BLPOP and BRPOP for blocking pop in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the BLPOP command do in Redis?

BLPOP removes and returns the first element of the first non-empty list from the given lists. If all lists are empty, it blocks the connection until an element is available or the timeout is reached.

beginner

How does BRPOP differ from BLPOP in Redis?

BRPOP removes and returns the last element of the first non-empty list from the given lists, blocking if all lists are empty until an element is available or timeout occurs. BLPOP removes from the start (left), BRPOP from the end (right).

intermediate

What happens if the timeout parameter in BLPOP or BRPOP is set to 0?

If timeout is 0, BLPOP or BRPOP will block indefinitely until an element becomes available in one of the lists.

beginner

What is the return format of BLPOP or BRPOP when an element is popped?

The return is a two-element array: the first element is the key (list name) where the element was popped from, and the second element is the popped value.

intermediate

Why would you use BLPOP or BRPOP instead of LPOP or RPOP?

BLPOP and BRPOP block and wait for elements if lists are empty, making them useful for queue-like behavior where you want to wait for new data instead of polling repeatedly.

What does the 'B' in BLPOP and BRPOP stand for?

A Batch

B Blocking

C Binary

D Buffered

The 'B' stands for Blocking, meaning the command waits if no elements are available.

If you want to remove the last element from a list and wait if empty, which command do you use?

A BRPOP

B BLPOP

C LPOP

D RPOP

BRPOP removes the last element and blocks if the list is empty.

What does BLPOP return if it successfully pops an element?

A Nothing

B The list length after pop

C Just the popped element

D An array with the list name and popped element

BLPOP returns an array: [list name, popped element].

What happens if all lists are empty and timeout is set to 5 seconds in BLPOP?

A Returns immediately with null

B Throws an error

C Blocks for 5 seconds or until an element is available

D Returns the first list name

BLPOP blocks up to the timeout waiting for an element.

Which command would you use to implement a queue consumer that waits for new items?

A BLPOP

B LPOP

C RPOP

D LRANGE

BLPOP blocks and waits for new items, ideal for queue consumers.

Explain how BLPOP works and when you would use it.

Think about waiting for data in a list like waiting in line.

Describe the difference between BLPOP and BRPOP.

Focus on which end of the list each command pops from.

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