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How do you cancel a queued Redis transaction before it executes, and what command is used for this? How does the WATCH command enable optimistic locking within a Redis transaction? What does it mean for Redis transactions to lack full rollback support, and how does this differ from transactions in a traditional relational database? What is a practical real world example where a Redis transaction ensures data consistency across multiple related keys? How do Redis transactions interact with Redis Cluster when the queued commands involve keys located on different shards?
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When would you choose a Lua script over a MULTI and EXEC transaction to achieve atomicity in Redis?

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A Lua script is often a better choice when your atomic operation needs conditional logic based on values read partway through, since the entire script runs as a single atomic unit on the server with full access to intermediate results, whereas a plain transaction only queues commands blindly without letting you branch based on values retrieved earlier in the same transaction.
EVAL "local val = redis.call('GET', KEYS[1]) if val == ARGV[1] then return redis.call('DEL', KEYS[1]) else return 0 end" 1 mykey expected_value
Real-world example A distributed locking system uses a Lua script instead of a plain transaction to atomically check that a lock's value matches the expected owner before deleting it, something a simple MULTI and EXEC sequence cannot express.

Common follow-ups: What are the performance differences between Lua scripts and transactions for simple use cases?;Can a Lua script call WATCH internally?

Lua Scripting;Distributed Locks with Redis
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