WATCH for optimistic locking in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the WATCH command do in Redis?

The WATCH command monitors one or more keys for changes. If any watched key changes before the transaction executes, the transaction is aborted to avoid conflicts.

intermediate

Explain optimistic locking in Redis using WATCH.

Optimistic locking assumes no conflicts will happen. Redis uses WATCH to monitor keys. If keys change before EXEC, the transaction fails, so you can retry safely.

beginner

What happens if a watched key is modified before EXEC in Redis?

The transaction is aborted and EXEC returns null. This prevents conflicting updates and ensures data consistency.

intermediate

How do you retry a transaction after a WATCH aborts in Redis?

You can retry by re-watching the keys and re-executing the transaction commands in a loop until EXEC succeeds.

advanced

Why is WATCH considered optimistic locking instead of pessimistic locking?

Because it does not lock keys upfront. It only checks for changes at commit time, assuming conflicts are rare and handling them if they occur.

What does Redis do if a key watched by WATCH changes before EXEC?

A Automatically retries the transaction

B Locks the key until transaction finishes

C Ignores the change and commits anyway

D Aborts the transaction and returns null

If a watched key changes, Redis aborts the transaction to avoid conflicts and returns null.

Which Redis command is used to start monitoring keys for optimistic locking?

A EXEC

B WATCH

C MULTI

D DISCARD

WATCH is used to monitor keys for changes before running a transaction.

What is the purpose of optimistic locking in Redis?

A To detect conflicts only at commit time

B To lock keys before any changes

C To prevent any concurrent access

D To automatically merge conflicting changes

Optimistic locking assumes no conflicts and checks for them only when committing.

Which command executes all commands in a Redis transaction after WATCH?

A EXEC

B MULTI

C WATCH

D DISCARD

EXEC runs all queued commands in the transaction if no watched keys changed.

If a Redis transaction fails due to WATCH, what should you do next?

A Use <code>DISCARD</code> to force commit

B Ignore and continue without retry

C Retry the transaction after re-watching keys

D Delete the keys and try again

You should retry by watching keys again and re-executing the transaction.

Describe how Redis uses the WATCH command to implement optimistic locking.

Think about how Redis checks for conflicts only at commit time.

Explain the difference between optimistic locking with WATCH and pessimistic locking.

Focus on when locking happens and how conflicts are handled.