Introduction to Redis sets
A Redis set is an unordered collection of unique strings (members). You can use Redis sets to efficiently:
- Track unique items (e.g., track all unique IP addresses accessing a given blog post).
- Represent relations (e.g., the set of all users with a given role).
- Perform common set operations such as intersection, unions, and differences.
- Store the set of favorited book IDs for users 123 and 456:
> SADD user:123:favorites 347 (integer) 1 > SADD user:123:favorites 561 (integer) 1 > SADD user:123:favorites 742 (integer) 1 > SADD user:456:favorites 561 (integer) 1
- Check whether user 123 likes books 742 and 299
> SISMEMBER user:123:favorites 742 (integer) 1 > SISMEMBER user:123:favorites 299 (integer) 0
- Do user 123 and 456 have any favorite books in common?
> SINTER user:123:favorites user:456:favorites 1) "561"
- How many books has user 123 favorited?
> SCARD user:123:favorites (integer) 3
The max size of a Redis set is 2^32 - 1 (4,294,967,295) members.
SADDadds a new member to a set.
SREMremoves the specified member from the set.
SISMEMBERtests a string for set membership.
SINTERreturns the set of members that two or more sets have in common (i.e., the intersection).
SCARDreturns the size (a.k.a. cardinality) of a set.
See the complete list of set commands.
Most set operations, including adding, removing, and checking whether an item is a set member, are O(1).
This means that they're highly efficient.
However, for large sets with hundreds of thousands of members or more, you should exercise caution when running the
This command is O(n) and returns the entire set in a single response.
As an alternative, consider the
SSCAN, which lets you retrieve all members of a set iteratively.
Sets membership checks on large datasets (or on streaming data) can use a lot of memory. If you're concerned about memory usage and don't need perfect precision, consider a Bloom filter or Cuckoo filter as an alternative to a set.
Redis sets are frequently used as a kind of index. If you need to index and query your data, consider RediSearch and RedisJSON.
- Redis Sets Explained and Redis Sets Elaborated are two short but thorough video explainers covering Redis sets.
- Redis University's RU101 explores Redis sets in detail.