Transaction execution model 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:

MULTI
SET key1 value1
INCR counter
EXEC

Assuming counter was not set before, what will be the result of the EXEC command?

Redis

MULTI
SET key1 value1
INCR counter
EXEC

A ["OK", 0]

B ["OK", "1"]

C ["OK", "0"]

D ["OK", 1]

💡 Hint

Remember that INCR initializes the key to 0 before incrementing if it does not exist.

The SET command returns OK. The INCR command increments counter from 0 to 1 and returns the new value as an integer 1. So the EXEC returns an array with ["OK", 1].

🧠 Conceptual

intermediate

Which statement best describes Redis transactions?

Which of the following statements about Redis transactions is true?

A Redis transactions guarantee atomicity and durability but not isolation.

B Redis transactions guarantee atomicity, isolation, and durability.

C Redis transactions guarantee atomicity and isolation but not durability.

D Redis transactions guarantee isolation and durability but not atomicity.

💡 Hint

Think about what Redis transactions do and do not guarantee.

Redis transactions guarantee atomicity (all commands run as one unit) and isolation (no other commands run between them). However, Redis does not guarantee durability because data may be lost if the server crashes before persistence.

📝 Syntax

advanced

Identify the syntax error in this Redis transaction block

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

MULTI
SET key1 value1
INCR
EXEC

A

MULTI
SET key1 value1
INCR
EXEC

B

MULTI
SET key1 value1
INCR counter
EXEC

C

MULTI
SET key1 value1
INCR counter
DISCARD

D

CEXE
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ITLUM

💡 Hint

Check the command arguments carefully.

The INCR command requires a key argument. Option A misses the key argument, causing a syntax error.

🔧 Debug

advanced

Why does this Redis transaction not execute any commands?

Given the commands:

WATCH key1
MULTI
SET key1 value1
EXEC

Assuming another client modifies key1 after WATCH but before EXEC, what will be the result of EXEC?

A EXEC returns null and no commands are executed.

B EXEC executes all commands and returns their results.

C EXEC returns an error because the watched key changed.

D EXEC retries the transaction automatically.

💡 Hint

Think about what happens when a watched key is modified before EXEC.

If a watched key changes before EXEC, the transaction is aborted and EXEC returns null, meaning no commands run.

❓ optimization

expert

How to optimize Redis transactions to avoid race conditions?

You want to increment a counter only if a key exists, using a Redis transaction. Which approach best avoids race conditions?

A Use WATCH on the key, then MULTI, check existence with GET, then INCR, finally EXEC.

B Use Lua scripting to check and increment atomically without WATCH or MULTI.

C Use MULTI, then GET, then INCR, then EXEC without WATCH.

D Use MULTI, then INCR, then EXEC, ignoring key existence.

💡 Hint

Think about atomic operations and avoiding race conditions.

Lua scripting runs atomically on Redis server, so checking and incrementing in one script avoids race conditions better than WATCH/MULTI.