Redis sets

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.

Examples

  • 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

Limits

The max size of a Redis set is 2^32 - 1 (4,294,967,295) members.

Basic commands

  • SADD adds a new member to a set.
  • SREM removes the specified member from the set.
  • SISMEMBER tests a string for set membership.
  • SINTER returns the set of members that two or more sets have in common (i.e., the intersection).
  • SCARD returns the size (a.k.a. cardinality) of a set.

See the complete list of set commands.

Performance

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 SMEMBERS command. 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.

Alternatives

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.

Learn more

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