[[indices-flush]]
=== Flush

Flushing an index is the process of making sure that any data that is currently
only stored in the <<index-modules-translog,transaction log>> is also
permanently stored in the Lucene index. When restarting, {es} replays any
unflushed operations from the transaction log into the Lucene index to bring it
back into the state that it was in before the restart. {es} automatically
triggers flushes as needed, using heuristics that trade off the size of the
unflushed transaction log against the cost of performing each flush.

Once each operation has been flushed it is permanently stored in the Lucene
index. This may mean that there is no need to maintain an additional copy of it
in the transaction log, unless <<index-modules-translog-retention,it is retained
for some other reason>>. The transaction log is made up of multiple files,
called _generations_, and {es} will delete any generation files once they are no
longer needed, freeing up disk space.

It is also possible to trigger a flush on one or more indices using the flush
API, although it is rare for users to need to call this API directly. If you
call the flush API after indexing some documents then a successful response
indicates that {es} has flushed all the documents that were indexed before the
flush API was called.

[source,js]
--------------------------------------------------
POST twitter/_flush
--------------------------------------------------
// CONSOLE
// TEST[setup:twitter]

[float]
[[flush-parameters]]
==== Request Parameters

The flush API accepts the following request parameters:

[horizontal]
`wait_if_ongoing`:: If set to `true` the flush operation will block until the
flush can be executed if another flush operation is already executing. If set to
`false` then an exception will be thrown on the shard level if another flush
operation is already running. Defaults to `true`.

`force`:: Whether a flush should be forced even if it is not necessarily needed
i.e. if no changes will be committed to the index. This can be used to force
the generation number of the transaction log to be incremented even if no
uncommitted changes are present. This parameter should be considered internal.

[float]
[[flush-multi-index]]
==== Multi Index

The flush API can be applied to more than one index with a single call, or even
on `_all` the indices.

[source,js]
--------------------------------------------------
POST kimchy,elasticsearch/_flush

POST _flush
--------------------------------------------------
// CONSOLE
// TEST[s/^/PUT kimchy\nPUT elasticsearch\n/]

[[synced-flush-api]]
==== Synced Flush

{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 copy of the shard, and then adds a marker known
as the `sync_id` to each copy 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 copies, {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.

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,sh]
--------------------------------------------------
GET twitter/_stats?filter_path=**.commit&level=shards <1>
--------------------------------------------------
// CONSOLE
// TEST[s/^/PUT twitter\nPOST twitter\/_flush\/synced\n/]
<1> `filter_path` is used to reduce the verbosity of the response, but is entirely optional


which returns something similar to:

[source,js]
--------------------------------------------------
{
   "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
                 }
               }
            ]
         }
      }
   }
}
--------------------------------------------------
// TESTRESPONSE[s/"id" : "3M3zkw2GHMo2Y4h4\/KFKCg=="/"id": $body.indices.twitter.shards.0.0.commit.id/]
// TESTRESPONSE[s/"translog_uuid" : "hnOG3xFcTDeoI_kvvvOdNA"/"translog_uuid": $body.indices.twitter.shards.0.0.commit.user_data.translog_uuid/]
// TESTRESPONSE[s/"history_uuid" : "XP7KDJGiS1a2fHYiFL5TXQ"/"history_uuid": $body.indices.twitter.shards.0.0.commit.user_data.history_uuid/]
// TESTRESPONSE[s/"sync_id" : "AVvFY-071siAOuFGEO9P"/"sync_id": $body.indices.twitter.shards.0.0.commit.user_data.sync_id/]
<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.

[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 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.

[source,sh]
--------------------------------------------------
POST twitter/_flush/synced
--------------------------------------------------
// CONSOLE
// TEST[setup:twitter]

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

Here is what it looks like when all shards of a two shards and one replica
index successfully sync-flushed:

[source,js]
--------------------------------------------------
{
   "_shards": {
      "total": 2,
      "successful": 2,
      "failed": 0
   },
   "twitter": {
      "total": 2,
      "successful": 2,
      "failed": 0
   }
}
--------------------------------------------------
// TESTRESPONSE[s/"successful": 2/"successful": 1/]

Here is what it looks like when one shard group failed due to pending
operations:

[source,js]
--------------------------------------------------
{
   "_shards": {
      "total": 4,
      "successful": 2,
      "failed": 2
   },
   "twitter": {
      "total": 4,
      "successful": 2,
      "failed": 2,
      "failures": [
         {
            "shard": 1,
            "reason": "[2] ongoing operations on primary"
         }
      ]
   }
}
--------------------------------------------------
// NOTCONSOLE

NOTE: The above error is shown when the synced flush fails 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:

[source,js]
--------------------------------------------------
{
   "_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"
            }
         }
      ]
   }
}
--------------------------------------------------
// NOTCONSOLE

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.

[source,js]
--------------------------------------------------
POST kimchy,elasticsearch/_flush/synced

POST _flush/synced
--------------------------------------------------
// CONSOLE
