Question to the interviewer: Is there a synchronization mechanism between the Lucene index and the main SQL storage? How is consistency addressed?

Lucene itself does not provide a built-in mechanism for synchronization with SQL storage, as it is a library for full-text search, not a DBMS. The issue of consistency between the Lucene index and the database is usually addressed at the application or infrastructure level.

Common approaches include:

• Transactional synchronization: first update the database, then the index. If the database update is successful, the index is updated. In case of failure, changes can be rolled back or the index update can be retried.
• Message queues: changes in the database are published to a queue (e.g., Kafka), and a separate process updates the Lucene index asynchronously. This reduces load but introduces temporary inconsistency.
• Periodic reindexing: the index is updated on a schedule, which guarantees eventual consistency but not instant accuracy.

For critical systems, these methods are often combined to balance performance and consistency.