HTTP 409 status code

409 Conflict

The request cannot be completed due to a conflict with the current state of the requested resource. This code should only be used if the user is believed to be able to resolve the conflict and resubmit a new request. The response should contain enough information for the user to discover the source of the conflict.

Conflicts usually occur in the processing of PUT requests. For example, in an environment using version checking, if the version information attached to a modification request for a specific resource submitted by a PUT conflicts with a previous (third-party) request, then the server should return a 409 error at this time. Inform the user that the request cannot be completed. At this time, the response entity is likely to contain a difference comparison between the two conflicting versions, so that the user can resubmit the new version after merging.