Synced flush API
editSynced flush API
editDeprecated in 7.6.
Synced-flush is deprecated and will be removed in 8.0. Use 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.
POST /twitter/_flush/synced
Request
editPOST /<index>/_flush/synced
GET /<index>/_flush/synced
POST /_flush/synced
GET /_flush/synced
Description
editUse the synced flush API
editUse 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
Elasticsearch 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
editElasticsearch 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 Elasticsearch performs a special kind of flush
known as a synced flush. A synced flush performs a normal
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, Elasticsearch 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 Elasticsearch 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.
Check for sync_id
markers
editTo 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 API:
The API returns the following response:
{ "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", "max_unsafe_auto_id_timestamp" : "-1", "min_retained_seq_no" : "0" }, "num_docs" : 0 } } ] } } } }
The sync_id
marker is removed as soon as the shard is flushed again, and
Elasticsearch 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.
Path parameters
edit-
<index>
-
(Optional, string) Comma-separated list or wildcard expression of index names used to limit the request.
To sync-flush all indices, omit this parameter or use a value of
_all
or*
.
Query parameters
edit-
allow_no_indices
-
(Optional, boolean) If
true
, the request does not return an error if a wildcard expression or_all
value retrieves only missing or closed indices.This parameter also applies to index aliases that point to a missing or closed index.
-
expand_wildcards
-
(Optional, string) Controls what kind of indices that wildcard expressions can expand to. Valid values are:
-
all
- Expand to open and closed indices.
-
open
- Expand only to open indices.
-
closed
- Expand only to closed indices.
-
none
- Wildcard expressions are not accepted.
Defaults to
open
. -
-
ignore_unavailable
-
(Optional, boolean) If
true
, missing or closed indices are not included in the response. Defaults tofalse
.
Response codes
edit-
200
- All shards successfully sync-flushed.
-
409
- A replica shard failed to sync-flush.
Examples
editSync-flush a specific index
editPOST /kimchy/_flush/synced
Synch-flush several indices
editPOST /kimchy,elasticsearch/_flush/synced
Sync-flush all indices
editPOST /_flush/synced
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:
{ "_shards": { "total": 2, "successful": 2, "failed": 0 }, "twitter": { "total": 2, "successful": 2, "failed": 0 } }
The following response indicates one shard group failed due to pending operations:
{ "_shards": { "total": 4, "successful": 2, "failed": 2 }, "twitter": { "total": 4, "successful": 2, "failed": 2, "failures": [ { "shard": 1, "reason": "[2] ongoing operations on primary" } ] } }
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:
{ "_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" } } ] } }