docs(omo-codex): batch 98 (15 files)

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+# RPC — Connect-Go (default) + grpc-go (fallback) + protovalidate
+
+`connectrpc/connect-go` is the default. It is wire-compatible with gRPC, also speaks Connect protocol + gRPC-Web from browsers, and uses ordinary `net/http` so middleware (logging, auth, tracing) composes the same way as REST. Reach for raw `grpc-go` only when you need a gRPC-specific feature Connect lacks.
+
+---
+
+## When Connect vs grpc-go
+
+| Need | Use |
+|---|---|
+| Standard unary + server-streaming + client-streaming | **Connect** |
+| Browser client without `grpc-web` proxy | **Connect** (native gRPC-Web support) |
+| HTTP/1.1 fallback for hostile networks | **Connect** (gRPC requires HTTP/2 end-to-end) |
+| Server reflection for `grpcurl` | grpc-go (Connect has reflection too, but ecosystem smaller) |
+| Bidirectional streaming with frame-level control | grpc-go |
+| Strict gRPC environment (Envoy with gRPC filters, Istio strict mode) | grpc-go |
+
+**Default**: Connect. The default has been correct since 2024.
+
+---
+
+## Toolchain — Buf, not protoc
+
+```bash
+go install github.com/bufbuild/buf/cmd/buf@latest
+go install google.golang.org/protobuf/cmd/protoc-gen-go@latest
+go install connectrpc.com/connect/cmd/protoc-gen-connect-go@latest
+go install github.com/bufbuild/protovalidate/cmd/protoc-gen-go-vtproto@latest
+```
+
+Buf replaces `protoc` for everything: linting, breaking-change detection, codegen, formatting. The `protoc` toolchain is dead-letter walking — every modern proto project uses Buf.
+
+---
+
+## Project layout
+
+```
+proto/
+  buf.yaml
+  buf.gen.yaml
+  buf.lock
+  myservice/v1/
+    user.proto
+    auth.proto
+
+gen/
+  myservice/v1/
+    user.pb.go               # protoc-gen-go output
+    auth.pb.go
+    myservicev1connect/      # protoc-gen-connect-go output
+      user.connect.go
+      auth.connect.go
+```
+
+**`gen/` is committed.** Generated code is part of the API contract; CI proves it is up-to-date.
+
+---
+
+## `buf.yaml`
+
+```yaml
+version: v2
+modules:
+  - path: proto
+lint:
+  use:
+    - STANDARD
+breaking:
+  use:
+    - FILE
+```
+
+## `buf.gen.yaml`
+
+```yaml
+version: v2
+managed:
+  enabled: true
+  override:
+    - file_option: go_package_prefix
+      value: github.com/your-org/myservice/gen
+plugins:
+  - remote: buf.build/protocolbuffers/go
+    out: gen
+    opt:
+      - paths=source_relative
+  - remote: buf.build/connectrpc/go
+    out: gen
+    opt:
+      - paths=source_relative
+  - remote: buf.build/bufbuild/validate-go
+    out: gen
+    opt:
+      - paths=source_relative
+```
+
+The `buf.build/...` plugin URIs use Buf's hosted remote registry — no local plugin installation needed.
+
+## Taskfile target
+
+```yaml
+gen:proto:
+  cmds:
+    - buf lint
+    - buf format -w
+    - buf generate
+  sources:
+    - proto/**/*.proto
+    - buf.yaml
+    - buf.gen.yaml
+```
+
+Run `task gen:proto` after editing any `.proto`. CI runs `buf generate` then `git diff --exit-code` to catch stale generated code.
