Priority queue pattern in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add an item with a priority to a Redis sorted set.

Redis

ZADD tasks [1] "task1"

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

Atasks

B"task1"

CZADD

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Putting the member before the score

Using the key name as the score

The ZADD command adds an item with a score (priority) to a sorted set. Here, 1 is the priority score.

2fill in blank

medium

Complete the code to get the highest priority item from the Redis sorted set.

Redis

ZRANGE tasks [1] [2] WITHSCORES

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

A1

B0

C-1

D-2

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 0 returns the lowest priority item

Using positive indexes to get the last element

ZRANGE with start and stop as -1 returns the last element, which has the highest score (priority) in ascending order.

3fill in blank

hard

Fix the error in the code to remove the highest priority item from the sorted set.

Redis

ZREM tasks [1]

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

A"task1"

BZRANGE tasks -1 -1

CZPOPMAX tasks

DZREM tasks -1

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to remove by index or command instead of member name

Using ZREM with a negative index

ZREM removes a member by name. You must specify the member's name, not a command or index.

4fill in blank

hard

Fill both blanks to atomically pop the highest priority item from the sorted set.

Redis

ZPOPMAX [1] [2]

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

Atasks

B1

C0

Dtask1

Attempts:

3 left

💡 Hint

Common Mistakes

Using member name instead of key as first argument

Using 0 as count which pops nothing

ZPOPMAX removes and returns the highest score members. The first argument is the key, the second is the count of items to pop.

5fill in blank

hard

Fill all three blanks to add a task with priority, then get and remove the highest priority task.

Redis

ZADD [1] [2] "task2"
result = ZPOPMAX [3] 1

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

Atasks

B5

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using different keys for add and pop

Using a string instead of a number for priority

First, ZADD adds "task2" with priority 5 to the sorted set named "tasks". Then, ZPOPMAX removes and returns the highest priority task from the same set.

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