[[indices-synced-flush-api]]
=== Synced flush API
++++
<titleabbrev>Synced flush</titleabbrev>
++++

deprecated::[7.6, "Synced-flush is deprecated and will be removed in 8.0. Use <<indices-flush,flush>> instead. A flush has the same effect as a synced flush on Elasticsearch 7.6 or later."]

Performs a synced flush on one or more indices.

[source,console]
--------------------------------------------------
POST /twitter/_flush/synced
--------------------------------------------------
// TEST[skip: Synced flush can conflict with scheduled flushes in doc tests]


[[synced-flush-api-request]]
==== {api-request-title}

`POST /<index>/flush/synced`

`GET /<index>/flush/synced`

`POST /flush/synced`

`GET /flush/synced`


[[synced-flush-api-desc]]
==== {api-description-title}

[[synced-flush-using-api]]
===== Use the synced flush API

Use the synced flush API to manually initiate a synced flush.
This can be useful for a planned cluster restart where
you can stop indexing but don't want to wait for 5 minutes until all indices
are marked as inactive and automatically sync-flushed.

You can request a synced flush even if there is ongoing indexing activity, and
{es} will perform the synced flush on a "best-effort" basis: shards that do not
have any ongoing indexing activity will be successfully sync-flushed, and other
shards will fail to sync-flush. The successfully sync-flushed shards will have
faster recovery times as long as the `sync_id` marker is not removed by a
subsequent flush.


[[synced-flush-overview]]
===== Synced flush overview

{es} keeps track of which shards have received indexing activity recently, and
considers shards that have not received any indexing operations for 5 minutes to
be inactive.

When a shard becomes inactive {es} performs a special kind of flush
known as a *synced flush*. A synced flush performs a normal
<<indices-flush,flush>> on each replica of the shard, and then adds a marker known
as the `sync_id` to each replica to indicate that these copies have identical
Lucene indices. Comparing the `sync_id` markers of the two copies is a very
efficient way to check whether they have identical contents.

When allocating shard replicas, {es} must ensure that each replica contains the
same data as the primary. If the shard copies have been synced-flushed and the
replica shares a `sync_id` with the primary then {es} knows that the two copies
have identical contents. This means there is no need to copy any segment files
from the primary to the replica, which saves a good deal of time during
recoveries and restarts.

This is particularly useful for clusters having lots of indices which are very
rarely updated, such as with time-based indices. Without the synced flush
marker, recovery of this kind of cluster would be much slower.


[[synced-flush-sync-id-markers]]
===== Check for `sync_id` markers

To check whether a shard has a `sync_id` marker or not, look for the `commit`
section of the shard stats returned by the <<indices-stats,indices stats>> API:

[source,console]
--------------------------------------------------
GET /twitter/_stats?filter_path=**.commit&level=shards <1>
--------------------------------------------------
// TEST[skip: Synced flush can conflict with scheduled flushes in doc tests]

<1> `filter_path` is used to reduce the verbosity of the response, but is entirely optional

The API returns the following response:

[source,console-result]
--------------------------------------------------
{
   "indices": {
      "twitter": {
         "shards": {
            "0": [
               {
                 "commit" : {
                   "id" : "3M3zkw2GHMo2Y4h4/KFKCg==",
                   "generation" : 3,
                   "user_data" : {
                     "translog_uuid" : "hnOG3xFcTDeoI_kvvvOdNA",
                     "history_uuid" : "XP7KDJGiS1a2fHYiFL5TXQ",
                     "local_checkpoint" : "-1",
                     "translog_generation" : "2",
                     "max_seq_no" : "-1",
                     "sync_id" : "AVvFY-071siAOuFGEO9P", <1>
                     "max_unsafe_auto_id_timestamp" : "-1",
                     "min_retained_seq_no" : "0"
                   },
                   "num_docs" : 0
                 }
               }
            ]
         }
      }
   }
}
--------------------------------------------------
// TEST[skip: Synced flush can conflict with scheduled flushes in doc tests]
<1> the `sync id` marker

NOTE: The `sync_id` marker is removed as soon as the shard is flushed again, and
{es} may trigger an automatic flush of a shard at any time if there are
unflushed operations in the shard's translog. In practice this means that one
should consider any indexing operation on an index as having removed its
`sync_id` markers.


[[synced-flush-api-path-params]]
==== {api-path-parms-title}

include::{docdir}/rest-api/common-parms.asciidoc[tag=index]
+
To sync-flush all indices,
omit this parameter
or use a value of `_all` or `*`.


[[synced-flush-api-query-params]]
==== {api-query-parms-title}

include::{docdir}/rest-api/common-parms.asciidoc[tag=allow-no-indices]

include::{docdir}/rest-api/common-parms.asciidoc[tag=expand-wildcards]
+
Defaults to `open`.

include::{docdir}/rest-api/common-parms.asciidoc[tag=index-ignore-unavailable]


[[synced-flush-api-response-codes]]
==== {api-response-codes-title}

`200`::
All shards successfully sync-flushed.

`409`::
A replica shard failed to sync-flush.


[[synced-flush-api-example]]
==== {api-examples-title}


[[synced-flush-api-specific-ex]]
===== Sync-flush a specific index

[source,console]
----
POST /kimchy/_flush/synced
----
// TEST[skip: Synced flush can conflict with scheduled flushes in doc tests]


[[synced-flush-api-multi-ex]]
===== Synch-flush several indices

[source,console]
--------------------------------------------------
POST /kimchy,elasticsearch/_flush/synced
--------------------------------------------------
// TEST[skip: Synced flush can conflict with scheduled flushes in doc tests]


[[synced-flush-api-all-ex]]
===== Sync-flush all indices

[source,console]
--------------------------------------------------
POST /_flush/synced
--------------------------------------------------
// TEST[skip: Synced flush can conflict with scheduled flushes in doc tests]

The response contains details about how many shards were successfully
sync-flushed and information about any failure.

The following response indicates two shards
and one replica shard
successfully sync-flushed:

[source,console-result]
--------------------------------------------------
{
   "_shards": {
      "total": 2,
      "successful": 2,
      "failed": 0
   },
   "twitter": {
      "total": 2,
      "successful": 2,
      "failed": 0
   }
}
--------------------------------------------------
// TEST[skip: Synced flush can conflict with scheduled flushes in doc tests]

The following response indicates one shard group failed
due to pending operations:

[source,console-result]
--------------------------------------------------
{
   "_shards": {
      "total": 4,
      "successful": 2,
      "failed": 2
   },
   "twitter": {
      "total": 4,
      "successful": 2,
      "failed": 2,
      "failures": [
         {
            "shard": 1,
            "reason": "[2] ongoing operations on primary"
         }
      ]
   }
}
--------------------------------------------------
// TEST[skip: Synced flush can conflict with scheduled flushes in doc tests]

Sometimes the failures are specific to a shard replica. The copies that failed
will not be eligible for fast recovery but those that succeeded still will be.
This case is reported as follows:

[source,console-result]
--------------------------------------------------
{
   "_shards": {
      "total": 4,
      "successful": 1,
      "failed": 1
   },
   "twitter": {
      "total": 4,
      "successful": 3,
      "failed": 1,
      "failures": [
         {
            "shard": 1,
            "reason": "unexpected error",
            "routing": {
               "state": "STARTED",
               "primary": false,
               "node": "SZNr2J_ORxKTLUCydGX4zA",
               "relocating_node": null,
               "shard": 1,
               "index": "twitter"
            }
         }
      ]
   }
}
--------------------------------------------------
// TEST[skip: Synced flush can conflict with scheduled flushes in doc tests]
