Pattern for rich error handling in gRPC

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Include additional error details in the response Metadata. However, still make sure to provide a useful status code and message. In this case, you can add RegisterUserResponse to the Metadata.

In gRPC Java, that would look like:

Metadata.Key<RegisterUserResponse> REGISTER_USER_RESPONSE_KEY =
    ProtoUtils.keyForProto(RegisterUserResponse.getDefaultInstance());
...
Metadata metadata = new Metadata();
metadata.put(REGISTER_USER_RESPONSE_KEY, registerUserResponse);
responseObserver.onError(
    Status.INVALID_ARGUMENT.withDescription("Email or password malformed")
      .asRuntimeException(metadata));

Another option is to use the google.rpc.Status proto which includes an additional Any for details. Support is coming to each language to handle the type. In Java, it’d look like:

// This is com.google.rpc.Status, not io.grpc.Status
Status status = Status.newBuilder()
    .setCode(Code.INVALID_ARGUMENT.getNumber())
    .setMessage("Email or password malformed")
    .addDetails(Any.pack(registerUserResponse))
    .build();
responseObserver.onError(StatusProto.toStatusRuntimeException(status));

google.rpc.Status is cleaner in some languages as the error details can be passed around as one unit. It also makes it clear what parts of the response are error-related. On-the-wire, it still uses Metadata to pass the additional information.

You may also be interested in error_details.proto which contains some common types of errors.

I discussed this topic during CloudNativeCon. You can check out the slides and linked recording on YouTube.

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