BLPOP and BRPOP for blocking pop in Redis - Step-by-Step Execution

Concept Flow - BLPOP and BRPOP for blocking pop

Client sends BLPOP/BRPOP command

↓

Check if list has elements?

No→Block client, wait for element

|Yes↓

Pop element from list (left for BLPOP, right for BRPOP)

↓

Return popped element to client

↓

Client receives element, command ends

The client sends a blocking pop command. If the list is empty, the client waits until an element is available. When an element appears, it is popped and returned.

Execution Sample

Redis

BLPOP mylist 5
BRPOP mylist 5

These commands block up to 5 seconds waiting to pop an element from the left or right of 'mylist'.

Execution Table

Step	Command	List State	Condition	Action	Output
1	BLPOP mylist 5	mylist = []	List empty	Block client, wait for element	No output yet
2	Element pushed to mylist	mylist = ['a']	Element available	Unblock client	No output yet
3	BLPOP resumes	mylist = ['a']	List not empty	Pop left element 'a'	Return ['mylist', 'a']
4	BRPOP mylist 5	mylist = []	List empty	Block client, wait for element	No output yet
5	Element pushed to mylist	mylist = ['b']	Element available	Unblock client	No output yet
6	BRPOP resumes	mylist = ['b']	List not empty	Pop right element 'b'	Return ['mylist', 'b']
7	BLPOP mylist 2	mylist = []	List empty	Block client, wait 2 seconds	Timeout reached, return nil
8	BRPOP mylist 0	mylist = []	List empty	Block client, wait indefinitely	Wait until element appears

💡 Execution stops when element is popped and returned or timeout occurs.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 5	After Step 6	After Step 7
mylist	[]	['a']	[]	['b']	[]	[]
Client State	Waiting	Waiting	Received 'a'	Waiting	Received 'b'	Timeout or Waiting

Key Moments - 3 Insights

Why does the client block when the list is empty?

Because BLPOP and BRPOP are blocking commands, they wait for an element to appear instead of returning immediately. See execution_table rows 1 and 4 where the client blocks waiting.

What happens if the timeout expires with no elements?

The command returns nil to indicate no element was popped. This is shown in execution_table row 7 where the timeout of 2 seconds is reached.

How does BLPOP differ from BRPOP in popping elements?

BLPOP pops from the left (start) of the list, BRPOP pops from the right (end). See rows 3 and 6 where 'a' is popped from left and 'b' from right.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output of BLPOP at step 3?

A nil

B ['mylist', 'a']

C ['mylist', 'b']

D No output yet

💡 Hint

Check the Output column at step 3 in execution_table.

At which step does the client receive nil due to timeout?

A Step 5

B Step 8

C Step 7

D Step 2

💡 Hint

Look for 'Timeout reached, return nil' in the Output column.

If an element is pushed to the list while client is blocked, what happens next?

A Client unblocks and pops the element

B Client remains blocked indefinitely

C Client returns nil immediately

D List is cleared

💡 Hint

See steps 2 and 5 where element push causes client to unblock.

Concept Snapshot

BLPOP and BRPOP block client if list is empty.
They wait until an element is available or timeout expires.
BLPOP pops from left; BRPOP pops from right.
If timeout is 0, block indefinitely.
Returns [listname, element] or nil on timeout.

Full Transcript

BLPOP and BRPOP are Redis commands that remove and return an element from a list. They block the client if the list is empty, waiting for an element to appear or until a timeout expires. BLPOP removes from the left side of the list, BRPOP from the right. If the timeout expires without an element, they return nil. This visual trace shows the client sending commands, blocking when the list is empty, unblocking when elements are pushed, and receiving the popped elements or nil on timeout.

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