Implementing Distributed Locks with Redis and Redisson Watchdog Mechanism

Redis can be used to implement a distributed lock by storing a unique key as the lock marker and a unique client identifier as its value. The lock is set only when the key does not exist, ensuring mutual exclusion, and an expiration time prevents deadlocks.

The article discusses the problem when the lock expiration (e.g., 30 seconds) is shorter than the business processing time, leading to lock loss and potential contention, and outlines the trade‑offs of setting the lock timeout too short or too long.

One solution is automatic renewal: a watchdog thread periodically extends the lock’s TTL before it expires. Redisson provides such a watchdog mechanism.

Redisson’s public boolean tryLock(long waitTime, long leaseTime, TimeUnit unit) throws InterruptedException { ... } method is shown, illustrating how it attempts to acquire the lock, subscribes to lock‑release events, waits with a semaphore, and retries within the configured wait time.

The watchdog implementation is explained: after a lock is acquired, Redisson registers the owning thread in RedissonLock.EXPIRATION_RENEWAL_MAP and a background task checks every internalLockLeaseTime / 3 (≈10 seconds) to extend the key’s TTL using a Lua script if the thread still holds the lock.

If the client crashes, the watchdog thread stops, the lock expires after the default 30 seconds, and other clients can acquire it.

The article also includes promotional material for a technical learning group, which is not part of the technical explanation.