The reindexing process will not impact:

• Users' ability to log in
• User registration for applications

However, there may be a small increase in load times if the reindex occurs during a high-traffic period for your application. The reindexing primarily affects search and user count operations in the administrative console, not end-user authentication flows.

If you do choose to reindex (for other reasons such as search optimization), it can be done safely without blocking user authentication.

Additional Context

When Would Reindexing Be Needed?

While upgrading FusionAuth typically does not require reindexing, there are specific scenarios where it may be necessary:

• Specific version upgrades: Certain version upgrades may require a reindex—this is always noted in the release notes. For example, version 1.17.2 noted: "A reindex may be necessary depending on how you have upgraded your Elasticsearch cluster."
• Database restoration: If you restore FusionAuth from a database dump without migrating the search index, you will need to reindex.
• User imports: If you import users via the User Import API, a reindex may be needed.
• Search engine switching: When switching from the database search engine to Elasticsearch, a reindex is required.

In general, even after a temporary Elasticsearch outage, the index will sync up automatically without manual reindexing.

Why Authentication Is Unaffected

Elasticsearch is only used for search operations in the admin UI and search APIs—it is not queried during authentication. FusionAuth does attempt to update the Elasticsearch index during authentication, but this is done asynchronously (since version 1.30.2) and is not required to complete the login. This means that even if Elasticsearch is temporarily unavailable or reindexing, authentication should not be blocked.

Performance Considerations

Reindexing is an expensive operation and should be avoided unless necessary. It causes additional CPU and I/O overhead until complete. However, performance has been significantly improved in recent versions, and beginning in version 1.48.0, index aliases are used to minimize disruption to API requests during a reindex.

Related Documentation

• Upgrade FusionAuth Guide - Best practices for upgrading FusionAuth
• Release Notes - Always check for version-specific reindexing requirements
• Re-indexing Documentation - When and why reindexing might be needed
• Rebuild the Elasticsearch Index API - How to trigger a reindex programmatically
• Switching Search Engines - Guide for switching between database and Elasticsearch search