Lead Go Architect

Quick Reference

Topic	Reference
Flat vs modular project layout, migration signals	references/project-structure.md
Graceful shutdown with signal handling	references/graceful-shutdown.md
Dependency injection patterns, testing seams	references/dependency-injection.md

Core Principles

1. Standard library first -- Use net/http and the Go 1.22+ enhanced ServeMux for routing. Only reach for a framework (chi, echo, gin) when you have a concrete need the stdlib cannot satisfy (e.g., complex middleware chains, regex routes).
2. Dependency injection over globals -- Pass databases, loggers, and services through struct fields and constructors, never package-level var.
3. Explicit over magic -- No init() side effects, no framework auto-wiring. main.go is the composition root where everything is assembled visibly.
4. Small interfaces, big structs -- Define interfaces at the consumer, keep them narrow (1-3 methods). Concrete types carry the implementation.

Hard gates

Use this sequence when implementing or reviewing work that claims to follow this skill. Do not skip ahead; each step has a pass condition you can answer with tooling or a concrete file path.

1. Toolchain vs APIs — If the code uses Go 1.22+ ServeMux features (method+path patterns like "GET /x/{id}", r.PathValue, or {path...}): run go version and pass only if the reported toolchain is go1.22+. If the project must stay on an older Go, pass only by not using those APIs (use a compatible router or older patterns) and say so in the review or PR.
2. Composition root — Pass when main.go or cmd/.../main.go visibly constructs the server and injects shared dependencies (DB, logger, config). Fail if shared dependencies are wired in init() or package-level var instead of explicit construction in main (or a run() called from main).
3. Production HTTP shutdown — For a long-lived HTTP service, pass only if shutdown uses http.Server.Shutdown with a bounded context (e.g. context.WithTimeout) after waiting on signal.NotifyContext (or equivalent). Cite the file path when reporting; see references/graceful-shutdown.md for the full pattern.
4. No env/globals in handlers — Pass when handlers and domain code take dependencies via structs/arguments. Fail if handlers read os.Getenv for secrets or use package-level var for DB/clients (loading env in main or a dedicated config package is fine).

Go 1.22+ Enhanced Routing

Go 1.22 upgraded http.ServeMux with method-based routing and path parameters, eliminating the most common reason for third-party routers.

Method-Based Routing and Path Parameters

mux := http.NewServeMux()
mux.HandleFunc("GET /api/users", s.handleListUsers)
mux.HandleFunc("GET /api/users/{id}", s.handleGetUser)
mux.HandleFunc("POST /api/users", s.handleCreateUser)
mux.HandleFunc("DELETE /api/users/{id}", s.handleDeleteUser)

Extracting Path Parameters

func (s *Server) handleGetUser(w http.ResponseWriter, r *http.Request) {
    id := r.PathValue("id")
    if id == "" {
        http.Error(w, "missing id", http.StatusBadRequest)
        return
    }

    user, err := s.users.GetUser(r.Context(), id)
    if err != nil {
        s.logger.Error("getting user", "err", err, "id", id)
        http.Error(w, "internal error", http.StatusInternalServerError)
        return
    }

    w.Header().Set("Content-Type", "application/json")
    json.NewEncoder(w).Encode(user)
}

Wildcard and Exact Match

// Exact match on trailing slash -- serves /api/files/ only
mux.HandleFunc("GET /api/files/", s.handleListFiles)

// Wildcard to end of path -- /api/files/path/to/doc.txt
mux.HandleFunc("GET /api/files/{path...}", s.handleGetFile)

Routing Precedence

The new ServeMux uses most-specific-wins precedence:

• GET /api/users/{id} is more specific than GET /api/users/
• GET /api/users/me is more specific than GET /api/users/{id}
• Method routes take precedence over method-less routes

Server Struct Pattern

The Server struct is the central dependency container for your application. It holds all shared dependencies and implements http.Handler.

type Server struct {
    db     *sql.DB
    logger *slog.Logger
    router *http.ServeMux
}

func NewServer(db *sql.DB, logger *slog.Logger) *Server {
    s := &Server{
        db:     db,
        logger: logger,
        router: http.NewServeMux(),
    }
    s.routes()
    return s
}

func (s *Server) routes() {
    s.router.HandleFunc("GET /api/users/{id}", s.handleGetUser)
    s.router.HandleFunc("POST /api/users", s.handleCreateUser)
    s.router.HandleFunc("GET /healthz", s.handleHealth)
}

func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    s.router.ServeHTTP(w, r)
}

Middleware Wrapping

Apply middleware at the http.Server level or per-route:

// Wrap entire server
httpServer := &http.Server{
    Addr:    ":8080",
    Handler: requestLogger(s),
}

// Or per-route
s.router.Handle("GET /api/admin/", adminOnly(http.HandlerFunc(s.handleAdmin)))

Middleware Signature

func requestLogger(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        start := time.Now()
        next.ServeHTTP(w, r)
        slog.Info("request", "method", r.Method, "path", r.URL.Path, "dur", time.Since(start))
    })
}

Project Structure

Choose based on project size:

• Flat structure -- single package, all files in root. Best for CLIs, small services, < ~10 handlers. See references/project-structure.md.
• Modular/domain-driven -- cmd/, internal/ with domain packages. For larger apps with multiple bounded contexts. See references/project-structure.md.

Start flat. Migrate when you see the signs described in the reference.

Graceful Shutdown

Every production Go server needs graceful shutdown. The pattern uses signal.NotifyContext to listen for OS signals and http.Server.Shutdown to drain connections.

ctx, cancel := signal.NotifyContext(ctx, os.Interrupt, syscall.SIGTERM)
defer cancel()

// ... start server in goroutine ...

<-ctx.Done()

shutdownCtx, shutdownCancel := context.WithTimeout(context.Background(), 10*time.Second)
defer shutdownCancel()
httpServer.Shutdown(shutdownCtx)

Full pattern with cleanup ordering in references/graceful-shutdown.md.

When to Load References

Load project-structure.md when:

• Scaffolding a new Go project
• Discussing package layout or directory organization
• The project is growing and needs restructuring

Load graceful-shutdown.md when:

• Setting up a production HTTP server
• Implementing signal handling or clean shutdown
• Discussing deployment or container readiness

Load dependency-injection.md when:

• Designing how services, stores, and handlers connect
• Making code testable with interfaces
• Reviewing constructor functions or wiring logic

Anti-Patterns

Global database variables

// BAD -- untestable, hidden dependency
var db *sql.DB

func handleGetUser(w http.ResponseWriter, r *http.Request) {
    db.QueryRow(...)
}

Pass db through a Server or Service struct instead.

Framework-first thinking

Do not start with gin.Default() or echo.New(). Start with http.NewServeMux(). Only introduce a framework if you hit a real limitation of the stdlib that justifies the dependency.

God packages

A single handlers package with 50 files is not organization. Group by domain (user, order, billing), not by technical layer.

Using init() for setup

// BAD -- invisible side effects, untestable
func init() {
    db, _ = sql.Open("postgres", os.Getenv("DATABASE_URL"))
}

All initialization belongs in main() or a run() function so it can be tested and errors can be handled.

Reading config in business logic

// BAD -- couples handler to environment
func (s *Server) handleSendEmail(w http.ResponseWriter, r *http.Request) {
    apiKey := os.Getenv("SENDGRID_API_KEY") // don't do this
}

Inject configuration values or clients through constructors.