BLPOP and BRPOP for blocking pop in Redis

Introduction

BLPOP and BRPOP let you wait for items to appear in a list and remove them immediately. This helps when you want to get data as soon as it is available without checking repeatedly.

You have a task queue and want workers to wait for new tasks without wasting CPU.

You want to process messages as soon as they arrive in a list.

You need to coordinate multiple clients where one waits for data from another.

You want to avoid polling a list repeatedly to check for new items.

You want to build a simple chat system where clients wait for new messages.

Syntax

Redis

BLPOP key [key ...] timeout
BRPOP key [key ...] timeout

BLPOP removes and returns the first element (left) from the first non-empty list among the given keys.

BRPOP removes and returns the last element (right) from the first non-empty list among the given keys.

timeout is the number of seconds to wait if all lists are empty. Use 0 to wait forever.

Examples

Waits up to 5 seconds for an item from 'mylist' and removes it from the left.

Redis

BLPOP mylist 5

Waits forever for an item from 'queue1' or 'queue2' and removes it from the right of the first non-empty list.

Redis

BRPOP queue1 queue2 0

Waits forever for an item from 'tasks' list and removes it from the left.

Redis

BLPOP tasks 0

Sample Program

First, we add 'apple' and 'banana' to 'mylist'. Then BLPOP removes and returns 'apple' from the left immediately.

Redis

RPUSH mylist "apple"
RPUSH mylist "banana"
BLPOP mylist 1

Important Notes

If all lists are empty and timeout expires, BLPOP/BRPOP return nil.

You can pass multiple keys to wait on several lists at once.

BLPOP and BRPOP block the client connection until an element is available or timeout happens.

Summary

BLPOP and BRPOP wait for and remove items from lists, blocking if empty.

BLPOP removes from the left; BRPOP removes from the right.

They help build efficient waiting queues without polling.

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