# Go Microservice — Project CLAUDE.md

> Real-world example for a Go microservice with PostgreSQL, gRPC, and Docker.
> Copy this to your project root and customize for your service.

## Project Overview

**Stack:** Go 1.22+, PostgreSQL, gRPC + REST (grpc-gateway), Docker, sqlc (type-safe SQL), Wire (dependency injection)

**Architecture:** Clean architecture with domain, repository, service, and handler layers. gRPC as primary transport with REST gateway for external clients.

## Critical Rules

### Go Conventions

- Follow Effective Go and the Go Code Review Comments guide
- Use `errors.New` / `fmt.Errorf` with `%w` for wrapping — never string matching on errors
- No `init()` functions — explicit initialization in `main()` or constructors
- No global mutable state — pass dependencies via constructors
- Context must be the first parameter and propagated through all layers

### Database

- All queries in `queries/` as plain SQL — sqlc generates type-safe Go code
- Migrations in `migrations/` using golang-migrate — never alter the database directly
- Use transactions for multi-step operations via `pgx.Tx`
- All queries must use parameterized placeholders (`$1`, `$2`) — never string formatting

### Error Handling

- Return errors, don't panic — panics are only for truly unrecoverable situations
- Wrap errors with context: `fmt.Errorf("creating user: %w", err)`
- Define sentinel errors in `domain/errors.go` for business logic
- Map domain errors to gRPC status codes in the handler layer

```go
// Domain layer — sentinel errors
var (
    ErrUserNotFound  = errors.New("user not found")
    ErrEmailTaken    = errors.New("email already registered")
)

// Handler layer — map to gRPC status
func toGRPCError(err error) error {
    switch {
    case errors.Is(err, domain.ErrUserNotFound):
        return status.Error(codes.NotFound, err.Error())
    case errors.Is(err, domain.ErrEmailTaken):
        return status.Error(codes.AlreadyExists, err.Error())
    default:
        return status.Error(codes.Internal, "internal error")
    }
}
```

### Code Style

- No emojis in code or comments
- Exported types and functions must have doc comments
- Keep functions under 50 lines — extract helpers
- Use table-driven tests for all logic with multiple cases
- Prefer `struct{}` for signal channels, not `bool`

## File Structure

```
cmd/
  server/
    main.go              # Entrypoint, Wire injection, graceful shutdown
internal/
  domain/                # Business types and interfaces
    user.go              # User entity and repository interface
    errors.go            # Sentinel errors
  service/               # Business logic
    user_service.go
    user_service_test.go
  repository/            # Data access (sqlc-generated + custom)
    postgres/
      user_repo.go
      user_repo_test.go  # Integration tests with testcontainers
  handler/               # gRPC + REST handlers
    grpc/
      user_handler.go
    rest/
      user_handler.go
  config/                # Configuration loading
    config.go
proto/                   # Protobuf definitions
  user/v1/
    user.proto
queries/                 # SQL queries for sqlc
  user.sql
migrations/              # Database migrations
  001_create_users.up.sql
  001_create_users.down.sql
```

## Key Patterns

### Repository Interface

```go
type UserRepository interface {
    Create(ctx context.Context, user *User) error
    FindByID(ctx context.Context, id uuid.UUID) (*User, error)
    FindByEmail(ctx context.Context, email string) (*User, error)
    Update(ctx context.Context, user *User) error
    Delete(ctx context.Context, id uuid.UUID) error
}
```

### Service with Dependency Injection

```go
type UserService struct {
    repo   domain.UserRepository
    hasher PasswordHasher
    logger *slog.Logger
}

func NewUserService(repo domain.UserRepository, hasher PasswordHasher, logger *slog.Logger) *UserService {
    return &UserService{repo: repo, hasher: hasher, logger: logger}
}

func (s *UserService) Create(ctx context.Context, req CreateUserRequest) (*domain.User, error) {
    existing, err := s.repo.FindByEmail(ctx, req.Email)
    if err != nil && !errors.Is(err, domain.ErrUserNotFound) {
        return nil, fmt.Errorf("checking email: %w", err)
    }
    if existing != nil {
        return nil, domain.ErrEmailTaken
    }

    hashed, err := s.hasher.Hash(req.Password)
    if err != nil {
        return nil, fmt.Errorf("hashing password: %w", err)
    }

    user := &domain.User{
        ID:       uuid.New(),
        Name:     req.Name,
        Email:    req.Email,
        Password: hashed,
    }
    if err := s.repo.Create(ctx, user); err != nil {
        return nil, fmt.Errorf("creating user: %w", err)
    }
    return user, nil
}
```

### Table-Driven Tests

```go
func TestUserService_Create(t *testing.T) {
    tests := []struct {
        name    string
        req     CreateUserRequest
        setup   func(*MockUserRepo)
        wantErr error
    }{
        {
            name: "valid user",
            req:  CreateUserRequest{Name: "Alice", Email: "alice@example.com", Password: "secure123"},
            setup: func(m *MockUserRepo) {
                m.On("FindByEmail", mock.Anything, "alice@example.com").Return(nil, domain.ErrUserNotFound)
                m.On("Create", mock.Anything, mock.Anything).Return(nil)
            },
            wantErr: nil,
        },
        {
            name: "duplicate email",
            req:  CreateUserRequest{Name: "Alice", Email: "taken@example.com", Password: "secure123"},
            setup: func(m *MockUserRepo) {
                m.On("FindByEmail", mock.Anything, "taken@example.com").Return(&domain.User{}, nil)
            },
            wantErr: domain.ErrEmailTaken,
        },
    }

    for _, tt := range tests {
        t.Run(tt.name, func(t *testing.T) {
            repo := new(MockUserRepo)
            tt.setup(repo)
            svc := NewUserService(repo, &bcryptHasher{}, slog.Default())

            _, err := svc.Create(context.Background(), tt.req)

            if tt.wantErr != nil {
                assert.ErrorIs(t, err, tt.wantErr)
            } else {
                assert.NoError(t, err)
            }
        })
    }
}
```

## Environment Variables

```bash
# Database
DATABASE_URL=postgres://user:pass@localhost:5432/myservice?sslmode=disable

# gRPC
GRPC_PORT=50051
REST_PORT=8080

# Auth
JWT_SECRET=           # Load from vault in production
TOKEN_EXPIRY=24h

# Observability
LOG_LEVEL=info        # debug, info, warn, error
OTEL_ENDPOINT=        # OpenTelemetry collector
```

## Testing Strategy

```bash
/go-test             # TDD workflow for Go
/go-review           # Go-specific code review
/go-build            # Fix build errors
```

### Test Commands

```bash
# Unit tests (fast, no external deps)
go test ./internal/... -short -count=1

# Integration tests (requires Docker for testcontainers)
go test ./internal/repository/... -count=1 -timeout 120s

# All tests with coverage
go test ./... -coverprofile=coverage.out -count=1
go tool cover -func=coverage.out  # summary
go tool cover -html=coverage.out  # browser

# Race detector
go test ./... -race -count=1
```

## ECC Workflow

```bash
# Planning
/plan "Add rate limiting to user endpoints"

# Development
/go-test                  # TDD with Go-specific patterns

# Review
/go-review                # Go idioms, error handling, concurrency
/security-scan            # Secrets and vulnerabilities

# Before merge
go vet ./...
staticcheck ./...
```

## Git Workflow

- `feat:` new features, `fix:` bug fixes, `refactor:` code changes
- Feature branches from `main`, PRs required
- CI: `go vet`, `staticcheck`, `go test -race`, `golangci-lint`
- Deploy: Docker image built in CI, deployed to Kubernetes