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.