Transaction Log and Lucene Index

• Lucene index is organized during commit phase - relatively heavy operation

• Lucene does not have built-in transaction log capabilities

• Change made between two commit operations is lost in case of a failure

• To minimize data loss each shard uses write ahead log or transaction log

• In case of crash recent operations can be replayed back to Lucene index

• Flush is performing Lucene commit and is starting new transaction log in the background

• Flush and transaction log settings are configured in elasticsearch.yml