background-jobs-and-cachinglisted

# Background jobs & caching In-process async work (Bull + Redis) and a Redis cache for a backend service. Examples NestJS/TS, neutral `listing`/`payment` domain. principle → **▸ Example** → **▸ Other stacks**. Cross-service async (vs this in-process Bull) → the SQS events pattern in `write-service-code` §6. ## When to use Time-critical, in-process async (reminders, expiry/timeout actions, retries, fan-out) and caching hot reads. **Bull (this skill) = in-process, Redis-backed, supports delays + per-queue retry; SQS = cross-service.** Complementary — don't merge them. ## 1. Bull job queues - **One named queue per job type**, not a single mega-queue — independent concurrency + retry: ```ts BullModule.registerQueue({ name: REMINDER_QUEUE }, { name: PAYMENT_EXPIRY_QUEUE }); ``` Set **default job options** centrally: `removeOnComplete: true`, `removeOnFail: { age: <1h>, count: <N> }` (so Redis doesn't fill with finished jobs), plus a **retry policy** — `attempts: <n>` with `backoff: { type: 'exponential', delay: <ms> }` — so a transient failure retries with growing delay instead of dying on the first error or hammering the dependency instantly. - **Producer** injects the queue; **processor** handles it: ```ts @InjectQueue(PAYMENT_EXPIRY_QUEUE) private queue: Queue; await this.queue.add(JOB.expirePayment, { paymentId }, opts); @Processor(PAYMENT_EXPIRY_QUEUE) class PaymentExpiryProcessor { @Process(JOB.expirePayment) async handle(job: Job) { await thi