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What is a distributed lock, and why would an application need one when using Redis? How do you implement a basic distributed lock in Redis using the SET command with NX and EX options? What happens if the process holding a distributed lock crashes or hangs before it finishes its work and releases the lock? What are some common mistakes developers make when implementing distributed locks with Redis, and how do you avoid them? What is the Redlock algorithm, and why was it proposed for implementing distributed locks across multiple independent Redis instances? How would you design a distributed lock implementation that supports safely extending the lock's expiration time if the protected operation needs more time than originally expected?
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Why is it important to release a distributed lock using a script that verifies you are actually the current lock owner, rather than a simple DELETE command?

Intermediate

Answer

If you simply delete the lock key without checking first, you risk accidentally releasing a lock that was actually acquired by a different client after your own lock expired, so you should use a Lua script that checks the lock's value matches your unique identifier before deleting it, ensuring you only ever release a lock you actually still own.
if redis.call('get', KEYS[1]) == ARGV[1] then
  return redis.call('del', KEYS[1])
else
  return 0
end
Real-world example A payment processing service uses a Lua script to safely release its distributed lock, preventing a rare but serious bug where a slow process could have accidentally deleted a lock that had already been acquired by a different, newer process.

Common follow-ups: Why must this check and delete operation happen atomically in a single script?;What happens if the lock has already expired by the time you try to release it?

Lua Scripting;Distributed Locks with Redis
All 7 Distributed Locks with Redis questions