Signals Challenge (Node.js + Fastify)

Build a minimal production-leaning service that can handle load, rate limit, and avoid duplicates via idempotency.

Endpoints (to keep)

• POST /v1/signals
  • body: { "userId": "string", "type": "string", "payload": "string" }
  • headers: X-API-Key, Idempotency-Key (optional)
  • behaviors:
    • Rate limit per userId: RATE_LIMIT_PER_MIN per minute (default 5).
    • Idempotency: same Idempotency-Key should not create duplicates.
• GET /v1/signals?userId=...&limit=...
• GET /healthz

Your Tasks

1. Implement a robust rate limiter in src/rateLimit.js.
2. Make idempotency safe across scale in src/signals.js.
3. Handle DB failure gracefully with retry/backoff.
4. Think for 10k RPS. Add a SCALE.md.
5. Finish the tests in tests/*.test.js.

Deliverables

• Working service, passing tests, updated README, SCALE.md.
• Optional deploy link.

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Extra Production Constraints (must pass)

• Atomic Idempotency: Survive concurrent requests and restarts. Avoid check-then-insert races; use a DB-level unique constraint or atomic upsert pattern. Return the same resource for identical Idempotency-Key.
• Concurrency-Safe Rate Limit: Must behave correctly under burst and parallel calls. Naive in-memory counters that race will fail hidden checks. Explain how this becomes multi-instance safe.
• Transient DB Failures: Implement retry/backoff (with jitter) or circuit breaker when DB errors occur (we simulate via DB_FAIL_RATE). No duplicates on retry.
• Scale Plan (10k RPS): Fill SCALE.md with a clear, concise approach (indexes, pooling, caching, queues, horizontal scale, idempotency store).

We will run additional hidden concurrency/multi-instance tests during evaluation.