Key expiry for memory management 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 session:12345 user1
EXPIRE session:12345 10
TTL session:12345

What will the TTL command return immediately after setting the expiry?

Redis

SET session:12345 user1
EXPIRE session:12345 10
TTL session:12345

A 0

B -1

C -2

D 10

💡 Hint

TTL returns the remaining time to live in seconds for a key with expiry.

The EXPIRE command sets the key to expire in 10 seconds. TTL immediately after returns the remaining seconds, which is 10.

🧠 Conceptual

intermediate

Which Redis key expiry behavior is correct?

In Redis, what happens when a key with an expiry time reaches zero?

A The key remains until a client tries to access it, then it is deleted.

B The key is deleted only during periodic eviction cycles.

C The key is immediately deleted from memory.

D The key's expiry time resets automatically to prevent deletion.

💡 Hint

Redis uses lazy expiration to save CPU resources.

Redis deletes expired keys lazily when accessed or during periodic checks, not immediately at expiry time.

📝 Syntax

advanced

Which command correctly sets a key to expire in 5 minutes?

Choose the correct Redis command to set the key cache:item to expire in 5 minutes.

A SETEX cache:item 5 value

B EXPIRE cache:item 300

C SETEX cache:item 300 value

D EXPIRE cache:item 5m

💡 Hint

SETEX sets a key with a value and expiry in seconds.

SETEX requires the expiry time in seconds and the value. 5 minutes = 300 seconds.

❓ optimization

advanced

How to efficiently manage memory with many expiring keys?

You have millions of keys with expiry in Redis. Which approach optimizes memory and CPU usage best?

A Set expiry and use Redis active expiration cycle to remove expired keys regularly.

B Set expiry on all keys and rely on Redis lazy expiration only.

C Manually delete keys with a background script instead of using expiry.

D Never set expiry and let keys accumulate to avoid CPU overhead.

💡 Hint

Redis combines lazy and active expiration for efficiency.

Redis uses active expiration cycles to proactively remove expired keys, reducing memory and CPU spikes.

🔧 Debug

expert

Why does this Redis key never expire?

A developer runs these commands:

SET temp:data 100
EXPIRE temp:data 60
TTL temp:data

But the TTL command returns -1. What is the most likely cause?

A Redis does not support expiry on string keys.

B The key was overwritten by another SET command without expiry after EXPIRE was set.

C The EXPIRE command syntax is incorrect and was ignored.

D The TTL command was run on a different key by mistake.

💡 Hint

Setting a key again removes its expiry unless set with SETEX or EXPIRE after.

If a key is overwritten by SET after EXPIRE, the expiry is removed, so TTL returns -1.