BLPOP and BRPOP for blocking pop in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to pop the first element from the list 'tasks' using a blocking pop.

Redis

BLPOP tasks [1]

Drag options to blanks, or click blank then click option'

Atimeout

B1

C0

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using a non-numeric string as timeout causes an error.

Using a positive number causes the command to timeout after that many seconds.

The BLPOP command requires a timeout value in seconds. Using 0 means it will block indefinitely until an element is available.

2fill in blank

medium

Complete the code to pop the last element from the list 'queue' with a 10-second timeout.

Redis

BRPOP queue [1]

Drag options to blanks, or click blank then click option'

A0

B5

Ctimeout

D10

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 causes indefinite blocking, not a 10-second wait.

Using a string like 'timeout' causes an error.

The BRPOP command pops the last element and waits up to the specified timeout in seconds. Here, 10 means it waits 10 seconds before giving up.

3fill in blank

hard

Fix the error in the command to block pop from lists 'list1' and 'list2' with a 5-second timeout.

Redis

BLPOP [1] 5

Drag options to blanks, or click blank then click option'

Alist1 list2

Blist1,list2

Clist1;list2

Dlist1:list2

Attempts:

3 left

💡 Hint

Common Mistakes

Using commas or semicolons between keys causes syntax errors.

Using colons is invalid syntax.

The BLPOP command accepts multiple keys separated by spaces, not commas or other punctuation.

4fill in blank

hard

Fill both blanks to pop from 'mylist' with a 0-second timeout using BRPOP.

Redis

BRPOP [1] [2]

Drag options to blanks, or click blank then click option'

Amylist

B0

C5

Dtimeout

Attempts:

3 left

💡 Hint

Common Mistakes

Using a non-numeric timeout causes errors.

Using a positive timeout causes the command to stop waiting after that time.

The command BRPOP mylist 0 blocks indefinitely until an element is available in 'mylist'.

5fill in blank

hard

Fill all three blanks to pop from lists 'a', 'b' with a 3-second timeout using BLPOP.

Redis

BLPOP [1] [2] [3]

Drag options to blanks, or click blank then click option'

Aa

Bb

C3

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using commas between list names causes errors.

Using 0 as timeout blocks indefinitely, not 3 seconds.

The command BLPOP a b 3 blocks up to 3 seconds waiting for elements in lists 'a' or 'b'.

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