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📜 WAYPOINT LESSON

Checkpoint — The Parallel Fetcher

⭐⭐ intermediate⏳ 25 min read📍 Lesson 116 of 265

Combine fan-out, per-call timeouts, cooperative cancellation, and honest failure aggregation into one resilient pipeline.

The gate (mini-build)

A resilient concurrent fetch pipeline over CjFlaky.FetchAsync(key, delayMs, ct) (provided in your starter code; slow* keys hang for 5 s):

  1. FetchAll(IEnumerable<string> keys, TimeSpan perCall, CancellationToken ct) → starts EVERY simulated fetch at once (fan-out).
  2. Each call is time-boxed with perCall — a hung backend is ABANDONED, not waited out. Outcomes: "key=ok" or "key=timeout".
  3. Caller cancellation aborts immediately with OperationCanceledException even if fetches are still pending.
  4. Report(results) formats results sorted by key: "a=ok|b=timeout".

Tests observe timing generously (< 1 s for a 5 s hang) — the discipline is structural: start-all-then-await, WaitAsync per call, token through.