Go grpc

Go gRPC

Treat gRPC as a transport. Keep .proto-generated code and business logic separated. The official Go implementation is google.golang.org/grpc; pair it with protoc-gen-go + protoc-gen-go-grpc (or buf generate).

Core Rules

1. One concern per layer. .proto defines the contract; generated code lives in gen/; service implementation lives in internal/. Never edit generated files.
2. Always wrap RPC arguments in Request/Response messages. Bare scalars (string, int32) cannot be evolved without breaking callers.
3. Return typed status codes, never raw errors. A fmt.Errorf becomes codes.Unknown on the wire — the client cannot decide whether to retry.
4. Every client call has a deadline. No context.Background() to a remote service. Set context.WithTimeout per call.
5. Reuse connections. HTTP/2 multiplexes; creating a new grpc.ClientConn per request is a TLS handshake leak.
6. Disable reflection in production. Reflection is a developer convenience that doubles as an API enumeration tool for attackers.

When to Use What

Need	Use
Define service	.proto file in proto/<service>/v1/
Generate stubs	buf generate or protoc --go_out --go-grpc_out
Cross-cutting (auth, logging, recovery)	grpc.ChainUnaryInterceptor / ChainStreamInterceptor
Health probes (Kubernetes)	grpc_health_v1 from google.golang.org/grpc/health
Errors with details	status.Errorf(codes.X, ...) + WithDetails(errdetails.BadRequest{...})
Tests	google.golang.org/grpc/test/bufconn
Service mesh / mTLS	credentials.NewTLS or delegate to Istio/Linkerd

Read references/proto-and-codegen.md when organizing .proto packages or wiring buf. Read references/status-and-errors.md when mapping domain errors to gRPC codes.

Server Bootstrap

import (
    "google.golang.org/grpc"
    "google.golang.org/grpc/health"
    healthpb "google.golang.org/grpc/health/grpc_health_v1"
)

srv := grpc.NewServer(
    grpc.ChainUnaryInterceptor(recoveryUnary, loggingUnary, authUnary),
    grpc.ChainStreamInterceptor(recoveryStream, loggingStream),
)
pb.RegisterUserServiceServer(srv, &userService{...})
healthpb.RegisterHealthServer(srv, health.NewServer())

go func() { _ = srv.Serve(lis) }()

// Graceful shutdown bounded by a hard timeout.
<-shutdownSignal
stopped := make(chan struct{})
go func() { srv.GracefulStop(); close(stopped) }()
select {
case <-stopped:
case <-time.After(15 * time.Second):
    srv.Stop()
}

Three pieces are non-negotiable: interceptors for cross-cutting concerns, health service for Kubernetes probes, and a bounded graceful shutdown.

Client Bootstrap

conn, _ := grpc.NewClient("dns:///user-service:50051",
    grpc.WithTransportCredentials(credentials.NewTLS(tlsCfg)),
    grpc.WithDefaultServiceConfig(`{
      "loadBalancingPolicy": "round_robin",
      "methodConfig": [{
        "name": [{"service": "user.v1.UserService"}],
        "timeout": "5s",
        "retryPolicy": {
          "maxAttempts": 3, "initialBackoff": "0.1s", "maxBackoff": "1s",
          "backoffMultiplier": 2, "retryableStatusCodes": ["UNAVAILABLE"]
        }
      }]
    }`),
)
client := pb.NewUserServiceClient(conn)
ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second); defer cancel()
resp, err := client.GetUser(ctx, &pb.GetUserRequest{Id: id})

The service config is the right place for retries — let the library handle the loop, backoff, and UNAVAILABLE-only filter.

Errors

A raw Go error returned from an RPC becomes codes.Unknown. The client cannot tell a 404 from a 500. Always use status.Errorf:

if errors.Is(err, ErrNotFound) {
    return nil, status.Errorf(codes.NotFound, "user %q not found", req.Id)
}
if errors.As(err, &validationErr) {
    st, _ := status.New(codes.InvalidArgument, "validation").WithDetails(
        &errdetails.BadRequest{FieldViolations: violations(validationErr)},
    )
    return nil, st.Err()
}
return nil, status.Errorf(codes.Internal, "lookup: %v", err)

Quick map:

Domain	Code
Missing/invalid field	InvalidArgument
Not found	NotFound
Already exists	AlreadyExists
Unauthenticated	Unauthenticated
Authenticated but forbidden	PermissionDenied
Rate-limited	ResourceExhausted
Dependency down, retriable	Unavailable
Bug, unexpected	Internal

Streaming

Pattern	Use case
Server streaming	Log tailing, paginated result sets, server-sent events
Client streaming	File upload, batch ingest
Bidirectional	Chat, real-time sync

Streams must respect ctx.Done(). A goroutine reading from a stream after the client disconnects is a slow leak.

func (s *server) ListUsers(req *pb.ListUsersRequest, stream pb.UserService_ListUsersServer) error {
    for _, u := range s.repo.All(stream.Context()) {
        if err := stream.Send(toProto(u)); err != nil {
            return err // includes ctx canceled
        }
    }
    return nil
}

Testing with bufconn

bufconn is an in-memory net.Listener. It exercises the real gRPC stack — interceptors, marshaling, metadata — without binding a TCP port. See references/testing.md for the full harness plus table-driven status-code assertions, metadata injection, and stream testing.

Security Notes

• TLS in production. Plaintext is only acceptable behind a confirmed-private network (and even then mTLS is preferable).
• For service-to-service auth, prefer a mesh (Istio/Linkerd) over hand-rolled token validation.
• For user auth, implement credentials.PerRPCCredentials to attach a token and validate inside an auth interceptor.
• Reflection: enable in dev, disable in prod via build tag or env flag.

Anti-Patterns

Anti-pattern	Why it hurts	Do this instead
return fmt.Errorf("not found")	Wire code is Unknown, clients can't retry-discriminate	status.Errorf(codes.NotFound, ...)
context.Background() to a client call	No deadline → goroutines pile up on a slow dependency	context.WithTimeout(parent, 5s)
New ClientConn per request	TLS handshake every call; sockets exhaust	One grpc.NewClient at startup, reuse
Bare string as RPC argument	Cannot add fields without breaking callers	Always Request/Response messages
Reflection on in production	Lets attackers enumerate every method	Compile-out with build tag in prod
codes.Internal for all errors	Client retry config can't distinguish bugs from outages	Map domain → specific codes
No health service	Kubernetes can't gate traffic; rolling deploys break	Register grpc_health_v1
Ignoring stream.Context().Done()	Goroutines run after client disconnect	Select on ctx.Done() in stream loops

Verification Checklist

• .proto packages are versioned (pkg/v1, not pkg)
• All RPCs take Request and return Response messages
• Generated code is in a separate directory, never edited
• Every error return uses status.Errorf with a specific code
• Every client call has a deadline via context.WithTimeout
• Server registers grpc_health_v1
• GracefulStop is bounded by a time.After fallback
• Reflection is gated to non-production builds
• Tests use bufconn and assert status.Code(err)

References

• references/proto-and-codegen.md — .proto layout, buf.yaml, codegen flags
• references/status-and-errors.md — code mapping, rich details with errdetails
• references/testing.md — bufconn, metadata, streaming assertions
• references/anti-patterns.md — detailed walkthrough of each anti-pattern