REDIS · PICTO

When to reach for a Redis SET

Uniqueness, membership checks, and tag fan-out — where sets beat lists and hashes.

4 min read May 2026

Redis has five core data structures. Lists, hashes, and sorted sets get most of the attention, but the plain SET is the right tool in more situations than most developers reach for it.

What a set gives you

A Redis SET is an unordered collection of unique strings. Adding a duplicate is a no-op. Membership checks are O(1). Set operations — union, intersection, difference — are O(N).

plaintext
SADD online:users "alice" "bob" "carol"
SISMEMBER online:users "alice"   # → 1
SADD online:users "alice"        # duplicate → no change, size stays 3
SCARD online:users               # → 3

REACH FOR A SET WHEN You need to track membership, deduplicate a stream, or fan out to tagged subscribers.

The fan-out pattern is especially useful: store a set of subscriber IDs per tag, then use SUNIONSTORE across tags to build a recipient list without hitting your database.