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Workshop 3: Distributed Locks

Learn how to implement Redis-backed distributed locks to prevent race conditions in distributed systems.

What You'll Learn

  • Common distributed lock scenarios
  • Implementing locks with Redisson
  • Preventing inventory oversell and duplicate processing

Run with Docker

# From repository root
cd java-springboot/workshop-hub

# Start Redis + Postgres + Workshop
docker compose -f docker-compose.local.yml --profile infrastructure up -d
docker compose -f docker-compose.local.yml --profile workshop-3_distributed_locks up -d

Open http://localhost:8082

Workshop Flow

Stage What You Do
1. / Reproduce the race-condition problem from the workshop home page
2. /reentrant Learn why a reentrant lock is required for nested checkout calls
3. /reentrant/implement Review the implementation guide before editing code
4. /reentrant/editor or your IDE Update build.gradle.kts, src/main/resources/application.properties, and src/main/java/com/redis/workshop/locks/service/LockManager.java
5. /reentrant/demo Rebuild from the Workshop Hub and verify the job, inventory, and checkout scenarios

Scenarios

  1. Single-Runner Job - Only one worker runs a scheduled job
  2. Cache Stampede - Only one worker rebuilds cache
  3. Event Deduplication - Only one worker processes an event
  4. Single Import - Only one worker runs batch import
  5. Inventory Oversell - Prevent concurrent stock updates

Your Tasks

The editable implementation path is:

  1. build.gradle.kts
  2. src/main/resources/application.properties
  3. src/main/java/com/redis/workshop/locks/service/LockManager.java

src/main/java/com/redis/workshop/locks/service/PurchaseService.java is included as a read-only review step so you can see how withLock() protects the inventory workflow.

1. application.properties - Enable lock mode

workshop.lock.mode=redisson

2. LockManager.java - Implement Redisson lock

public <T> T withLock(String lockKey, Supplier<T> onAcquired, Supplier<T> onBusy) {
    if (!isEnabled()) {
        return onAcquired.get();
    }

    RLock lock = redissonClient.getLock(lockKey);
    try {
        boolean acquired = lock.tryLock(waitTime, leaseTime, TimeUnit.MILLISECONDS);
        if (acquired) {
            return onAcquired.get();
        } else {
            return onBusy.get();
        }
    } catch (InterruptedException e) {
        Thread.currentThread().interrupt();
        return onBusy.get();
    } finally {
        if (lock.isHeldByCurrentThread()) {
            lock.unlock();
        }
    }
}

View Locks in Redis Insight

Open http://localhost:5540 and search for lock keys.

Stopping

docker compose -f docker-compose.local.yml --profile workshop-3_distributed_locks down

Resources