Temporary data with TTL in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this Redis command sequence?

Given the commands below, what will be the output of the final GET command?

Redis

SET session:12345 user1
EXPIRE session:12345 5
GET session:12345
# Wait 6 seconds
GET session:12345

A "user1" then "user1"

B nil then "user1"

C "user1" then nil

D nil then nil

💡 Hint

Think about what EXPIRE does and how TTL affects key availability.

The key session:12345 is set with value user1 and an expiration of 5 seconds. The first GET returns the value. After waiting 6 seconds, the key expires and the second GET returns nil.

🧠 Conceptual

intermediate

Which Redis command sets a key with a TTL in one atomic operation?

You want to store a temporary key that automatically expires after 10 seconds. Which command achieves this in one step?

A PSETEX key 10000 value

B

SET key value
EXPIRE key 10

C SETEX key 10 value

D SET key value EX 10

💡 Hint

Look for the command that combines setting and expiration in one call.

The SET command supports the EX option to set expiration in seconds atomically. SETEX also works but the question asks for one atomic command with TTL as an option.

📝 Syntax

advanced

Identify the syntax error in this Redis command for setting TTL

Which option contains a syntax error when trying to set a key with a TTL of 30 seconds?

Redis

SET mykey myvalue EXPIRE 30

A SETEX mykey 30 myvalue

B SET mykey myvalue EXPIRE 30

C SET mykey myvalue PX 30000

D SET mykey myvalue EX 30

💡 Hint

Check the correct option keyword for expiration in the SET command.

The SET command uses EX or PX to set expiration, not EXPIRE. Option B incorrectly uses EXPIRE as an argument.

❓ optimization

advanced

Best practice to update TTL without changing key value

You want to extend the TTL of an existing key without modifying its value. Which command is the best choice?

A EXPIRE key 60

B SET key value EX 60

C SETEX key 60 value

D PERSIST key

💡 Hint

Consider commands that only affect TTL without overwriting data.

EXPIRE updates the TTL of a key without changing its value. SET or SETEX overwrite the value. PERSIST removes TTL.

🔧 Debug

expert

Why does this Lua script fail to set TTL on a key in Redis?

Consider this Lua script run via EVAL: local key = KEYS[1] redis.call('EXPIRE', key, 10) redis.call('SET', key, 'temp') return redis.call('TTL', key) Why might the TTL returned be -1 (no expiration)?

A The key is set without expiration because SET overwrites TTL

B The EXPIRE command is ignored inside Lua scripts

C The script runs too fast for TTL to apply

D The key is overwritten after EXPIRE, removing TTL

💡 Hint

Think about the order of commands and how SET affects TTL.

The SET command without expiration removes any existing TTL. EXPIRE is called before SET, but then SET removes the TTL. Here, the script calls EXPIRE first, then SET, so TTL is lost. The question tests understanding that SET without expiration removes TTL.