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[7.4] [DOCS] Separate and reformat synced flush API docs (#46634) #46840

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[7.4] [DOCS] Separate and reformat synced flush API docs (#46634) #46840

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4 changes: 2 additions & 2 deletions client/rest-high-level/src/main/java/org/elasticsearch/client/IndicesClient.java
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Original file line number Diff line number Diff line change
Expand Up @@ -570,7 +570,7 @@ public void flushAsync(FlushRequest flushRequest, RequestOptions options, Action

/**
* Initiate a synced flush manually using the synced flush API.
* See <a href="https://www.elastic.co/guide/en/elasticsearch/reference/master/indices-flush.html#synced-flush-api">
* See <a href="https://www.elastic.co/guide/en/elasticsearch/reference/master/indices-synced-flush-api.html">
* Synced flush API on elastic.co</a>
* @param syncedFlushRequest the request
* @param options the request options (e.g. headers), use {@link RequestOptions#DEFAULT} if nothing needs to be customized
Expand All @@ -584,7 +584,7 @@ public SyncedFlushResponse flushSynced(SyncedFlushRequest syncedFlushRequest, Re

/**
* Asynchronously initiate a synced flush manually using the synced flush API.
* See <a href="https://www.elastic.co/guide/en/elasticsearch/reference/master/indices-flush.html#synced-flush-api">
* See <a href="https://www.elastic.co/guide/en/elasticsearch/reference/master/indices-synced-flush-api.html">
* Synced flush API on elastic.co</a>
* @param syncedFlushRequest the request
* @param options the request options (e.g. headers), use {@link RequestOptions#DEFAULT} if nothing needs to be customized
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3 changes: 3 additions & 0 deletions docs/reference/indices.asciidoc
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Expand Up @@ -69,6 +69,7 @@ index settings, aliases, mappings, and index templates.
* <<indices-clearcache>>
* <<indices-refresh>>
* <<indices-flush>>
* <<indices-synced-flush-api>>
* <<indices-forcemerge>>

include::indices/create-index.asciidoc[]
Expand Down Expand Up @@ -139,6 +140,8 @@ include::indices/clearcache.asciidoc[]

include::indices/flush.asciidoc[]

include::indices/synced-flush.asciidoc[]

include::indices/refresh.asciidoc[]

include::indices/forcemerge.asciidoc[]
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201 changes: 3 additions & 198 deletions docs/reference/indices/flush.asciidoc
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Expand Up @@ -62,204 +62,9 @@ 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
[[synced-flush-api]]
==== Synced Flush

POST _flush/synced
--------------------------------------------------
// CONSOLE
See <<indices-synced-flush-api>>.
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