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What is a Redis transaction, and how do the MULTI and EXEC commands work together to create one? How do you cancel a queued Redis transaction before it executes, and what command is used for this? 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? When would you choose a Lua script over a MULTI and EXEC transaction to achieve atomicity in Redis? How do Redis transactions interact with Redis Cluster when the queued commands involve keys located on different shards?
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Redis › Transactions

How does the WATCH command enable optimistic locking within a Redis transaction?

Intermediate

Answer

WATCH lets you monitor one or more keys for changes made by other clients before your transaction executes, and if any watched key is modified between calling WATCH and calling EXEC, the entire transaction is automatically aborted, letting your application detect the conflict and retry the operation safely rather than working with stale data.
WATCH account:1:balance
val = GET account:1:balance
MULTI
SET account:1:balance <new_value>
EXEC
Real-world example An inventory management system watches a product's stock count before decrementing it, so if another customer's purchase changes the stock count first, the transaction safely aborts and the application retries with the updated value.

Common follow-ups: What does EXEC return if a watched key was modified before it ran?;How many keys can be watched at the same time?

Distributed Locks with Redis;Rate Limiting with Redis
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