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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. 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:

  • INVALID_ARGUMENT
  • NOT_FOUND
  • ALREADY_EXISTS
  • FAILED_PRECONDITION
  • ABORTED
  • OUT_OF_RANGE
  • DATA_LOSS

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.