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Status codes and their use in gRPC

gRPC uses a set of well defined status codes as part of the RPC API. These statuses are defined as such:

Code Number Description
OK 0 Not an error; returned on success.
CANCELLED 1 The operation was cancelled, typically by the caller.
UNKNOWN 2 Unknown error. For example, this error may be returned when 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.
INVALID_ARGUMENT 3 The 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_EXCEEDED 4 The deadline expired before the 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
NOT_FOUND 5 Some requested entity (e.g., file or directory) was not found. Note to server developers: if a request is denied for an entire class of users, such as gradual feature rollout or undocumented allowlist, NOT_FOUND may be used. If a request is denied for some users within a class of users, such as user-based access control, PERMISSION_DENIED must be used.
ALREADY_EXISTS 6 The entity that a client attempted to create (e.g., file or directory) already exists.
PERMISSION_DENIED 7 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). This error code does not imply the request is valid or the requested entity exists or satisfies other pre-conditions.
RESOURCE_EXHAUSTED 8 Some resource has been exhausted, perhaps a per-user quota, or perhaps the entire file system is out of space.
FAILED_PRECONDITION 9 The operation was rejected because the system is not in a state required for the operation's execution. For example, the directory to be deleted is non-empty, an rmdir operation is applied to a non-directory, etc. Service implementors can use the following guidelines to decide between FAILED_PRECONDITION, ABORTED, and UNAVAILABLE: (a) Use UNAVAILABLE if the client can retry just the failing call. (b) Use ABORTED if the client should retry at a higher level (e.g., when a client-specified test-and-set fails, indicating the client should restart a read-modify-write sequence). (c) 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 the files are deleted from the directory.
ABORTED 10 The operation was aborted, typically due to a concurrency issue such as a sequencer check failure or transaction abort. See the guidelines above for deciding between FAILED_PRECONDITION, ABORTED, and UNAVAILABLE.
OUT_OF_RANGE 11 The 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.
UNIMPLEMENTED 12 The operation is not implemented or is not supported/enabled in this service.
INTERNAL 13 Internal errors. This means that some invariants expected by the underlying system have been broken. This error code is reserved for serious errors.
UNAVAILABLE 14 The service is currently unavailable. This is most likely a transient condition, which can be corrected by retrying with a backoff. Note that it is not always safe to retry non-idempotent operations.
DATA_LOSS 15 Unrecoverable data loss or corruption.
UNAUTHENTICATED 16 The request does not have valid authentication credentials for the operation.

All RPCs started at a client return a status object composed of an integer code and a string message. The server-side can choose the status it returns for a given RPC.

The gRPC client and server-side implementations may also generate and return status on their own when errors happen. Only a subset of the pre-defined status codes are generated by the gRPC libraries. This allows applications to be sure that any other code it sees was actually returned by the application (although it is also possible for the server-side to return one of the codes generated by the gRPC libraries).

The following table lists the codes that may be returned by the gRPC libraries (on either the client-side or server-side) and summarizes the situations in which they are generated.

Case Code Generated at Client or Server
Client Application cancelled the request CANCELLED Both
Deadline expires before server returns status DEADLINE_EXCEEDED Both
Method not found at server UNIMPLEMENTED Server
Server shutting down UNAVAILABLE Server
Server side application throws an exception (or does something other than returning a Status code to terminate an RPC) UNKNOWN Server
No response received before Deadline expires. This may occur either when the client is unable to send the request to the server or when the server fails to respond in time. DEADLINE_EXCEEDED Both
Some data transmitted (e.g., request metadata written to TCP connection) before connection breaks UNAVAILABLE Client
Could not decompress, but compression algorithm supported (Client -> Server) INTERNAL Server
Could not decompress, but compression algorithm supported (Server -> Client) INTERNAL Client
Compression mechanism used by client not supported at server UNIMPLEMENTED Server
Server temporarily out of resources (e.g., Flow-control resource limits reached) RESOURCE_EXHAUSTED Server
Client does not have enough memory to hold the server response RESOURCE_EXHAUSTED Client
Flow-control protocol violation INTERNAL Both
Error parsing returned status UNKNOWN Client
Incorrect Auth metadata ( Credentials failed to get metadata, Incompatible credentials set on channel and call, Invalid host set in :authority metadata, etc.) UNAUTHENTICATED Both
Request cardinality violation (method requires exactly one request but client sent some other number of requests) UNIMPLEMENTED Server
Response cardinality violation (method requires exactly one response but server sent some other number of responses) UNIMPLEMENTED Client
Error parsing response proto INTERNAL Client
Error parsing request proto INTERNAL Server
Sent or received message was larger than configured limit RESOURCE_EXHAUSTED Both
Keepalive watchdog times out UNAVAILABLE Both

The following status codes are never generated by the library:


Applications that may wish to retry failed RPCs must decide which status codes on which to retry. As shown in the table above, the gRPC library can generate the same status code for different cases. Server applications can also return those same status codes. Therefore, there is no fixed list of status codes on which it is appropriate to retry in all applications. As a result, individual applications must make their own determination as to which status codes should cause an RPC to be retried.

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