BLPOP and BRPOP for blocking pop in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this BLPOP command?

Given the Redis list mylist contains ["a", "b", "c"], what will be the result of BLPOP mylist 0?

Redis

BLPOP mylist 0

A ["mylist", "a"]

B ["mylist", "c"]

C nil

D ["mylist", "b"]

💡 Hint

BLPOP removes and returns the first element of the list.

BLPOP removes the first element from the left of the list and returns it with the key name.

❓ query_result

intermediate

What happens when BRPOP is used on an empty list with timeout 2?

If the Redis list tasks is empty, what will BRPOP tasks 2 return after 2 seconds?

Redis

BRPOP tasks 2

A nil

B ["tasks", ""]

C ["tasks", null]

D error

💡 Hint

BRPOP waits for an element or times out.

If the list is empty and no element is pushed within 2 seconds, BRPOP returns nil.

📝 Syntax

advanced

Which BLPOP command syntax is correct to pop from multiple lists?

You want to pop the first element from either list1 or list2 with a 5-second timeout. Which command is correct?

A BLPOP 5 list1 list2

B BLPOP list1 list2 5

C BLPOP list1 5 list2

D BLPOP list1, list2 5

💡 Hint

Timeout is the last argument after all keys.

The correct syntax lists all keys first, then the timeout at the end.

❓ optimization

advanced

How to efficiently wait for elements from multiple lists?

You want to wait for elements from queue1, queue2, and queue3 and pop the first available element. Which command is the most efficient?

A BRPOP queue1 queue2 queue3 0

B BLPOP queue1 0; BLPOP queue2 0; BLPOP queue3 0

C BLPOP queue1 queue2 queue3 0

D LPOP queue1; LPOP queue2; LPOP queue3

💡 Hint

Use a single blocking command with multiple keys.

BLPOP with multiple keys and zero timeout waits efficiently for any list to have an element.

🧠 Conceptual

expert

What is the difference between BLPOP and BRPOP in Redis?

Choose the option that best describes the difference between BLPOP and BRPOP commands.

A BLPOP returns elements without blocking; BRPOP always blocks.

B BLPOP blocks only on empty lists; BRPOP blocks only on non-empty lists.

C BLPOP works only on strings; BRPOP works only on lists.

D BLPOP removes and returns the first element from the left; BRPOP removes and returns the last element from the right.

💡 Hint

Think about which end of the list each command pops from.

BLPOP pops from the left (start) of the list; BRPOP pops from the right (end). Both can block if the list is empty.