gRPC defined 18 status codes for returning different types of errors.

I think they’re a pretty good reference if you want to design clear error handling for API.

Status Codes

OK = 0

Not an error; returned on success.


The operation was cancelled (typically by the caller).


Unknown error. An example of where this error may be returned is if a Status value received from another address space belongs to an error-space that is not known in this address space. Also errors raised by APIs that do not return enough error information may be converted to this error.


Client specified an invalid argument. Note that this differs from FAILED_PRECONDITION. INVALID_ARGUMENT indicates arguments that are problematic regardless of the state of the system (e.g., a malformed file name).


Deadline expired before operation could complete. For operations that change the state of the system, this error may be returned even if the operation has completed successfully. For example, a successful response from a server could have been delayed long enough for the deadline to expire.


Some requested entity (e.g., file or directory) was not found.


Some entity that we attempted to create (e.g., file or directory) already exists.


The caller does not have permission to execute the specified operation. PERMISSION_DENIED must not be used for rejections caused by exhausting some resource (use RESOURCE_EXHAUSTED instead for those errors). PERMISSION_DENIED must not be used if the caller can not be identified (use UNAUTHENTICATED instead for those errors).


The request does not have valid authentication credentials for the operation.


Some resource has been exhausted, perhaps a per-user quota, or perhaps the entire file system is out of space.


Operation was rejected because the system is not in a state required for the operation’s execution. For example, directory to be deleted may be non-empty, an rmdir operation is applied to a non-directory, etc.

A litmus test that may help a service implementor in deciding between FAILED_PRECONDITION, ABORTED, and UNAVAILABLE:

  1. Use UNAVAILABLE if the client can retry just the failing call.
  2. Use ABORTED if the client should retry at a higher-level (e.g., restarting a read-modify-write sequence).
  3. Use FAILED_PRECONDITION if the client should not retry until the system state has been explicitly fixed. E.g., if an “rmdir” fails because the directory is non-empty, FAILED_PRECONDITION should be returned since the client should not retry unless they have first fixed up the directory by deleting files from it.
  4. Use FAILED_PRECONDITION if the client performs conditional REST Get/Update/Delete on a resource and the resource on the server does not match the condition. E.g., conflicting read-modify-write on the same resource.


The operation was aborted, typically due to a concurrency issue like sequencer check failures, transaction aborts, etc.

See litmus test above for deciding between FAILED_PRECONDITION, ABORTED, and UNAVAILABLE.


Operation was attempted past the valid range. E.g., seeking or reading past end of file.

Unlike INVALID_ARGUMENT, this error indicates a problem that may be fixed if the system state changes. For example, a 32-bit file system will generate INVALID_ARGUMENT if asked to read at an offset that is not in the range [0,2^32-1], but it will generate OUT_OF_RANGE if asked to read from an offset past the current file size.

There is a fair bit of overlap between FAILED_PRECONDITION and OUT_OF_RANGE. We recommend using OUT_OF_RANGE (the more specific error) when it applies so that callers who are iterating through a space can easily look for an OUT_OF_RANGE error to detect when they are done.


Operation is not implemented or not supported/enabled in this service.


Internal errors. Means some invariants expected by underlying System has been broken. If you see one of these errors, Something is very broken.


The service is currently unavailable. This is a most likely a transient condition and may be corrected by retrying with a backoff.

warning Although data MIGHT not have been transmitted when this status occurs, there is NOT A GUARANTEE that the server has not seen anything. So in general it is unsafe to retry on this status code if the call is non-idempotent.

See litmus test above for deciding between FAILED_PRECONDITION, ABORTED, and UNAVAILABLE.


Unrecoverable data loss or corruption.