Redis Sets

8. Redis Sets

What is a Redis Set?

A Redis Set is an unordered collection of unique strings. Duplicates are automatically ignored.

Analogy: A bag of tickets where each ticket has a unique number. You can add tickets, check if a ticket exists, or remove a ticket. No duplicates allowed.

Set Operations at a Glance

flowchart TB
    subgraph TeamA[Team A: Alice, Bob, Charlie]
        TA["{Alice, Bob, Charlie}"]
    end
    subgraph TeamB[Team B: Bob, Diana, Eve]
        TB["{Bob, Diana, Eve}"]
    end

    TeamA --> Intersection
    TeamB --> Intersection
    Intersection["SINTER = {Bob} 🟣<br/>In BOTH sets"]

    TeamA --> Union
    TeamB --> Union
    Union["SUNION = {Alice, Bob, Charlie, Diana, Eve} 🟢<br/>All members, no duplicates"]

    TeamA --> Diff["SDIFF = {Alice, Charlie} 🔵<br/>In A but NOT in B"]

    style TeamA fill:#3b82f6,color:#fff
    style TeamB fill:#7c3aed,color:#fff
    style Intersection fill:#ec4899,color:#fff
    style Union fill:#059669,color:#fff
    style Diff fill:#f59e0b,color:#fff

Core Commands

# Add members to a set
SADD tags:post:101 "redis" "database" "caching" "backend"

# Try adding a duplicate (ignored silently)
SADD tags:post:101 "redis"
# Output: (integer) 0  (nothing was added)

# Get all members (order not guaranteed)
SMEMBERS tags:post:101
# Output: {"redis", "database", "caching", "backend"}

# Check if a member exists
SISMEMBER tags:post:101 "redis"
# Output: (integer) 1 (yes)

SISMEMBER tags:post:101 "python"
# Output: (integer) 0 (no)

# Count members
SCARD tags:post:101
# Output: (integer) 4

# Remove a member
SREM tags:post:101 "caching"

# Pop a random member (removes it)
SPOP tags:post:101

Set Operations

Redis sets support powerful mathematical operations:

SADD devs:teamA "Alice" "Bob" "Charlie"
SADD devs:teamB "Bob" "Diana" "Eve"

# INTERSECTION — members in BOTH sets
SINTER devs:teamA devs:teamB
# Output: {"Bob"}

# UNION — all members from both sets (no duplicates)
SUNION devs:teamA devs:teamB
# Output: {"Alice", "Bob", "Charlie", "Diana", "Eve"}

# DIFFERENCE — in A but NOT in B
SDIFF devs:teamA devs:teamB
# Output: {"Alice", "Charlie"}

Real-World Examples

Example 1: Unique Visitors Per Day

# Track unique visitors by user ID
SADD visitors:2024-01-15 "user:101" "user:202" "user:303"
SADD visitors:2024-01-15 "user:101"   # Duplicate, ignored

# Count unique visitors
SCARD visitors:2024-01-15
# Output: (integer) 3

Example 2: Online Users

# User goes online
SADD online:users "user:42"

# User goes offline
SREM online:users "user:42"

# Who is online?
SMEMBERS online:users