Task Scheduling & Jobs

Priority: P1 (OPERATIONAL)

Background job processing and scheduled task patterns.

Workflow: Add a Scheduled Task

1. Register ScheduleModule — Import ScheduleModule.forRoot() in AppModule.
2. Create cron handler — Decorate a service method with @Cron(CronExpression.*).
3. Add distributed lock — Apply a Redis lock decorator to prevent multi-pod duplication.
4. Offload heavy work — Push job IDs to BullMQ; let workers process them.
5. Wrap in try/catch — Uncaught exceptions in cron handlers crash the entire Node process.
6. Verify — Test with 2+ instances to confirm only one acquires the lock.

Problem & Solution

• Problem: @Cron() runs on every instance. In K8s with 3 pods, your "Daily Report" runs 3 times.
• Solution: Distributed Locking using Redis.
  • Pattern: Using a decorator to wrap the cron method.
  • Logic: SET resource_name my_random_value NX PX 30000 (Redis Atomic Set).

Cron Decorator Pattern

• Implementation:

  See implementation examples

• Tools: Use nestjs-redlock or custom Redis wrapper via redlock library.

Cron-to-Queue Offload

See implementation examples

Job Robustness

• Isolation: Never perform heavy processing inside the Cron handler.
  • Pattern: Cron -> Push Job ID to Queue (BullMQ) -> Worker processes it.
  • Why: Cron schedulers can get blocked by the Event Loop; Workers are scalable.
• Error Handling: Wrap ALL cron logic in try/catch. Uncaught exceptions in a Cron job can crash the entire Node process.

Anti-Patterns

• No unguarded cron logic: Always wrap in try/catch; uncaught exceptions crash the entire Node process.
• No direct cron processing: Push to BullMQ queue; workers are scalable, cron handlers are not.
• No bare @Cron in multi-pod: Use distributed locking (redlock) to prevent duplicate concurrent runs.