EXEC to execute 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 in a transaction using MULTI and EXEC:

MULTI
SET key1 value1
INCR counter
GET key1
EXEC

Assuming counter was initially 5, what will EXEC return?

Redis

MULTI
SET key1 value1
INCR counter
GET key1
EXEC

A ["OK", 6, "value1"]

B ["OK", "6", "value1"]

C ["OK", 5, "value1"]

D ["OK", 6, null]

💡 Hint

Remember that INCR returns the new incremented value as an integer.

The SET command returns OK, INCR increments counter from 5 to 6 and returns 6, GET returns the string value1. EXEC returns an array of these results.

🧠 Conceptual

intermediate

What does EXEC do in a Redis transaction?

In Redis, after starting a transaction with MULTI and queuing commands, what is the role of the EXEC command?

A It executes all queued commands atomically and returns their results.

B It cancels the transaction and discards all queued commands.

C It queues another command to be executed later.

D It resets the database to its initial state.

💡 Hint

Think about what happens after queuing commands with MULTI.

EXEC runs all commands queued after MULTI in a single atomic operation and returns their results as an array.

📝 Syntax

advanced

Which option shows correct syntax to start and execute a Redis transaction?

Select the option that correctly starts a transaction, queues two commands, and executes them.

A

SET key1 val1
MULTI
INCR counter
EXEC

B

MULTI
SET key1 val1
EXEC
INCR counter

C

MULTI
SET key1 val1
INCR counter
EXEC

D

MULTI
EXEC
SET key1 val1
INCR counter

💡 Hint

Commands must be queued between MULTI and EXEC.

Only option C queues commands between MULTI and EXEC properly. Other options execute or queue commands out of order.

🔧 Debug

advanced

Why does EXEC return null in this Redis transaction?

Given the commands:

MULTI
SET key1 value1
WATCH key1
INCR counter
EXEC

Why might EXEC return null?

A Because INCR counter is invalid syntax inside MULTI.

B Because SET key1 value1 failed due to wrong arguments.

C Because EXEC was called before queuing any commands.

D Because WATCH was called after MULTI, causing the transaction to fail.

💡 Hint

Remember the order of WATCH and MULTI matters.

WATCH must be called before MULTI. Calling WATCH after MULTI causes the transaction to fail and EXEC returns null.

❓ optimization

expert

How to optimize multiple Redis commands for atomic execution?

You want to increment a counter and set a key's value atomically in Redis. Which approach is best for performance and atomicity?

A Use MULTI, queue INCR and SET commands, then EXEC.

B Use Lua scripting to perform both commands atomically.

C Send INCR and SET commands separately without MULTI/EXEC.

D Use WATCH on the key, then send INCR and SET commands.

💡 Hint

Lua scripts run atomically and can combine commands efficiently.

Lua scripting allows combining multiple commands into one atomic operation with better performance than MULTI/EXEC.