d-subrahmanyam/cron-scheduling

Cron Scheduling

Cron Expression Syntax

┌─────── minute (0-59)
│ ┌────── hour (0-23)
│ │ ┌───── day of month (1-31)
│ │ │ ┌──── month (1-12)
│ │ │ │ ┌─── day of week (0-7, 0 and 7 = Sunday)
│ │ │ │ │
* * * * *

| Expression | Schedule | |------------|----------| | 0 * * * * | Every hour | | */15 * * * * | Every 15 minutes | | 0 9 * * 1-5 | Weekdays at 9:00 AM | | 0 0 1 * * | First day of month, midnight | | 0 */6 * * * | Every 6 hours |

Node.js (node-cron)

import cron from 'node-cron';

// Run every day at 2:00 AM
cron.schedule('0 2 * * *', async () => {
  console.log('Running daily cleanup...');
  await cleanupExpiredSessions();
}, {
  timezone: 'America/New_York',
});

// Run every 5 minutes
cron.schedule('*/5 * * * *', async () => {
  await syncExternalData();
});

With Distributed Locking (prevent duplicate runs)

import { Redlock } from 'redlock';

const redlock = new Redlock([redis]);

cron.schedule('0 * * * *', async () => {
  let lock;
  try {
    lock = await redlock.acquire(['cron:hourly-report'], 60000);
    await generateHourlyReport();
  } catch (err) {
    if (err instanceof Redlock.LockError) return; // Another instance has the lock
    throw err;
  } finally {
    await lock?.release();
  }
});

Spring @Scheduled (Java)

@Component
@EnableScheduling
public class ScheduledTasks {

    @Scheduled(cron = "0 0 2 * * *") // Daily at 2 AM
    public void dailyCleanup() {
        sessionRepo.deleteExpired();
    }

    @Scheduled(fixedRate = 300000) // Every 5 minutes
    public void syncData() {
        externalService.sync();
    }

    @Scheduled(fixedDelay = 60000) // 60s after last completion
    public void processQueue() {
        queueService.processNext();
    }
}

Distributed Scheduling (ShedLock)

@Scheduled(cron = "0 */10 * * * *")
@SchedulerLock(name = "syncInventory", lockAtLeastFor = "5m", lockAtMostFor = "10m")
public void syncInventory() {
    inventoryService.syncAll();
}

Python (APScheduler)

from apscheduler.schedulers.asyncio import AsyncIOScheduler
from apscheduler.triggers.cron import CronTrigger

scheduler = AsyncIOScheduler()

@scheduler.scheduled_job(CronTrigger(hour=2, minute=0))
async def daily_cleanup():
    await cleanup_expired_sessions()

@scheduler.scheduled_job('interval', minutes=5)
async def sync_data():
    await sync_external_data()

scheduler.start()

Anti-Patterns

| Anti-Pattern | Fix | |--------------|-----| | Cron on multiple instances without locking | Use distributed lock (Redlock, ShedLock) | | Long-running cron blocks next run | Use job queue for heavy work | | No error handling in cron | Wrap in try-catch, log errors, alert | | Hardcoded schedule | Use environment variable or config | | No monitoring of cron execution | Log start/end times, track failures |

Production Checklist

• [ ] Distributed locking for multi-instance deployments
• [ ] Error handling with alerting
• [ ] Execution time monitoring
• [ ] Timezone explicitly set
• [ ] Graceful shutdown (finish current job)
• [ ] Cron expressions documented