Multi-key transactions for consistency in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this Redis transaction?

Consider the following Redis commands executed as a transaction:

WATCH balance:alice balance:bob
MULTI
DECRBY balance:alice 50
INCRBY balance:bob 50
EXEC

Assuming initial balances are balance:alice = 100 and balance:bob = 200, what will be the balances after EXEC if no other client modifies these keys?

Redis

WATCH balance:alice balance:bob
MULTI
DECRBY balance:alice 50
INCRBY balance:bob 50
EXEC

A balance:alice = 100, balance:bob = 200

B balance:alice = 50, balance:bob = 200

C balance:alice = 50, balance:bob = 250

D balance:alice = 100, balance:bob = 250

💡 Hint

Think about what MULTI and EXEC do together with WATCH.

The WATCH command monitors keys for changes. Since no other client modifies the keys, EXEC runs the queued commands atomically, decreasing Alice's balance by 50 and increasing Bob's by 50.

🧠 Conceptual

intermediate

Why use WATCH in Redis transactions?

What is the main purpose of the WATCH command in Redis transactions?

A To monitor keys for changes and abort the transaction if they change

B To automatically retry the transaction if it fails

C To queue commands for atomic execution

D To lock keys so no other client can read them during the transaction

💡 Hint

Think about how Redis detects conflicts in transactions.

WATCH monitors keys and if any change before EXEC, the transaction aborts to avoid inconsistent updates.

📝 Syntax

advanced

Identify the syntax error in this Redis transaction script

Which option contains a syntax error that will cause the Redis transaction to fail?

Redis

WATCH key1 key2
MULTI
INCR key1
DECR key2
EXEC

A

WATCH key1 key2
MULTI
INCR key1
DECR key2
EXEC
DISCARD

B

WATCH key1 key2
MULTI
INCR key1
DECR key2
EXEC extra_arg

C

WATCH key1 key2
MULTI
INCR key1
DECR key2
EXEC

D

CEXE
2yek RCED
1yek RCNI
ITLUM
2yek 1yek HCTAW

💡 Hint

Check the EXEC command syntax carefully.

EXEC does not accept any arguments. Adding extra arguments causes a syntax error.

❓ optimization

advanced

How to optimize multi-key updates for consistency in Redis?

You want to atomically update multiple keys in Redis and ensure no other client modifies them during your update. Which approach is best?

A Update keys one by one without transactions

B Use MULTI/EXEC without WATCH and hope no conflicts occur

C Use Lua scripting to perform all updates atomically without WATCH

D Use WATCH on all keys, then MULTI/EXEC to queue updates, retry if EXEC returns null

💡 Hint

Consider how Redis handles optimistic locking and atomicity.

WATCH monitors keys for changes, MULTI/EXEC queues commands atomically, and retrying on EXEC null ensures consistency.

🔧 Debug

expert

Why does this Redis transaction fail to update balances consistently?

Given this Redis transaction:

WATCH balance:alice balance:bob
MULTI
DECRBY balance:alice 50
INCRBY balance:bob 50
EXEC

Sometimes, the balances are not updated as expected. What is the most likely cause?

A Another client modified balance:alice or balance:bob after WATCH but before EXEC, causing EXEC to return null

B WATCH locks the keys preventing any updates, so EXEC fails

C The DECRBY and INCRBY commands are not atomic inside MULTI/EXEC

D Redis does not support multi-key transactions

💡 Hint

Think about what happens if keys change after WATCH.

If keys change after WATCH, EXEC returns null and the transaction does not apply updates, causing inconsistent balances.