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