Redis sets are unordered collections of unique strings. They provide fast membership testing and set operations like union, intersection, and difference.
SADD
Adds one or more members to a set. Creates the key if it doesn’t exist.
Syntax
SADD key member [member ...]
The set key. Created if it doesn’t exist.
One or more members to add to the set.
The number of elements that were added to the set, not including elements already present.
- 2.4.0: Accepts multiple
member arguments.
Examples
redis> SADD myset "Hello"
(integer) 1
redis> SADD myset "World"
(integer) 1
redis> SADD myset "World"
(integer) 0
redis> SMEMBERS myset
1) "Hello"
2) "World"
redis> SADD myset "foo" "bar"
(integer) 2
SMEMBERS
Returns all members of a set.
Syntax
All members of the set, in no particular order.
SMEMBERS loads all members into memory. For large sets, use SSCAN to iterate incrementally.
Examples
redis> SADD myset "Hello"
(integer) 1
redis> SADD myset "World"
(integer) 1
redis> SMEMBERS myset
1) "Hello"
2) "World"
SINTER
Returns the intersection of multiple sets.
Syntax
One or more set keys to intersect.
Members present in all specified sets.
Examples
redis> SADD set1 "a" "b" "c"
(integer) 3
redis> SADD set2 "c" "d" "e"
(integer) 3
redis> SINTER set1 set2
1) "c"
redis> SADD set3 "c" "e" "f"
(integer) 3
redis> SINTER set1 set2 set3
1) "c"
SUNION
Returns the union of multiple sets.
Syntax
One or more set keys to union.
Members present in any of the specified sets.
Examples
redis> SADD set1 "a" "b" "c"
(integer) 3
redis> SADD set2 "c" "d" "e"
(integer) 3
redis> SUNION set1 set2
1) "a"
2) "b"
3) "c"
4) "d"
5) "e"
SDIFF
Returns the difference between sets.
Syntax
First key is the set to subtract from. Following keys are sets to subtract.
Members of the first set that don’t exist in any of the other sets.
Examples
redis> SADD set1 "a" "b" "c"
(integer) 3
redis> SADD set2 "c" "d" "e"
(integer) 3
redis> SDIFF set1 set2
1) "a"
2) "b"
redis> SDIFF set2 set1
1) "d"
2) "e"
Additional Set Commands
- SREM: Remove members from set
- SCARD: Get set cardinality (size)
- SISMEMBER: Check if member exists
- SMISMEMBER: Check multiple members
- SPOP: Remove and return random member(s)
- SRANDMEMBER: Get random member(s) without removing
- SMOVE: Move member between sets
- SSCAN: Iterate set members
- SINTERSTORE: Store intersection result
- SUNIONSTORE: Store union result
- SDIFFSTORE: Store difference result
- SINTERCARD: Get intersection cardinality
Use Cases
Tagging System
Implement tags for articles:
# Add tags to article
redis> SADD article:1000:tags "redis" "database" "nosql"
(integer) 3
redis> SADD article:1001:tags "redis" "performance"
(integer) 2
# Find articles tagged with "redis"
redis> SADD tag:redis:articles "1000" "1001"
(integer) 2
# Find common tags between articles
redis> SINTER article:1000:tags article:1001:tags
1) "redis"
Unique Visitors
Track unique visitors:
redis> SADD visitors:2024-03-03 "user:1" "user:2" "user:3"
(integer) 3
redis> SADD visitors:2024-03-03 "user:1"
(integer) 0
redis> SCARD visitors:2024-03-03
(integer) 3
Friend Recommendations
Find mutual friends:
redis> SADD friends:alice "bob" "charlie" "dave"
(integer) 3
redis> SADD friends:bob "alice" "charlie" "eve"
(integer) 3
# Find mutual friends
redis> SINTER friends:alice friends:bob
1) "charlie"
# Find potential friends (friends of friends)
redis> SDIFF friends:bob friends:alice
1) "eve"
Access Control
