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docs/reference/indices/flush.asciidoc

@@ -49,25 +49,25 @@ POST /_flush
 [[indices-synced-flush]]
 === Synced Flush
 
-Elasticsearch tracks the indexing activity of each shards. Shards that have not
-received any indexing operations for 30 minutes (configurable) are automatically marked as inactive. This presents
+Elasticsearch tracks the indexing activity of each shard. Shards that have not
+received any indexing operations for 30 minutes are automatically marked as inactive. This presents
 an opportunity for Elasticsearch to reduce shard resources and also perform
-a special kind of flush, called `synced flush`. A synced flush performs normal
-flushing and adds a special uniquely generated marker (`sync_id`) to all shards.
+a special kind of flush, called `synced flush`. A synced flush performs a normal flush, then adds
+a generated unique marker (sync_id) to all shards.
 
 Since the sync id marker was added when there were no ongoing indexing operations, it can
-be used as a quick way to check if two shards indices are identical. This quick sync id
+be used as a quick way to check if the two shards' lucene indices are identical. This quick sync id
 comparison (if present) is used during recovery or restarts to skip the first and
 most costly phase of the process. In that case, no segment files need to be copied and
 the transaction log replay phase of the recovery can start immediately. Note that since the sync id
-marker was applied together with a flush, it is highly likely that the transaction log will be empty,
+marker was applied together with a flush, it is very likely that the transaction log will be empty,
 speeding up recoveries even more.
 
 This is particularly useful for use cases having lots of indices which are
 never or very rarely updated, such as time based data. This use case typically generates lots of indices whose
 recovery without the synced flush marker would take a long time.
 
-To check whether a shard has a marker or not, one can use the `commit` section of shard stats returned by
+To check whether a shard has a marker or not, look for the `commit` section of shard stats returned by
 the <<indices-stats,indices stats>> API:
 
 [source,bash]
@@ -76,23 +76,64 @@ GET /twitter/_stats/commit?level=shards
 --------------------------------------------------
 // AUTOSENSE
 
+
+which returns something similar to:
+
+[source,js]
+--------------------------------------------------
+{
+   ...
+   "indices": {
+      "twitter": {
+         "primaries": {},
+         "total": {},
+         "shards": {
+            "0": [
+               {
+                  "routing": {
+                     ...
+                  },
+                  "commit": {
+                     "id": "te7zF7C4UsirqvL6jp/vUg==",
+                     "generation": 2,
+                     "user_data": {
+                        "sync_id": "AU2VU0meX-VX2aNbEUsD" <1>,
+                        ...
+                     },
+                     "num_docs": 0
+                  }
+               }
+               ...
+            ],
+            ...
+         }
+      }
+   }
+}
+--------------------------------------------------
+<1> the `sync id` marker
+
 [float]
 === Synced Flush API
 
 The Synced Flush API allows an administrator to initiate a synced flush manually. This can be particularly useful for
-a planned (rolling) cluster restart where one can stop indexing and doesn't want to wait for the default 30 minutes to pass
-when the synced flush will be performed automatically.
+a planned (rolling) cluster restart where you can stop indexing and don't want to wait the default 30 minutes for
+idle indices to be sync-flushed automatically.
 
 While handy, there are a couple of caveats for this API:
 
 1. Synced flush is a best effort operation. Any ongoing indexing operations will cause
-the synced flush to fail. This means that some shards may be synced flushed while others aren't. See below for more.
+the synced flush to fail on that shard. This means that some shards may be synced flushed while others aren't. See below for more.
 2. The `sync_id` marker is removed as soon as the shard is flushed again. That is because a flush replaces the low level
 lucene commit point where the marker is stored. Uncommitted operations in the transaction log do not remove the marker.
 In practice, one should consider any indexing operation on an index as removing the marker as a flush can be triggered by Elasticsearch
 at any time.
 
 
+NOTE: It is harmless to request a synced flush while there is ongoing indexing. Shards that are idle will succeed and shards
+ that are not will fail. Any shards that succeeded will have faster recovery times.
+
+
 [source,bash]
 --------------------------------------------------
 POST /twitter/_flush/synced
@@ -145,6 +186,8 @@ Here is what it looks like when one shard group failed due to pending operations
 }
 --------------------------------------------------
 
+NOTE: The above error is shown when the synced flush failes due to concurrent indexing operations. The HTTP
+status code in that case will be `409 CONFLICT`.
 
 Sometimes the failures are specific to a shard copy. The copies that failed will not be eligible for
 fast recovery but those that succeeded still will be. This case is reported as follows:
@@ -180,6 +223,8 @@ fast recovery but those that succeeded still will be. This case is reported as f
 --------------------------------------------------
 
 
+NOTE: When a shard copy fails to sync-flush, the HTTP status code returned will be `409 CONFLICT`.
+
 The synced flush API can be applied to more than one index with a single call,
 or even on `_all` the indices.