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.