Manage user permissions:
redis> SADD user:1000:permissions "read" "write" "delete"
(integer) 3
redis> SISMEMBER user:1000:permissions "write"
(integer) 1
redis> SISMEMBER user:1000:permissions "admin"
(integer) 0
Online Users
Track currently online users:
# User logs in
redis> SADD online:users "user:1000"
(integer) 1
# User logs out
redis> SREM online:users "user:1000"
(integer) 1
# Count online users
redis> SCARD online:users
(integer) 0
Best Practices
- Membership Testing: SISMEMBER is O(1) - very fast
- Set Operations: SINTER can be expensive on large sets
- Iteration: Use SSCAN for large sets instead of SMEMBERS
- Cardinality: SCARD is O(1) - always use instead of counting SMEMBERS
Memory Optimization
- Use integer encoding when possible (more efficient)
- Avoid very large sets (consider splitting)
- Use expiration for temporary sets
- Store results of expensive operations with SINTERSTORE
Set Operations Strategy
Intersection
Union
Difference
Find common elements:# Direct operation
redis> SINTER set1 set2 set3
1) "common"
# Store result for reuse
redis> SINTERSTORE result set1 set2 set3
(integer) 1
Combine all elements:# Direct operation
redis> SUNION set1 set2 set3
1) "a"
2) "b"
3) "c"
# Store result
redis> SUNIONSTORE result set1 set2 set3
(integer) 3
Find unique elements:# Elements in set1 but not in set2
redis> SDIFF set1 set2
1) "unique"
# Store result
redis> SDIFFSTORE result set1 set2
(integer) 1
Patterns
Bloom Filter Alternative
Simple membership testing:
redis> SADD seen:urls "https://example.com"
(integer) 1
redis> SISMEMBER seen:urls "https://example.com"
(integer) 1
redis> SISMEMBER seen:urls "https://other.com"
(integer) 0
Vote System
Implement upvote/downvote:
# User upvotes post
redis> SADD post:100:upvotes "user:1000"
(integer) 1
redis> SREM post:100:downvotes "user:1000"
(integer) 0
# Count votes
redis> SCARD post:100:upvotes
(integer) 1
# Check if user voted
redis> SISMEMBER post:100:upvotes "user:1000"
(integer) 1
Random Selection
Select random items:
redis> SADD lottery "ticket1" "ticket2" "ticket3" "ticket4" "ticket5"
(integer) 5
redis> SPOP lottery 3
1) "ticket3"
2) "ticket1"
3) "ticket5"
redis> SMEMBERS lottery
1) "ticket2"
2) "ticket4"
Game Matchmaking
Find players with similar skills:
redis> SADD skill:beginner "player1" "player2" "player3"
(integer) 3
redis> SADD skill:intermediate "player4" "player5"
(integer) 2
redis> SADD queue:waiting "player1" "player4"
(integer) 2
# Find beginner in queue
redis> SINTER skill:beginner queue:waiting
1) "player1"
Set vs Other Data Structures
| Feature | Set | List | Sorted Set |
|---|
| Duplicates | No | Yes | No |
| Order | Unordered | Ordered | Score-ordered |
| Membership | O(1) | O(N) | O(log N) |
| Use Case | Unique items | Sequences | Rankings |
| Set Ops | Yes | No | Limited |
Common Patterns
Real-Time Analytics
# Track unique events
redis> SADD analytics:pageviews:2024-03-03 "session:1" "session:2"
(integer) 2
redis> SCARD analytics:pageviews:2024-03-03
(integer) 2
# Daily active users
redis> SADD dau:2024-03-03 "user:1" "user:2" "user:3"
(integer) 3
redis> SCARD dau:2024-03-03
(integer) 3
Social Graph
# Follow relationship
redis> SADD following:alice "bob" "charlie"
(integer) 2
redis> SADD followers:bob "alice" "dave"
(integer) 2
# Mutual follows
redis> SINTER following:alice followers:alice
(empty array)
For large-scale social graphs, consider using a dedicated graph database alongside Redis.