PERSIST to remove expiry in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of the PERSIST command?

Given a Redis key session_token with an expiry set, what will be the output of the command PERSIST session_token if the key exists and has an expiry?

Redis

SET session_token abc123
EXPIRE session_token 300
PERSIST session_token

A 0

B Error: wrong number of arguments

C nil

D 1

💡 Hint

PERSIST removes the expiry from a key if it exists and has a timeout.

The PERSIST command returns 1 if the timeout was removed successfully, meaning the key existed and had an expiry. It returns 0 if the key does not exist or did not have an expiry.

❓ query_result

intermediate

What happens when PERSIST is called on a key without expiry?

If a Redis key user:100 exists but has no expiry set, what will PERSIST user:100 return?

Redis

SET user:100 "John Doe"
PERSIST user:100

A Error: key does not exist

B 1

C 0

D nil

💡 Hint

PERSIST only removes expiry if one exists.

Since the key exists but has no expiry, PERSIST returns 0 indicating no timeout was removed.

📝 Syntax

advanced

Which PERSIST command syntax is correct?

Identify the correct syntax to remove the expiry from a Redis key named cache_data.

A PERSIST cache_data

B PERSIST(cache_data)

C PERSIST 'cache_data'

D PERSIST cache_data EXPIRE

💡 Hint

Redis commands do not use parentheses or extra keywords for PERSIST.

The correct syntax is simply PERSIST key. Parentheses or extra keywords cause syntax errors.

🧠 Conceptual

advanced

Why use PERSIST instead of deleting and recreating a key?

Which of the following is the best reason to use PERSIST to remove expiry from a key instead of deleting and recreating it?

A Deleting and recreating is faster and safer.

B PERSIST preserves the key's value and metadata without interruption.

C PERSIST automatically backs up the key to disk.

D Deleting a key does not remove its expiry.

💡 Hint

Think about what happens to the key's data and connections.

PERSIST removes expiry without deleting the key, so the value and metadata remain intact and accessible without interruption.

🔧 Debug

expert

Why does PERSIST return 0 unexpectedly?

You run PERSIST mykey but it returns 0 even though you expect the key to have an expiry. Which is the most likely cause?

A The key <code>mykey</code> exists but has no expiry set.

B The Redis server is down.

C The key <code>mykey</code> does not exist in Redis.

D PERSIST command is deprecated and no longer works.

💡 Hint

PERSIST returns 0 if no expiry was removed.

If the key exists but has no expiry, PERSIST returns 0 indicating no timeout was removed. This is the most common cause of unexpected 0 returns.