{"record":{"id":"151ddd6b8a31d17e","repo":"gofiber/fiber","slug":"failed-to-write-cached-response-at-fastpath-w","errorCode":null,"errorMessage":"failed to write cached response at fastpath: %w","messagePattern":"failed to write cached response at fastpath: %w","errorType":"exception","errorClass":null,"httpStatus":null,"severity":"error","filePath":"middleware/idempotency/idempotency.go","lineNumber":120,"sourceCode":"\t\t// Don't execute middleware if Next returns true\n\t\tif cfg.Next != nil && cfg.Next(c) {\n\t\t\treturn c.Next()\n\t\t}\n\n\t\t// Don't execute middleware if the idempotency key is empty\n\t\tif c.Get(cfg.KeyHeader) == \"\" {\n\t\t\treturn c.Next()\n\t\t}\n\n\t\t// Validate key\n\t\tkey := utils.CopyString(c.Get(cfg.KeyHeader))\n\t\tif err := cfg.KeyHeaderValidate(key); err != nil {\n\t\t\treturn err\n\t\t}\n\n\t\t// First-pass: if the idempotency key is in the storage, get and return the response\n\t\tif ok, err := maybeWriteCachedResponse(c, key); err != nil {\n\t\t\treturn fmt.Errorf(\"failed to write cached response at fastpath: %w\", err)\n\t\t} else if ok {\n\t\t\treturn nil\n\t\t}\n\n\t\tif err := cfg.Lock.Lock(key); err != nil {\n\t\t\treturn fmt.Errorf(\"failed to lock: %w\", err)\n\t\t}\n\t\tdefer func() {\n\t\t\tif err := cfg.Lock.Unlock(key); err != nil {\n\t\t\t\tlog.Errorf(\"[IDEMPOTENCY] failed to unlock key %q: %v\", maskKey(key), err)\n\t\t\t}\n\t\t}()\n\n\t\t// Lock acquired. If the idempotency key now is in the storage, get and return the response\n\t\tif ok, err := maybeWriteCachedResponse(c, key); err != nil {\n\t\t\treturn fmt.Errorf(\"failed to write cached response while locked: %w\", err)\n\t\t} else if ok {\n\t\t\treturn nil","sourceCodeStart":102,"sourceCodeEnd":138,"githubUrl":"https://github.com/gofiber/fiber/blob/a105acad6c1e4576a77f01e02973f67e962bb58d/middleware/idempotency/idempotency.go#L102-L138","documentation":"Wraps any error from maybeWriteCachedResponse on the pre-lock fastpath — i.e. the first lookup attempt before acquiring the idempotency lock. It is a thin context layer over error 168 (failed to read response) or 169 (failed to unmarshal response), identifying that the failure happened during the unlocked fast lookup.","triggerScenarios":"A non-safe request with a valid X-Idempotency-Key enters the handler; the first maybeWriteCachedResponse call (before Lock.Lock) returns an error from Storage.GetWithContext or UnmarshalMsg. The error is re-wrapped and returned to Fiber's error handler.","commonSituations":"Storage backend briefly unavailable at request time, transient network error to Redis/DB, or schema-mismatched cached blob encountered on the fastpath. Distinct from error 172 which signals the same failure after acquiring the lock.","solutions":["Resolve the underlying storage/marshalling error (see errors 168/169).","If transient, a client retry with the same idempotency key is safe.","Add metrics on fastpath failures to detect storage flapping.","Consider a short-circuit fallback if idempotency is best-effort in your system."],"exampleFix":null,"handlingStrategy":"retry","validationCode":null,"typeGuard":null,"tryCatchPattern":null,"preventionTips":["Keep the storage backend healthy to avoid fastpath failures.","Track fastpath vs locked-path error rates separately.","Make client retries idempotent so the same key is safe to retry."],"tags":["idempotency","storage","fastpath","context-wrap"],"backgroundTag":null,"analyzedSha":"a105acad6c1e4576a77f01e02973f67e962bb58d","analyzedAt":"2026-08-11T17:33:26.942Z","contentChangedAt":null,"schemaVersion":2},"datasetVersion":"2026-09-23T08:17:48.524Z"}