Nestjs Scheduling

Task Scheduling & Jobs

Priority: P1 (OPERATIONAL)

Background job processing and scheduled task patterns.

• 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:

  @Cron(CronExpression.EVERY_MINUTE)
  @DistributedLock({ key: 'send_emails', ttl: 5000 })
  async handleCron() {
    // Only runs if lock acquired
  }
  

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

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.