+
+---
+
+## A `.proto` with validation
+
+```proto
+syntax = "proto3";
+
+package myservice.v1;
+
+import "buf/validate/validate.proto";
+
+option go_package = "github.com/your-org/myservice/gen/myservice/v1;myservicev1";
+
+service UserService {
+  rpc CreateUser(CreateUserRequest) returns (CreateUserResponse);
+  rpc GetUser(GetUserRequest)       returns (GetUserResponse);
+  rpc StreamEvents(StreamEventsRequest) returns (stream Event);
+}
+
+message CreateUserRequest {
+  string email    = 1 [(buf.validate.field).string.email = true];
+  string username = 2 [
+    (buf.validate.field).string.min_len = 3,
+    (buf.validate.field).string.max_len = 32,
+    (buf.validate.field).string.pattern = "^[a-zA-Z0-9_]+$"
+  ];
+  int32 age       = 3 [
+    (buf.validate.field).int32.gte = 13,
+    (buf.validate.field).int32.lte = 130
+  ];
+}
+
+message CreateUserResponse {
+  User user = 1;
+}
+
+message User {
+  string id       = 1;
+  string email    = 2;
+  string username = 3;
+  google.protobuf.Timestamp created_at = 4;
+}
+```
+
+`protovalidate` replaces the abandoned `protoc-gen-validate` — it is the official Buf-backed successor as of 2024, supported by Connect's interceptor pipeline.
+
+---
+
+## Server — Connect
+
+```go
+package main
+
+import (
+    "context"
+    "log/slog"
+    "net/http"
+
+    "connectrpc.com/connect"
+    "buf.build/go/protovalidate"
+    validateinterceptor "connectrpc.com/validate"
+    "golang.org/x/net/http2"
+    "golang.org/x/net/http2/h2c"
+
+    myservicev1 "github.com/your-org/myservice/gen/myservice/v1"
+    "github.com/your-org/myservice/gen/myservice/v1/myservicev1connect"
+)
+
+type UserServer struct {
+    svc *UserService
+}
+
+func (s *UserServer) CreateUser(
+    ctx context.Context,
+    req *connect.Request[myservicev1.CreateUserRequest],
+) (*connect.Response[myservicev1.CreateUserResponse], error) {
+
+    // protovalidate already ran via the interceptor below.
+    // req.Msg is guaranteed to satisfy the .proto constraints.
+
+    user, err := s.svc.Create(ctx, req.Msg.Email, req.Msg.Username, req.Msg.Age)
+    if err != nil {
+        return nil, mapError(err)
+    }
+    return connect.NewResponse(&myservicev1.CreateUserResponse{
+        User: userToProto(user),
+    }), nil
+}
+
+func main() {
+    validator, _ := protovalidate.New()
+    interceptors := connect.WithInterceptors(
+        loggingInterceptor(),
+        validateinterceptor.NewInterceptor(validator),
+    )
+
+    mux := http.NewServeMux()
+    mux.Handle(myservicev1connect.NewUserServiceHandler(
+        &UserServer{svc: newUserService()},
+        interceptors,
+    ))
+
+    // h2c lets the server speak HTTP/2 cleartext for gRPC clients.
+    srv := &http.Server{
+        Addr:    ":8080",
+        Handler: h2c.NewHandler(mux, &http2.Server{}),
+    }
+    slog.Info("rpc server listening", slog.String("addr", srv.Addr))
+    if err := srv.ListenAndServe(); err != nil { slog.Error("rpc", slog.Any("err", err)) }
+}
+```
+
+The handler is **just an `http.Handler`** — mount it in the same `http.ServeMux` as your REST routes if you want one binary serving both.
+
+---
+
+## Error mapping — Connect codes
+
+```go
+func mapError(err error) error {
+    if err == nil { return nil }
+
+    switch {
+    case errors.Is(err, domain.ErrInvalidEmail),
+         errors.Is(err, domain.ErrInvalidUsername):
+        return connect.NewError(connect.CodeInvalidArgument, err)
+    case errors.Is(err, ErrNotFound):
+        return connect.NewError(connect.CodeNotFound, err)
+    case errors.Is(err, ErrUnauthorized):
+        return connect.NewError(connect.CodeUnauthenticated, err)
+    case errors.Is(err, ErrConflict):
+        return connect.NewError(connect.CodeAlreadyExists, err)
+    default:
+        slog.Error("unmapped rpc error", slog.Any("err", err))
+        return connect.NewError(connect.CodeInternal, errors.New("internal"))
+    }
+}
+```
+
+Connect codes map 1:1 to gRPC codes. Clients see canonical error semantics.
