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 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.

Elasticsearch 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.

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 API:

GET /twitter/_stats?filter_path=**.commit&level=shards 

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
                 }
               }
            ]
         }
      }
   }
}

POST /_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"
            }
         }
      ]
   }
}