go-api-design

Go API Design

APIs are contracts. Once published, they're promises. Design them as if you'll maintain them for a decade — because you probably will.

Community default. A project skill or agent-config rule that explicitly supersedes go-api-design takes precedence over this one.

Detailed reference material, loaded on demand:

• references/handlers-and-middleware.md — handler structs, JSON response helpers, request ID and recoverer middleware, decode-and-validate.

Read a reference file only when the summary below is not enough.

1. HTTP Handler Structure

• Route with the standard http.ServeMux (Go 1.22+): "GET /users/{id}" patterns match the method, r.PathValue("id") reads the wildcard, and a wrong method gets a 405 with Allow. A router dependency earns its place for middleware groups and sub-routers, not for path parameters.
• Handlers are methods on a struct that holds their dependencies (store, logger) — never package globals.
• Handler methods return nothing: they write the response and handle the error themselves, mapping domain errors (errors.Is(err, ErrNotFound)) to status codes and logging only the unexpected ones.
• Route through one JSON helper that sets Content-Type, writes the status once, and logs encode failures.

2. Middleware Pattern

Middleware is func(http.Handler) http.Handler. Order, outside → inside:

Recoverer → RequestID → Logger → Auth → RateLimit → Handler

Recover MUST be outermost. Auth before business logic. Logger captures timing.

3. Request Validation

• Cap the body before decoding: r.Body = http.MaxBytesReader(w, r.Body, 1<<20).
• Decode into a typed request struct, then validate it in the same step — a successful decode is not a valid value.
• Reply 400 with a stable message; never echo the decoder's error text.

4. URL and Naming Conventions

GET    /api/v1/users          → list users
POST   /api/v1/users          → create user
GET    /api/v1/users/{id}     → get user
PUT    /api/v1/users/{id}     → replace user
PATCH  /api/v1/users/{id}     → partial update
DELETE /api/v1/users/{id}     → delete user

GET    /api/v1/users/{id}/orders → list user orders (nested resource)

Rules:

• Plural nouns for resources: /users, not /user
• Kebab-case for multi-word paths: /order-items
• One JSON field convention per API, never mixed — camelCase ("createdAt") is the default here; match an existing API if one exists
• Version in URL path: /api/v1/...
• No verbs in URLs: /users/search?q=alice, NOT /searchUsers

5. Pagination

type PageRequest struct {
    Cursor string `json:"cursor"`
    Limit  int    `json:"limit"`
}

type PageResponse[T any] struct {
    Items      []T    `json:"items"`
    NextCursor string `json:"nextCursor,omitempty"`
    HasMore    bool   `json:"hasMore"`
}

Prefer cursor-based pagination over offset/limit for large datasets. Offset pagination breaks under concurrent writes.

6. Graceful Shutdown

func main() {
    if err := run(context.Background()); err != nil {
        slog.Error("server stopped", slog.Any("error", err))
        os.Exit(1)
    }
}

func run(ctx context.Context) error {
    ctx, stop := signal.NotifyContext(ctx, os.Interrupt, syscall.SIGTERM)
    defer stop()

    srv := &http.Server{
        Addr:              ":8080",
        Handler:           router,
        ReadHeaderTimeout: 5 * time.Second, // Slowloris guard
        ReadTimeout:       10 * time.Second,
        WriteTimeout:      15 * time.Second,
        IdleTimeout:       120 * time.Second,
    }

    errc := make(chan error, 1)
    go func() { errc <- srv.ListenAndServe() }()

    select {
    case err := <-errc:
        return fmt.Errorf("listen: %w", err) // e.g. port already in use
    case <-ctx.Done():
    }

    // ctx is already cancelled; shutdown needs its own deadline.
    shutdownCtx, cancel := context.WithTimeout(context.WithoutCancel(ctx), 30*time.Second)
    defer cancel()
    if err := srv.Shutdown(shutdownCtx); err != nil {
        return fmt.Errorf("shutdown: %w", err)
    }
    if err := <-errc; !errors.Is(err, http.ErrServerClosed) {
        return err
    }
    return nil
}

• ReadHeaderTimeout is the Slowloris defence: without it a client can hold a connection open by trickling header bytes. gosec G112 flags a server that lacks it.
• Compare http.ErrServerClosed with errors.Is, never ==.
• Exit only from main. A log.Fatal inside the serving goroutine skips every deferred cleanup, including the shutdown itself.

7. Health Check Endpoints

// Liveness: is the process alive? GET /healthz → 200, no dependency checks
// Readiness: should it receive traffic? GET /readyz → 200 or 503
func (h *HealthHandler) handleReady(w http.ResponseWriter, r *http.Request) {
    if h.draining.Load() { // atomic.Bool, set when shutdown starts
        h.respondError(w, http.StatusServiceUnavailable, "shutting down")
        return
    }
    if err := h.db.PingContext(r.Context()); err != nil {
        h.respondError(w, http.StatusServiceUnavailable, "database unavailable")
        return
    }
    h.respondJSON(w, http.StatusOK, map[string]string{"status": "ready"})
}

On SIGTERM behind a load balancer: set draining, wait one readiness period so the balancer stops routing, then call srv.Shutdown. Shutting down first rejects requests the balancer is still sending. Keep liveness free of dependency checks — a database outage should not make the orchestrator restart every replica.

8. Error Response Format

Consistent error responses across the entire API:

{
    "error": {
        "code": "VALIDATION_ERROR",
        "message": "invalid request parameters",
        "details": [
            {"field": "email", "message": "must be a valid email"}
        ]
    }
}

Map internal errors to HTTP status codes at the handler boundary. Internal errors should NEVER leak to clients. For a standard shape, use RFC 9457 problem details (application/problem+json) — the go-openapi skill has the type.

9. Idempotency and Browser Clients

• POST endpoints that create or charge accept an Idempotency-Key header: store key → response for a bounded time, replay the stored response for a repeated key, and reject a concurrent duplicate with 409. Clients and proxies retry; without this, a retry is a double charge.
• APIs called from browsers with cookies need CSRF protection — http.CrossOriginProtection (Go 1.25+); the go-security-audit skill owns the details.