go-grpc

Go gRPC Services

gRPC's contract-first model only pays off if the contract is treated as an API: versioned packages, deliberate error codes, deadlines everywhere, and interceptors for everything cross-cutting.

1. Proto Design Rules

syntax = "proto3";

package payment.v1;                            // version IN the package
option go_package = "github.com/acme/payment-service/gen/payment/v1;paymentv1";

service PaymentService {
  rpc CreatePayment(CreatePaymentRequest) returns (CreatePaymentResponse);
}

message CreatePaymentRequest {                 // one request/response pair
  string order_id = 1;                         // per RPC, always — even if
  int64 amount_cents = 2;                      // empty today
}

message CreatePaymentResponse {
  Payment payment = 1;
}

• Version in the package (payment.v1); breaking change = payment.v2.
• Never reuse or renumber field tags; reserved 3, 7; deleted ones.
• Dedicated Request/Response messages per RPC — adding a field later is free; changing a shared message breaks every RPC using it.
• Generate with buf or a pinned protoc in make generate; commit generated code so builds don't depend on toolchain drift.

2. Errors: Status Codes, Not Strings

Return status.Error, mapping domain errors in ONE place:

func (s *Server) CreatePayment(ctx context.Context, req *pb.CreatePaymentRequest) (*pb.CreatePaymentResponse, error) {
    p, err := s.svc.Create(ctx, toDomain(req))
    if err != nil {
        return nil, toStatus(err)
    }
    return &pb.CreatePaymentResponse{Payment: fromDomain(p)}, nil
}

func toStatus(err error) error {
    switch {
    case errors.Is(err, domain.ErrNotFound):
        return status.Error(codes.NotFound, "payment not found")
    case errors.Is(err, domain.ErrDuplicate):
        return status.Error(codes.AlreadyExists, "payment already exists")
    case errors.Is(err, context.DeadlineExceeded):
        return status.Error(codes.DeadlineExceeded, "timed out")
    default:
        return status.Error(codes.Internal, "internal error") // no details leak
    }
}