HTTP 409 – Conflict
The request conflicts with the current state of the target resource.
Common use case
Common in editing workflows, e.g. rejecting a PUT that would overwrite a resource someone else has already modified since it was last read.
Example
HTTP/1.1 409 Conflict
{"error":"Resource was modified by another user"}History
Part of the original HTTP/1.1 spec, RFC 2068 (1997).
Did you know?
It is a favourite of version-control-style APIs and optimistic-locking systems, where two clients racing to update the same record need a clear signal that one of them lost.