Leaderboard implementation in Redis

Introduction

A leaderboard shows who is winning by ranking players or items by their scores. It helps track top performers easily.

You want to show top players in a game.

You want to rank salespeople by their sales numbers.

You want to display the most popular articles by views.

You want to track the highest scores in a quiz app.

You want to keep a list of best customers by points earned.

Syntax

Redis

ZADD leaderboard_name score member

ZADD adds or updates a member with a score in a sorted set.

The sorted set automatically keeps members ordered by score.

Examples

Adds player1 with score 1500 to the leaderboard.

Redis

ZADD game_leaderboard 1500 player1

Adds player2 with score 2000, who will rank higher than player1.

Redis

ZADD game_leaderboard 2000 player2

Gets top 3 players with their scores, sorted from lowest to highest.

Redis

ZRANGE game_leaderboard 0 2 WITHSCORES

Gets top 3 players with their scores, sorted from highest to lowest.

Redis

ZREVRANGE game_leaderboard 0 2 WITHSCORES

Sample Program

This adds three players with scores and then shows the top 3 players from highest to lowest score.

Redis

ZADD leaderboard 100 Alice
ZADD leaderboard 150 Bob
ZADD leaderboard 120 Carol
ZREVRANGE leaderboard 0 2 WITHSCORES

Important Notes

Scores can be any number, including decimals.

Members must be unique strings; adding the same member updates its score.

Use ZREVRANGE to get highest scores first, which is common for leaderboards.

Summary

Leaderboards use Redis sorted sets to store members with scores.

ZADD adds or updates scores.

ZREVRANGE fetches top members sorted by highest score.

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