409 Conflict

HTTP 409 Conflict indicates that the request could not be completed because of a conflict with the current state of the resource — such as a version mismatch in optimistic concurrency control or a duplicate resource error.

Version mismatches in PUT requests, duplicate creation attempts, edit conflicts in collaborative editing.

• Two clients editing the same record simultaneously (lost-update problem)
• Attempting to create a resource with a duplicate unique key
• ETag mismatch in conditional PUT
• Git-style merge conflict in WebDAV

• Fetch the latest resource version and re-apply the change
• Use If-Match with ETag for optimistic locking
• Show the conflict to the user and ask them to resolve it
• Implement retry with backoff after refresh

HTTP/1.1 409 Conflict
Content-Type: application/json

{"error":"Resource version mismatch"}