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How do you implement a basic distributed lock in Redis using the SET command with NX and EX options? 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? 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? 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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What is the Redlock algorithm, and why was it proposed for implementing distributed locks across multiple independent Redis instances?

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Answer

The Redlock algorithm acquires a lock across a majority of several independent Redis instances rather than relying on just one, aiming to provide stronger guarantees against a single Redis instance failure incorrectly causing a lock to be lost or duplicated, though it has also been the subject of debate among distributed systems experts regarding whether it fully guarantees correctness in every failure scenario.
-- Acquire the same lock key across five independent
-- Redis instances, requiring a majority to succeed
-- before considering the lock successfully acquired
Real-world example A financial trading system requiring extremely strong locking guarantees implements the Redlock algorithm across five independent Redis instances, accepting the added complexity in exchange for greater resilience against a single instance failure.

Common follow-ups: What are the criticisms that have been raised against the Redlock algorithm's correctness guarantees?;Is Redlock necessary for most typical applications, or is a simpler single instance lock usually sufficient?

Distributed Locks with Redis;Redis Sentinel & High Availability
All 7 Distributed Locks with Redis questions