SET with expiry (EX, PX) in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this Redis command sequence?

Consider the following Redis commands executed in order:

SET mykey "Hello" EX 10
TTL mykey

What will the TTL mykey command return immediately after setting the key?

Redis

SET mykey "Hello" EX 10
TTL mykey

A An integer close to 10 (e.g., 9 or 10)

B -1 (key exists but has no expiry)

C -2 (key does not exist)

D Syntax error

💡 Hint

EX sets the expiry time in seconds when setting the key.

The SET command with EX 10 sets the key to expire in 10 seconds. TTL returns the remaining time to live in seconds, so it will be close to 10 immediately after setting.

❓ query_result

intermediate

What happens when you use PX with SET in Redis?

You run the command:

SET session "abc123" PX 5000

What does the PX 5000 option do?

Redis

SET session "abc123" PX 5000

A Sets the key without expiry

B Sets the key to expire after 5000 seconds

C Sets the key to expire after 5000 milliseconds (5 seconds)

D Causes a syntax error

💡 Hint

PX sets expiry in milliseconds, EX sets expiry in seconds.

PX option sets the expiry time in milliseconds. PX 5000 means the key expires after 5 seconds.

📝 Syntax

advanced

Which SET command syntax is invalid in Redis?

Identify the invalid Redis SET command syntax among the options below:

A SET user "Alice" EX 20 PX 1000

B SET user "Alice" PX 1000

C SET user "Alice" EX 20

D SET user "Alice"

💡 Hint

You cannot use both EX and PX options together in one SET command.

Using both EX and PX options together in a single SET command is invalid and causes a syntax error.

❓ query_result

advanced

What is the result of this Redis command sequence?

Given the commands:

SET key1 "value1" EX 1
GET key1
WAIT 2000
GET key1

What will be the output of the second GET key1?

Redis

SET key1 "value1" EX 1
GET key1
WAIT 2000
GET key1

A "value1"

B nil

C Syntax error

D "" (empty string)

💡 Hint

EX 1 sets expiry to 1 second, WAIT 2000 waits 2 seconds.

The key expires after 1 second. After waiting 2 seconds, the key no longer exists, so GET returns nil.

🧠 Conceptual

expert

Why might you prefer PX over EX when setting expiry in Redis?

Choose the best reason why a developer might choose PX (milliseconds) instead of EX (seconds) when using the SET command with expiry.

A PX automatically renews the key expiry on access

B PX is faster to execute than EX

C PX stores expiry time as a timestamp instead of duration

D PX allows more precise expiry times for short-lived keys

💡 Hint

Think about the difference between seconds and milliseconds.

PX sets expiry in milliseconds, allowing finer control over expiry times, especially for very short durations.