Skill: anderskev/go-concurrency-web (version 2.3.1)

--- name: go-concurrency-web description: Go concurrency patterns for high-throughput web applications including worker pools, rate limiting, race detection, and safe shared state management. Use when implementing background task processing, rate limiters, or concurrent request handling. --- # Go Concurrency for Web Applications ## Quick Reference | Topic | Reference | |-------|-----------| | Worker Pools & errgroup | [references/worker-pools.md](references/worker-pools.md) | | Rate Limiting | [references/rate-limiting.md](references/rate-limiting.md) | | Race Detection & Fixes | [references/race-detection.md](references/race-detection.md) | ## Core Rules 1. **Goroutines are cheap but not free** — each goroutine consumes ~2-8 KB of stack. Unbounded spawning under load leads to OOM. 2. **Always have a shutdown path** — every goroutine you start must have a way to exit. Use `context.Context`, channel closing, or `sync.WaitGroup`. 3. **Prefer channels for communication** — use channels to coordinate work between goroutines and signal completion. 4. **Use mutexes for state protection** — when goroutines share mutable state, protect it with `sync.Mutex`, `sync.RWMutex`, or `sync/atomic`. 5. **Never spawn raw goroutines in HTTP handlers** — use worker pools, `errgroup`, or other bounded concurrency primitives. ## Gates (check before merge or review) Use these **sequenced** checks for objective pass/fail; do not replace them with “I verified mentally.” 1. **Race detector** - Run `go test -race ./...` on packages that changed concurrent code, or `go build -race` for binaries under test. - **Pass:** exit code `0`. If you report “no races,” attach or cite CI output / saved terminal transcript—do not assert cleanliness without that artifact. 2. **Bounded background work from HTTP** - Inspect handlers and middleware that start work beyond the request goroutine. - **Pass:** every such path uses a bounded primitive (worker pool, buffered channel with documented capacity, `errgroup` with an explicit concurrency cap)—not unbounded `go` per incoming request. 3. **Graceful teardown** - For processes that start long-lived goroutines, trace from shutdown signal (or test `defer`) to `Wait()` / channel close / `context` cancel for each goroutine family. - **Pass:** you can point to the call chain or a test that proves shutdown completes without hang (no orphan goroutines). ## Worker Pool Pattern Use worker pools for background tasks dispatched from HTTP handlers. This bounds concurrency and provides graceful shutdown. ```go // Worker pool for background tasks (e.g., sending emails) type WorkerPool struct { jobs chan Job wg sync.WaitGroup logger *slog.Logger } type Job struct { ID string Execute func(ctx context.Context) error } func NewWorkerPool(numWorkers int, queueSize int, logger *slog.Logger) *WorkerPool { wp := &WorkerPool{ jobs: make(chan Job, queueSize), logger: logger, } for i := 0; i < numWorkers; i++ { wp.wg.Add(1) go wp.worker(i) } return wp } func (wp *WorkerPool) worker(id int) { defer wp.wg.Done() for job := range wp.jobs { wp.logger.Info("processing job", "worker", id, "job_id", job.ID) if err := job.Execute(context.Background()); err != nil { wp.logger.Error("job failed", "worker", id, "job_id", job.ID, "err", err) } } } func (wp *WorkerPool) Submit(job Job) { wp.jobs <- job } func (wp *WorkerPool) Shutdown() { close(wp.jobs) wp.wg.Wait() } ``` ### Usage in HTTP Handler ```go func (s *Server) handleCreateUser(w http.ResponseWriter, r *http.Request) { user, err := s.userService.Create(r.Context(), decodeUser(r)) if err != nil { handleError(w, r, err) return } // Dispatch background task — never spawn raw goroutines in handlers s.workers.Submit(Job{ ID: "welcome-email-" + user.ID, Execute: func(ctx context.Context) error { return s.emailService.SendWelcome(ctx, user) }, }) writeJSON(w, http.StatusCreated, user) } ``` See [references/worker-pools.md](references/worker-pools.md) for sizing guidance, backpressure, error handling, retry patterns, and `errgroup` as a simpler alternative. ## Rate Limiting Use `golang.org/x/time/rate` for token bucket rate limiting. Apply as middleware for global limits or per-IP/per-user