+
+---
+
+## Logging interceptor
+
+```go
+func loggingInterceptor() connect.UnaryInterceptorFunc {
+    return func(next connect.UnaryFunc) connect.UnaryFunc {
+        return func(ctx context.Context, req connect.AnyRequest) (connect.AnyResponse, error) {
+            start := time.Now()
+            res, err := next(ctx, req)
+            attrs := []slog.Attr{
+                slog.String("proc",    req.Spec().Procedure),
+                slog.Duration("elapsed", time.Since(start)),
+            }
+            if err != nil {
+                attrs = append(attrs, slog.Any("err", err))
+                slog.LogAttrs(ctx, slog.LevelWarn, "rpc failed", attrs...)
+            } else {
+                slog.LogAttrs(ctx, slog.LevelInfo, "rpc ok", attrs...)
+            }
+            return res, err
+        }
+    }
+}
+```
+
+For streaming, implement the full `connect.Interceptor` (`WrapStreamingClient`, `WrapStreamingHandler`). Pattern is identical.
+
+---
+
+## Server streaming
+
+```go
+func (s *UserServer) StreamEvents(
+    ctx context.Context,
+    req *connect.Request[myservicev1.StreamEventsRequest],
+    stream *connect.ServerStream[myservicev1.Event],
+) error {
+    events, errs := s.svc.Subscribe(ctx, req.Msg.UserId)
+    for {
+        select {
+        case <-ctx.Done():
+            return ctx.Err()
+        case e, ok := <-events:
+            if !ok { return nil }
+            if err := stream.Send(eventToProto(e)); err != nil {
+                return err
+            }
+        case err := <-errs:
+            return connect.NewError(connect.CodeInternal, err)
+        }
+    }
+}
+```
+
+Same shape as SSE in `backend-stack.md`. Connect handles HTTP/2 framing.
+
+---
+
+## Client
+
+```go
+client := myservicev1connect.NewUserServiceClient(
+    http.DefaultClient,
+    "https://api.example.com",
+    // Use connect.WithGRPC() if the server is grpc-go and you want strict gRPC framing.
+    // Default is Connect protocol — works with Connect or gRPC servers transparently.
+)
+
+res, err := client.CreateUser(ctx, connect.NewRequest(&myservicev1.CreateUserRequest{
+    Email:    "a@b.com",
+    Username: "alice",
+    Age:      30,
+}))
+if err != nil {
+    var connectErr *connect.Error
+    if errors.As(err, &connectErr) {
+        slog.Error("rpc failed",
+            slog.String("code", connectErr.Code().String()),
+            slog.String("msg",  connectErr.Message()))
+    }
+    return err
+}
+slog.Info("created", slog.String("id", res.Msg.User.Id))
+```
+
+---
+
+## When you genuinely need raw grpc-go
+
+```go
+import "google.golang.org/grpc"
+
+lis, _ := net.Listen("tcp", ":8080")
+srv := grpc.NewServer(
+    grpc.UnaryInterceptor(loggingUnaryInterceptor),
+)
+myservicev1.RegisterUserServiceServer(srv, &userServer{})
+_ = srv.Serve(lis)
+```
+
+The codegen is from `protoc-gen-go-grpc` (different binary from `protoc-gen-connect-go`). You can codegen **both** in the same `buf.gen.yaml` and switch by importing the right package. Most teams pick one.
+
+---
+
+## When NOT to use RPC at all
+
+If your callers are all browsers, mobile apps, third-party developers, or the long tail of "things humans curl": **stay with REST + OpenAPI**. RPC's overhead is justified for service-to-service inside a single org. Outside that boundary, JSON over HTTP wins on debuggability.
+
+`oapi-codegen/oapi-codegen/v2` generates Go server stubs and clients from OpenAPI 3 — the REST equivalent of what Connect does for proto. Same parse-don't-validate boundary discipline, different wire format.
+
+---
+
+## Sources
+
+- Connect docs: https://connectrpc.com/docs/go/getting-started
+- Buf: https://buf.build/docs
+- protovalidate: https://github.com/bufbuild/protovalidate
+- "Why we replaced protoc with buf" (Buf blog): https://buf.build/blog
+- gRPC vs Connect comparison: https://connectrpc.com/docs/introduction