The /api/user/registration endpoint can be problematic when called concurrently for the same user. FusionAuth recommends using sequential calls.

Recommendation: Make your registration calls sequentially, one after another, even though it may take slightly longer. This ensures data consistency and avoids potential conflicts or failures during the registration process.

There is currently no single API call to atomically register a user to multiple applications. Each registration to an application requires a separate API call, and sequential execution is the safest approach.

Related Documentation User Registration API - Overview of the registration API and available operations Create a User Registration (for an existing user) - API documentation for creating registrations Registrations Core Concept - Understanding the relationship between Users and Applications Applications - Core concepts about Applications in FusionAuth Registering a user to other applications during registration - Community discussion on this topic