Changes to Lucene are only persisted to disk during a Lucene commit, which is a relatively expensive operation and so cannot be performed after every index or delete operation. Changes that happen after one commit and before another will be removed from the index by Lucene in the event of process exit or hardware failure.

Lucene commits are too expensive to perform on every individual change, so each shard copy also writes operations into its transaction log known as the translog. All index and delete operations are written to the translog after being processed by the internal Lucene index but before they are acknowledged. In the event of a crash, recent operations that have been acknowledged but not yet included in the last Lucene commit are instead recovered from the translog when the shard recovers.

An Elasticsearch flush is the process of performing a Lucene commit and starting a new translog generation. Flushes are performed automatically in the background in order to make sure the translog does not grow too large, which would make replaying its operations take a considerable amount of time during recovery. The ability to perform a flush manually is also exposed through an API, although this is rarely needed.

Translog settingsedit

The data in the translog is only persisted to disk when the translog is fsynced and committed. In the event of a hardware failure or an operating system crash or a JVM crash or a shard failure, any data written since the previous translog commit will be lost.

By default, index.translog.durability is set to request meaning that Elasticsearch will only report success of an index, delete, update, or bulk request to the client after the translog has been successfully fsynced and committed on the primary and on every allocated replica. If index.translog.durability is set to async then Elasticsearch fsyncs and commits the translog only every index.translog.sync_interval which means that any operations that were performed just before a crash may be lost when the node recovers.

The following dynamically updatable per-index settings control the behaviour of the translog:

Translog retentionedit

If an index is not using soft deletes to retain historical operations then Elasticsearch recovers each replica shard by replaying operations from the primary’s translog. This means it is important for the primary to preserve extra operations in its translog in case it needs to rebuild a replica. Moreover it is important for each replica to preserve extra operations in its translog in case it is promoted to primary and then needs to rebuild its own replicas in turn. The following settings control how much translog is retained for peer recoveries.