limits. Key points: - Global rate limiting protects overall service capacity - Per-IP rate limiting prevents individual clients from monopolizing resources - Always return `429 Too Many Requests` with a `Retry-After` header See [references/rate-limiting.md](references/rate-limiting.md) for middleware implementation, per-IP limiting, stale limiter cleanup, and API key-based limiting. ## Race Detection Run the race detector in development and CI: ```bash go test -race ./... go build -race -o myserver ./cmd/server ``` The race detector catches concurrent reads and writes to shared memory. It does not catch logical races (e.g., TOCTOU bugs) or deadlocks. See [references/race-detection.md](references/race-detection.md) for common web handler races, fixing strategies, and CI integration. ## Handler Safety Every incoming HTTP request runs in its own goroutine. Any shared mutable state on the server struct is a potential data race. ```go // BAD — shared state without protection type Server struct { requestCount int // data race! } func (s *Server) handleRequest(w http.ResponseWriter, r *http.Request) { s.requestCount++ // concurrent writes = race condition } // GOOD — use atomic or mutex type Server struct { requestCount atomic.Int64 } func (s *Server) handleRequest(w http.ResponseWriter, r *http.Request) { s.requestCount.Add(1) } // GOOD — use mutex for complex state type Server struct { mu sync.RWMutex cache map[string]*CachedItem } func (s *Server) handleGetCached(w http.ResponseWriter, r *http.Request) { s.mu.RLock() item, ok := s.cache[r.PathValue("key")] s.mu.RUnlock() // ... } ``` ### Rules for Handler Safety - **Request-scoped data is safe** — `r.Context()`, request body, URL params are isolated per request. - **Server struct fields are shared** — any field on `*Server` accessed by handlers needs synchronization. - **Database connections are safe** — `*sql.DB` manages its own connection pool with internal locking. - **Maps are not safe** — use `sync.Map` or protect with a mutex. - **Slices are not safe** — concurrent append or read/write requires a mutex. ## Anti-Patterns ### Unbounded goroutine spawning ```go // BAD — no limit on concurrent goroutines func (s *Server) handleWebhook(w http.ResponseWriter, r *http.Request) { go func() { // What if 10,000 requests arrive at once? s.processWebhook(r.Context(), decodeWebhook(r)) }() w.WriteHeader(http.StatusAccepted) } // GOOD — use a worker pool func (s *Server) handleWebhook(w http.ResponseWriter, r *http.Request) { webhook := decodeWebhook(r) s.workers.Submit(Job{ ID: "webhook-" + webhook.ID, Execute: func(ctx context.Context) error { return s.processWebhook(ctx, webhook) }, }) w.WriteHeader(http.StatusAccepted) } ``` ### Forgetting to propagate context ```go // BAD — loses cancellation signal func (s *Server) handleSearch(w http.ResponseWriter, r *http.Request) { results, err := s.search(context.Background(), r.URL.Query().Get("q")) // ... } // GOOD — use request context func (s *Server) handleSearch(w http.ResponseWriter, r *http.Request) { results, err := s.search(r.Context(), r.URL.Query().Get("q")) // ... } ``` ### Goroutine leak from missing channel receiver ```go // BAD — goroutine blocks forever if nobody reads the channel func fetchWithTimeout(ctx context.Context, url string) (*Response, error) { ch := make(chan *Response) go func() { resp, _ := http.Get(url) // blocks forever if ctx cancels ch <- resp // stuck here if nobody reads }() select { case resp := <-ch: return resp, nil case <-ctx.Done(): return nil, ctx.Err() // goroutine leaked! } } // GOOD — use buffered channel so goroutine can exit func fetchWithTimeout(ctx context.Context, url string) (*Response, error) { ch := make(chan *Response, 1) // buffered — goroutine can always send go func() { resp, _ := http.Get(url) ch <- resp }() select { case resp := <-ch: return resp, nil case <-ctx.Done(): return nil, ctx.Err() } } ``` ### Using `time.Sleep` for coordination ```go // BAD — sleeping to wait for goroutines go doWork() time.Sleep(5 * time.Second) // hoping it finishes // GOOD — use sync primitives var wg sync.WaitGroup wg.Add(1) go func() { defer wg.Done() doWork() }() wg.Wait() ```


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