NOTE
Elasticsearch Transaction Log
The translog's role in write durability and shard recovery before Lucene changes are safely committed.
This is a historical learning note and may contain outdated or incomplete understanding.
The transaction log records shard operations so acknowledged writes can be recovered even when the corresponding Lucene index changes have not yet been included in a durable commit.
After a crash/restart or during recovery, Elasticsearch can replay required operations in addition to using Lucene segment state.
Durability/fsync behavior is configurable and version-specific. Do not confuse translog durability with search visibility: refresh and translog/flush solve different problems.