How Redisson Implements Distributed Locks Using Redis

The article introduces the use of Redisson, a Java client for Redis, to create distributed locks, starting with adding the Redisson Maven dependency and a simple Java test class that obtains a lock, prints a success message, sleeps, and then releases the lock.

It then dives into the internal implementation of RedissonLock.tryLock , showing how the method delegates to tryAcquire and uses Lua scripts to atomically check lock existence, create the lock key with a re‑entrancy counter, set expiration, or return the remaining TTL when the lock is held by another thread.

The tryLockInnerAsync method builds the Lua script that performs three cases: creating a new lock, incrementing the re‑entrancy count, or returning the lock's remaining time, and executes it asynchronously via evalWriteAsync .

When a lock expires without being released, Redisson’s watch‑dog mechanism automatically renews the lease. The article explains how the watch‑dog schedules periodic renewal tasks, stores expiration entries in a map, and uses renewExpirationAsync to extend the lock’s TTL, defaulting to a 30‑second interval.

To illustrate Lua’s role, the article provides a brief overview of Lua, its installation steps, and examples of Lua scripts that interact with Redis commands such as redis.call('set', ...) and redis.call('get', ...) . It also covers Redis’s EVAL and EVALSHA commands for executing Lua scripts and caching them by SHA‑1.

Further, the article shows how Redisson uses Lua for lock release, including a publish call that notifies waiting threads via Redis’s Pub/Sub mechanism. It explains the Pub/Sub model, the PUBLISH and SUBSCRIBE commands, and why Redisson employs them to wake up contending clients.

Overall, the piece provides a comprehensive view of Redisson’s distributed lock implementation, covering dependency setup, Java usage, Lua script logic, automatic lease renewal, and the underlying Redis Pub/Sub coordination.