Priority queue pattern in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Retrieve highest priority item from a Redis sorted set

Given a Redis sorted set named tasks where scores represent priority (lower score means higher priority), which command retrieves the task with the highest priority?

Redis

ZADD tasks 10 "task1" 5 "task2" 20 "task3"

A ZRANGE tasks 0 0 WITHSCORES

B ZPOPMIN tasks 1

C ZREVRANGE tasks 0 0 WITHSCORES

D ZRANGE tasks -1 -1 WITHSCORES

💡 Hint

Remember that lower scores mean higher priority in Redis sorted sets.

ZRANGE with 0 0 returns the element with the lowest score (highest priority). ZPOPMIN removes the element, which is not asked here.

📝 Syntax

intermediate

Correct syntax to add an item with priority to Redis sorted set

Which of the following commands correctly adds an item job42 with priority score 15 to a Redis sorted set named job_queue?

A ZADD job_queue job42 15

B ZADD job_queue 15 job42

C ZADD job_queue (15) job42

D ZADD job_queue "job42" 15

💡 Hint

The correct order is: ZADD key score member

ZADD expects the score first, then the member. Option B follows this order correctly.

❓ optimization

advanced

Efficiently pop the highest priority item from a Redis sorted set

You want to remove and get the highest priority item (lowest score) from a Redis sorted set tasks. Which command is the most efficient and atomic way to do this?

A ZPOPMIN tasks 1

B ZRANGE tasks 0 0 WITHSCORES followed by ZREM tasks <member>

C ZREVRANGE tasks 0 0 WITHSCORES

D ZPOP tasks 1

💡 Hint

Look for a command that both returns and removes the element in one step.

ZPOPMIN atomically pops the element with the lowest score, making it efficient and safe for concurrent use.

🧠 Conceptual

advanced

Understanding priority queue behavior in Redis sorted sets

In a Redis sorted set used as a priority queue, what does the score represent and how does it affect the order of elements?

A Score represents priority; elements are ordered randomly regardless of score.

B Score represents priority; higher scores mean higher priority and appear first.

C Score is ignored; elements are ordered by insertion time.

D Score represents priority; lower scores mean higher priority and appear first.

💡 Hint

Think about how Redis sorted sets order elements by score.

Redis sorted sets order elements by score in ascending order, so lower scores come first, representing higher priority.

🔧 Debug

expert

Identify the error in this Redis command sequence for priority queue

You want to add a job with priority 10 and then pop the highest priority job from job_queue. Which command sequence will cause an error?

Redis

ZADD job_queue 10 job100
ZPOPMIN job_queue 1

A ZADD job_queue 10 job100\nZPOPMIN job_queue 1

B ZADD job_queue 10 job100\nZRANGE job_queue 0 0

C ZADD job_queue job100 10\nZPOPMIN job_queue 1

D ZADD job_queue 10 job100\nZREM job_queue job100

💡 Hint

Check the order of arguments in ZADD command.

Option C uses incorrect argument order in ZADD (member before score), causing a syntax error.

Practice

1. What does a priority queue in Redis typically use to manage item importance?

easy

A. Lists with push and pop operations

B. Hashes with key-value pairs

C. Sorted sets with scores representing priority

D. Strings storing serialized data

Solution

1. Step 1: Understand Redis data structures

   Redis offers several data types; sorted sets allow ordering by score.

2. Step 2: Identify priority queue implementation

   Priority queues use sorted sets where scores represent priority levels.

3. Final Answer:

   Sorted sets with scores representing priority -> Option C

4. Quick Check:

   Priority queue = sorted sets with scores [OK]

Hint: Remember: sorted sets sort by score for priority [OK]

Common Mistakes:

• Confusing lists with priority queues
• Using hashes which don't order by score
• Thinking strings can manage priority

2. Which Redis command adds an item with a priority score to a priority queue?

easy

A. HSET

B. ZADD

C. LPUSH

D. SET

Solution

1. Step 1: Identify command to add to sorted sets

   ZADD adds members with scores to sorted sets in Redis.

2. Step 2: Match command to priority queue addition

   Priority queues use sorted sets, so ZADD is the correct command.

3. Final Answer:

   ZADD -> Option B

4. Quick Check:

   Add to priority queue = ZADD [OK]

Hint: Use ZADD to add with score in sorted sets [OK]

Common Mistakes:

• Using LPUSH which is for lists
• Using HSET which is for hashes
• Using SET which is for strings

3. Given the commands:

ZADD tasks 1 "task1" 3 "task3" 2 "task2"
ZPOPMIN tasks

What will be the output of ZPOPMIN tasks?

medium

A. ["task1", "1"]

B. ["task3", "3"]

C. ["task2", "2"]

D. Syntax error

Solution

1. Step 1: Understand ZADD insertion

   Three tasks added with scores 1, 3, and 2 respectively.

2. Step 2: Understand ZPOPMIN behavior

   ZPOPMIN removes and returns the member with the lowest score, which is "task1" with score 1.

3. Final Answer:

   ["task1", "1"] -> Option A

4. Quick Check:

   Lowest score item popped = task1 [OK]

Hint: ZPOPMIN returns lowest score member first [OK]

Common Mistakes:

• Picking highest score instead of lowest
• Confusing ZPOPMIN with ZPOPMAX
• Expecting syntax error from valid commands

4. What is wrong with this Redis command sequence for a priority queue?

ZADD queue "task1" 1
ZPOPMIN queue

medium

A. ZADD requires at least two members

B. ZPOPMIN does not remove items

C. Queue key must be a list, not a sorted set

D. ZADD syntax is incorrect; score must come before member

Solution

1. Step 1: Check ZADD command syntax

   ZADD syntax requires the score before the member, but here member is before score.

2. Step 2: Verify ZPOPMIN behavior

   ZPOPMIN removes the lowest score member correctly; no issue here.

3. Final Answer:

   ZADD syntax is incorrect; score must come before member -> Option D

4. Quick Check:

   ZADD syntax = score before member [OK]

Hint: Score always before member in ZADD [OK]

Common Mistakes:

• Swapping score and member in ZADD
• Thinking ZPOPMIN doesn't remove items
• Using wrong data type for priority queue

5. You want to implement a priority queue in Redis where lower scores mean higher priority. You add tasks:

ZADD jobs 5 "job5" 1 "job1" 3 "job3"

Then you run:

ZPOPMIN jobs 2

What will be the result and what remains in the queue?

hard

A. Returns ["job1", "1", "job3", "3"]; queue left with ["job5"]

B. Returns ["job5", "3", "job3", "1"]; queue left with ["job1"]

C. Returns ["job1", "5", "job3", "3"]; queue left with ["job5"]

D. Returns error because ZPOPMIN cannot pop multiple items

Solution

1. Step 1: Understand ZPOPMIN with count

   ZPOPMIN with argument 2 pops two lowest score members.

2. Step 2: Identify lowest scores and remaining items

   Lowest scores are 1 (job1) and 3 (job3), so these are popped; job5 with score 5 remains.

3. Final Answer:

   Returns ["job1", "1", "job3", "3"]; queue left with ["job5"] -> Option A

4. Quick Check:

   ZPOPMIN 2 pops two lowest score items [OK]

Hint: ZPOPMIN count pops that many lowest score items [OK]

Common Mistakes:

• Mixing up scores and members order
• Thinking ZPOPMIN pops highest scores
• Assuming ZPOPMIN can't pop multiple items