name: NestJS Scheduling description: Distributed cron jobs and locking patterns. metadata: labels: [nestjs, cron, scheduling, redis] triggers: files: ['*/.service.ts'] keywords: [@Cron, CronExpression, ScheduleModule]

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:

typescript @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.