Leaderboard implementation in Redis - Mini Project: Build & Apply

Leaderboard Implementation with Redis

📖 Scenario: You are building a simple leaderboard for a game. Players earn points, and you want to keep track of their scores in order from highest to lowest.

🎯 Goal: Create a Redis sorted set to store player scores, add some players with their scores, update scores, and retrieve the top players.

📋 What You'll Learn

Create a Redis sorted set called game_leaderboard.

Add three players with exact scores: "Alice" with 1500, "Bob" with 1200, and "Charlie" with 1800.

Add a variable score_increment with value 300 to increase scores.

Increase Bob's score by score_increment.

Retrieve the top 3 players with their scores in descending order.

💡 Why This Matters

🌍 Real World

Leaderboards are common in games and apps to show top users or scores in real time.

💼 Career

Knowing how to use Redis sorted sets for leaderboards is useful for backend developers working on gaming, social, or analytics platforms.

1

Create the leaderboard sorted set with initial players

Use the Redis command ZADD to create a sorted set called game_leaderboard and add these players with their scores: 1500 "Alice", 1200 "Bob", and 1800 "Charlie".

Redis

# Use ZADD to add players and scores to game_leaderboard
# Your code here

Hint

Use ZADD followed by the sorted set name, then pairs of score and member.

2

Add a score increment variable

Create a Redis string key called score_increment and set its value to 300.

Redis

ZADD game_leaderboard 1500 Alice 1200 Bob 1800 Charlie
# Set score_increment to 300
# Your code here

Hint

Use SET to create a string key with the value 300.

3

Increase Bob's score by the increment

Use the Redis command ZINCRBY to increase Bob's score in game_leaderboard by the value stored in score_increment. Use GET score_increment to retrieve the increment value.

Redis

ZADD game_leaderboard 1500 Alice 1200 Bob 1800 Charlie
SET score_increment 300
# Increase Bob's score by score_increment
# Your code here

Hint

Use ZINCRBY with the sorted set name, increment value, and member name.

4

Retrieve the top 3 players with scores

Use the Redis command ZREVRANGE on game_leaderboard to get the top 3 players with their scores. Include the WITHSCORES option to see scores.

Redis

ZADD game_leaderboard 1500 Alice 1200 Bob 1800 Charlie
SET score_increment 300
ZINCRBY game_leaderboard 300 Bob
# Retrieve top 3 players with scores
# Your code here

Hint

ZREVRANGE returns members in descending order by score. Use 0 2 to get top 3.

Practice

1. What Redis data structure is best suited for implementing a leaderboard where members have scores?

easy

A. Hash

B. Sorted Set

C. List

D. String

Solution

1. Step 1: Understand leaderboard requirements

   A leaderboard needs to store members with scores and retrieve them sorted by score.

2. Step 2: Identify Redis data structure

   Redis Sorted Sets store members with scores and keep them sorted automatically.

3. Final Answer:

   Sorted Set -> Option B

4. Quick Check:

   Leaderboard = Sorted Set [OK]

Hint: Leaderboards need sorted data, use Sorted Sets [OK]

Common Mistakes:

• Choosing List which is ordered by insertion, not score
• Using Hash which stores key-value but not sorted by score
• Using String which holds plain text, not multiple members

2. Which Redis command correctly adds or updates a member's score in a leaderboard stored as a sorted set?

easy

A. ZADD leaderboard 100 user1

B. ZADD leaderboard user1 100

C. ZINCRBY leaderboard 100 user1

D. ZREM leaderboard user1

Solution

1. Step 1: Recall ZADD syntax

   ZADD command syntax is: ZADD key score member.

2. Step 2: Match syntax to options

   ZADD leaderboard 100 user1 uses ZADD leaderboard 100 user1 which matches the correct syntax.

3. Final Answer:

   ZADD leaderboard 100 user1 -> Option A

4. Quick Check:

   ZADD syntax = ZADD key score member [OK]

Hint: ZADD syntax: key, score, then member [OK]

Common Mistakes:

• Swapping score and member order
• Using ZINCRBY which increments score, not sets it
• Using ZREM which removes member instead of adding

3. Given the following Redis commands executed in order:

ZADD leaderboard 50 user1
ZADD leaderboard 100 user2
ZADD leaderboard 75 user3
ZREVRANGE leaderboard 0 1 WITHSCORES

What is the output of the last command?

medium

A. ["user2", "100", "user3", "75"]

B. ["user1", "50", "user3", "75"]

C. ["user3", "75", "user2", "100"]

D. ["user2", "100", "user1", "50"]

Solution

1. Step 1: Add members with scores

   user1=50, user2=100, user3=75 added to sorted set.

2. Step 2: Use ZREVRANGE 0 1 WITHSCORES

   This fetches top 2 members by highest score descending: user2(100), user3(75).

3. Final Answer:

   ["user2", "100", "user3", "75"] -> Option A

4. Quick Check:

   Top 2 by score desc = user2, user3 [OK]

Hint: ZREVRANGE 0 1 gets top 2 highest scores [OK]

Common Mistakes:

• Confusing ZREVRANGE with ZRANGE (ascending order)
• Ignoring WITHSCORES option output format
• Mixing member and score order in output

4. You run the following Redis commands:

ZADD leaderboard 30 user1
ZADD leaderboard 60 user2
ZADD leaderboard 45 user3
ZREVRANGE leaderboard 0 2

But the output is empty. What is the most likely cause?

medium

A. ZREVRANGE returns ascending order by default

B. ZREVRANGE syntax is incorrect, it needs WITHSCORES

C. ZADD commands used wrong order of arguments

D. The leaderboard key was deleted before ZREVRANGE

Solution

1. Step 1: Check command correctness

   ZADD commands have correct syntax and add members with scores.

2. Step 2: Analyze empty output from ZREVRANGE

   Empty output means the key 'leaderboard' likely does not exist or was deleted before ZREVRANGE.

3. Final Answer:

   The leaderboard key was deleted before ZREVRANGE -> Option D

4. Quick Check:

   Empty output = missing key [OK]

Hint: Empty ZREVRANGE means key missing or deleted [OK]

Common Mistakes:

• Thinking WITHSCORES is required for output (it's optional)
• Assuming ZREVRANGE returns ascending order
• Believing ZADD argument order was wrong (it was correct)

5. You want to implement a leaderboard that shows the top 3 users but excludes any user with a score below 50. Which Redis commands or approach should you use?

hard

A. Use ZRANGEBYSCORE leaderboard 50 +inf LIMIT 0 3

B. Use ZREVRANGE leaderboard 0 2 and filter scores < 50 in application

C. Use ZREVRANGEBYSCORE leaderboard +inf 50 LIMIT 0 3

D. Use ZRANGE leaderboard 0 2 WITHSCORES and exclude scores < 50

Solution

1. Step 1: Understand filtering by score

   We want top 3 users with scores >= 50, sorted descending.

2. Step 2: Choose correct Redis command

   ZREVRANGEBYSCORE leaderboard +inf 50 LIMIT 0 3 returns members with scores between +inf and 50 descending, limited to 3.

3. Final Answer:

   Use ZREVRANGEBYSCORE leaderboard +inf 50 LIMIT 0 3 -> Option C

4. Quick Check:

   Filter by score with ZREVRANGEBYSCORE [OK]

Hint: Use ZREVRANGEBYSCORE with score limits and LIMIT [OK]

Common Mistakes:

• Using ZREVRANGE without score filtering
• Using ZRANGEBYSCORE which sorts ascending, not descending
• Filtering scores in app instead of Redis command