Caching Across the System

// cache-aside with coalescing, stale-while-revalidate and jittered TTLs
const inflight = new Map<string, Promise<unknown>>();

async function cached<T>(key: string, load: () => Promise<T>, ttlS = 60, staleS = 300): Promise<T> {
  const hit = await redis.get(key).then(v => v && JSON.parse(v) as { v: T; at: number });
  const age = hit ? (Date.now() - hit.at) / 1000 : Infinity;
  if (hit && age < ttlS) return hit.v;                                   // fresh
  const refresh = () => {
    if (!inflight.has(key)) inflight.set(key, load().then(async v => {
      const jitter = Math.floor(Math.random() * ttlS * 0.2);
      await redis.set(key, JSON.stringify({ v, at: Date.now() }), 'EX', ttlS + staleS + jitter);
      return v;
    }).finally(() => inflight.delete(key)));
    return inflight.get(key) as Promise<T>;                               // one load per key
  };
  if (hit && age < ttlS + staleS) { refresh().catch(() => {}); return hit.v; }   // serve stale, refresh behind
  return refresh();
}

// writes: database first, then delete the key
await db.update(product); await redis.del(`product:${product.id}`);