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01-context-cancellation-concurrency/07-rate-limited-fanout/README.md

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+# Kata 07: The Rate-Limited Fan-Out Client
+**Target Idioms:** Rate Limiting (`x/time/rate`), Bounded Concurrency (`x/sync/semaphore`), HTTP Client Hygiene, Context Cancellation  
+**Difficulty:** 🟡 Intermediate
+
+## 🧠 The "Why"
+In many ecosystems, you slap a “rate limit middleware” in front of a thread pool and call it a day. In Go, people often:
+- spawn too many goroutines (no backpressure),
+- forget per-request cancellation,
+- misuse `http.DefaultClient` (timeouts/transport reuse),
+- implement “sleep-based” rate limiting (jittery, wasteful).
+
+This kata forces **explicit control** over *rate*, *in-flight concurrency*, and *cancellation*.
+
+## 🎯 The Scenario
+You’re building an internal service that needs to fetch user widgets from a downstream API:
+- API allows **10 requests/sec** with bursts up to **20**
+- Your service must also cap concurrency at **max 8 in-flight** requests
+- If any request fails, cancel everything immediately (fail-fast), and return the first error.
+
+## 🛠 The Challenge
+Implement `FanOutClient` with:
+- `FetchAll(ctx context.Context, userIDs []int) (map[int][]byte, error)`
+
+### 1. Functional Requirements
+- [ ] Requests must respect a **QPS rate limit** + **burst**.
+- [ ] Requests must run concurrently but never exceed **MaxInFlight**.
+- [ ] Results returned as `map[userID]payload`.
+- [ ] On first error, cancel remaining work and return immediately.
+
+### 2. The "Idiomatic" Constraints (Pass/Fail Criteria)
+- [ ] **Must** use `golang.org/x/time/rate.Limiter`.
+- [ ] **Must** use `golang.org/x/sync/semaphore.Weighted` (or equivalent semaphore pattern) for MaxInFlight.
+- [ ] **Must** use `http.NewRequestWithContext`.
+- [ ] **Must NOT** use `time.Sleep` for rate limiting.
+- [ ] **Must** reuse a single `http.Client` (with a configured `Transport` + `Timeout`).
+- [ ] Logging via `log/slog` (structured fields: userID, attempt, latency).
+
+## 🧪 Self-Correction (Test Yourself)
+- **If you spawn `len(userIDs)` goroutines:** you failed backpressure.
+- **If cancellation doesn’t stop waiting callers:** you failed context propagation.
+- **If QPS is enforced using `Sleep`:** you failed rate limiting.
+- **If you use `http.DefaultClient`:** you failed HTTP hygiene.
+
+## 📚 Resources
+- https://pkg.go.dev/golang.org/x/time/rate
+- https://pkg.go.dev/golang.org/x/sync/semaphore
+- https://go.dev/src/net/http/client.go
+- https://go.dev/src/net/http/transport.go