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.

Practice

1. What is the main difference between BLPOP and BRPOP commands in Redis?

easy

A. BLPOP removes items without blocking, BRPOP always blocks.

B. BLPOP blocks only if the list is empty, BRPOP never blocks.

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

D. BLPOP removes an item from the left of the list, BRPOP removes from the right.

Solution

1. Step 1: Understand the behavior of BLPOP and BRPOP

   Both commands block if the list is empty and wait for an item to appear.

2. Step 2: Identify the difference in removal side

   BLPOP removes from the left (start) of the list, while BRPOP removes from the right (end).

3. Final Answer:

   BLPOP removes an item from the left of the list, BRPOP removes from the right. -> Option D

4. Quick Check:

   Removal side difference = A [OK]

Hint: Remember L in BLPOP means left, R in BRPOP means right [OK]

Common Mistakes:

• Thinking BRPOP never blocks
• Confusing which side each command removes from
• Assuming BLPOP works only on strings

2. Which of the following is the correct syntax to block and pop an element from the right of list mylist with a timeout of 5 seconds?

easy

A. BRPOP mylist 5

B. BLPOP mylist 5

C. BRPOP 5 mylist

D. POPBR mylist 5

Solution

1. Step 1: Recall the syntax of BRPOP

   The correct syntax is BRPOP key [key ...] timeout, so the timeout is last.

2. Step 2: Match the command to the question

   To pop from the right of mylist with 5 seconds timeout, use BRPOP mylist 5.

3. Final Answer:

   BRPOP mylist 5 -> Option A

4. Quick Check:

   Correct syntax = D [OK]

Hint: Timeout is always the last argument in BLPOP/BRPOP [OK]

Common Mistakes:

• Putting timeout before the key
• Using BLPOP instead of BRPOP for right pop
• Using a non-existent command like POPBR

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

medium

A. nil

B. ["mylist", "c"]

C. ["mylist", "a"]

D. Error: timeout must be positive

Solution

1. Step 1: Understand BLPOP behavior with timeout 0

   Timeout 0 means block indefinitely until an element is available.

2. Step 2: Identify which element BLPOP removes

   BLPOP removes the first (leftmost) element, which is "a".

3. Final Answer:

   ["mylist", "a"] -> Option C

4. Quick Check:

   BLPOP left pop = A [OK]

Hint: BLPOP returns [key, popped_element] from left [OK]

Common Mistakes:

• Expecting the last element "c" instead of first
• Thinking timeout 0 causes error
• Expecting nil when list is not empty

4. You run the command BLPOP mylist 3 but it returns with nil. What is the most likely reason?

medium

A. The timeout value 3 is invalid and causes immediate return.

B. The list mylist does not exist or is empty.

C. BLPOP cannot be used with timeout less than 5 seconds.

D. The Redis server is down.

Solution

1. Step 1: Understand BLPOP behavior on empty or missing lists

   If the list is empty or missing, BLPOP waits up to the timeout for an element.

2. Step 2: Reason why it returns nil

   If the list is empty and no element arrives within 3 seconds, BLPOP returns nil.

3. Final Answer:

   The list mylist does not exist or is empty. -> Option B

4. Quick Check:

   Empty list causes nil return = B [OK]

Hint: Empty or missing list causes nil after timeout [OK]

Common Mistakes:

• Thinking timeout less than 5 is invalid
• Assuming server down causes nil instead of connection error
• Believing BLPOP always blocks indefinitely

5. You want to implement a task queue where workers wait for tasks to arrive. Which Redis command is best to use to efficiently wait for new tasks without polling?

hard

A. BRPOP with a timeout of 0 on the task list

B. LPOP in a loop with sleep delays

C. BLPOP with a timeout of 0 on the task list

D. GET on the task list key

Solution

1. Step 1: Understand blocking pop commands for waiting queues

   Both BLPOP and BRPOP block until an item is available, avoiding polling.

2. Step 2: Choose the best command for a queue

   Using BRPOP (pop from right) is common for queues where tasks are pushed left and workers pop right, preserving order.

3. Step 3: Avoid polling with LPOP and GET

   LPOP in a loop with sleep causes inefficient polling; GET is not for lists.

4. Final Answer:

   BRPOP with a timeout of 0 on the task list -> Option A

5. Quick Check:

   Blocking right pop for queue = C [OK]

Hint: Use BRPOP with zero timeout for efficient queue waiting [OK]

Common Mistakes:

• Using LPOP with polling instead of blocking pop
• Using GET which is for strings, not lists
• Choosing BLPOP which may not match push side