TTL-based expiry in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output after setting a TTL?

You run these Redis commands:

SET session:123 user1
EXPIRE session:123 10
TTL session:123

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

Redis

SET session:123 user1
EXPIRE session:123 10
TTL session:123

A 10

B -1

C 0

D -2

💡 Hint

TTL returns the remaining time to live in seconds.

After setting EXPIRE with 10 seconds, TTL returns 10 immediately.

🧠 Conceptual

intermediate

What does a TTL of -2 mean?

In Redis, what does a TTL command returning -2 indicate about the key?

A The key does not exist

B The key exists but has no expiry

C The key has expired but still exists

D The key has a persistent expiry

💡 Hint

Think about what happens when a key is missing.

TTL returns -2 when the key does not exist in Redis.

📝 Syntax

advanced

Which command correctly sets a key with a 5-second expiry?

Choose the Redis command that sets key 'temp' with value 'data' and expires it after 5 seconds in one atomic operation.

A SET temp data EX 5

B SETEX temp 5 data

C EXPIRE temp 5 SET data

D SET temp data PX 5000

💡 Hint

SETEX sets value and expiry in one command.

SETEX key seconds value sets the key with expiry atomically.

❓ optimization

advanced

How to efficiently check if a key will expire soon?

You want to check if a Redis key 'cache:item' will expire within the next 3 seconds. Which approach is best?

A Use TTL cache:item and check if result is between 0 and 3

B Use GET cache:item and check if value is null

C Use EXISTS cache:item and check if it returns 1

D Use PTTL cache:item and check if result is between 0 and 3000

💡 Hint

PTTL returns milliseconds remaining.

PTTL returns time to live in milliseconds, so checking if it's between 0 and 3000 is precise.

🔧 Debug

expert

Why does a key persist despite EXPIRE command?

You run these commands:

SET mykey value
EXPIRE mykey 10
DEL mykey
TTL mykey

The TTL command returns -2. But after 5 seconds, you check and the key still exists. Why?

A DEL command failed silently and key was never deleted

B TTL returns -2 only if key exists without expiry

C The key was recreated by another client after deletion

D Redis does not support expiry on deleted keys

💡 Hint

Consider concurrent clients and key recreation.

If another client recreated the key after deletion, TTL returns -2 because the original key was deleted, but the new key exists without expiry.