Each shard has a transaction log or write ahead log associated with it. It allows to guarantee that when an index/delete operation occurs, it is applied atomically, while not "committing" the internal Lucene index for each request. A flush ("commit") still happens based on several parameters:
index.translog.flush_threshold_ops-
After how many operations to flush. Defaults to
unlimited. index.translog.flush_threshold_size-
Once the translog hits this size, a flush will happen. Defaults to
200mb. index.translog.flush_threshold_period-
The period with no flush happening to force a flush. Defaults to
30m. index.translog.interval-
How often to check if a flush is needed, randomized between the interval value and 2x the interval value. Defaults to
5s. index.gateway.local.sync-
How often the translog is fsynced to disk. Defaults to
5s.
Note: these parameters can be updated at runtime using the Index Settings Update API (for example, these number can be increased when executing bulk updates to support higher TPS)