When to use it
Use 409 Stale Resource Version when implementing optimistic concurrency control (OCC) and the client attempts to update or delete a resource using an outdated version indicator (such as an ETag, version integer, or updated_at timestamp).
This indicates that another user or process has updated the resource since the client last fetched it, preventing the client from overwriting those changes (the “lost update” problem).
When not to use it
Do not use this for general data conflicts like duplicate unique values (use resource-conflict). Do not use this if the client is sending concurrent duplicate API calls using an idempotency key (use idempotency-key-conflict).
Example response
{ "type": "https://apiguide.dev/errors/stale-resource-version", "title": "Stale Resource Version", "status": 409, "detail": "The resource version '5' is stale. The resource has been updated by another process and is now version '6'.", "instance": "/v1/leads/lead_12345"}Return the current ETag so the client can re-fetch, alongside the If-Match value it sent that was rejected:
ETag: "6"If-Match: "5"