How do you prevent scheduled tasks from running on multiple instances (Distributed Locking)? शेड्यूल्ड टास्क्स को कई इंस्टेंस पर चलने से कैसे रोकें (डिस्ट्रिब्यूटेड लॉकिंग)?

@Scheduled runs independently on every JVM instance of an application, so a horizontally scaled service with 3 replicas would run the same scheduled job 3 times simultaneously by default — often causing duplicate emails, duplicate report generation, or race conditions on shared resources.

ShedLock is the standard solution: it uses a shared external store (a database table, Redis, or ZooKeeper) as a distributed lock, and only the instance that successfully acquires the lock for that execution window runs the task, while the others skip it. It's added declaratively with @SchedulerLock alongside the existing @Scheduled annotation, requiring minimal code changes.

@Scheduled(cron = "0 0 * * * *")
@SchedulerLock(name = "generateReport", lockAtMostFor = "10m", lockAtLeastFor = "1m")
public void generateReport() { ... }