Redis Sorted Sets

9. Redis Sorted Sets

What is a Sorted Set?

A Sorted Set is like a Set, but every member has an associated numeric score. Members are stored in ascending order by score, and duplicate members are not allowed (but duplicate scores are fine).

Analogy: A competition scoreboard where each player has a unique name and a score. The board is always sorted from lowest to highest score.

Core Commands

# ZADD <key> <score> <member>
ZADD leaderboard 1500 "Alice"
ZADD leaderboard 2800 "Bob"
ZADD leaderboard 2100 "Charlie"
ZADD leaderboard 950  "Diana"

# Get members sorted ascending (lowest score first)
ZRANGE leaderboard 0 -1
# Output: 1) "Diana" 2) "Alice" 3) "Charlie" 4) "Bob"

# Get with scores
ZRANGE leaderboard 0 -1 WITHSCORES
# Output: "Diana" 950, "Alice" 1500, "Charlie" 2100, "Bob" 2800

# Get top 3 (highest scores — descending)
ZREVRANGE leaderboard 0 2 WITHSCORES
# Output: "Bob" 2800, "Charlie" 2100, "Alice" 1500

# Get rank of a member (0-indexed, ascending)
ZRANK leaderboard "Alice"
# Output: (integer) 1

# Get rank from highest (descending)
ZREVRANK leaderboard "Bob"
# Output: (integer) 0  (Bob is #1)

# Get the score of a specific member
ZSCORE leaderboard "Charlie"
# Output: "2100"

# Update a score
ZADD leaderboard 3200 "Alice"

# Remove a member
ZREM leaderboard "Diana"

# Count members in a score range
ZCOUNT leaderboard 1000 3000

Leaderboard Diagram

flowchart TB
    Players[Players submit scores] --> ZADD["ZADD leaderboard score member<br/>Score determines rank"]
    ZADD --> Sorted[Scores stored in ascending order]

    Sorted --> ZRANGE["ZRANGE 0 -1 WITHSCORES<br/>Ascending: lowest score first<br/>Diana 950 → Alice 1500 → Charlie 2100 → Bob 2800"]
    Sorted --> ZREVRANGE["ZREVRANGE 0 -1 WITHSCORES<br/>Descending: highest score first<br/>Bob 2800 → Charlie 2100 → Alice 1500 → Diana 950"]
    Sorted --> ZRANK["ZRANK 'Alice'<br/>Returns: 1 (0-indexed rank)"]
    Sorted --> ZCOUNT["ZCOUNT 1000 3000<br/>Count players with score in range"]

    style Players fill:#3b82f6,color:#fff
    style ZADD fill:#7c3aed,color:#fff
    style Sorted fill:#059669,color:#fff
    style ZRANGE fill:#f59e0b,color:#fff
    style ZREVRANGE fill:#f59e0b,color:#fff
    style ZRANK fill:#ec4899,color:#fff
    style ZCOUNT fill:#06b6d4,color:#fff

Real-time leaderboard: Sorted sets keep the order as scores change. Use ZADD to update a score, and the member automatically moves to its correct position. Perfect for gaming, trending topics, and real-time rankings.

Real-World Examples

Example: Gaming Leaderboard

# Players complete a round
ZADD game:scores 4500 "player:alice"
ZADD game:scores 7200 "player:bob"
ZADD game:scores 6100 "player:charlie"

# Show top 10
ZREVRANGE game:scores 0 9 WITHSCORES

# Player's rank
ZREVRANK game:scores "player:alice"

Example: Trending Topics (Time-based scoring)

# Use timestamp as score
ZADD trending 1700000000 "redis"
ZADD trending 1700000100 "nodejs"

# Get most recently added (latest trending)
ZREVRANGE trending 0 4