Sets

What is a Redis Set?

A Redis Set is an unordered collection of unique strings. Adding a member that already exists is a no-op — Redis does not create duplicates. This makes Sets ideal for tracking tags, visited pages, or any group where each value should appear exactly once.

SADD and SREM — adding and removing members

SADD key member [member ...] adds one or more members. It returns the count of new members added (duplicates are not counted). SREM key member [member ...] removes members and returns how many were actually present.

127.0.0.1:6379> SADD article:1:tags "redis" "database" "caching"
(integer) 3
127.0.0.1:6379> SADD article:1:tags "redis"
(integer) 0
127.0.0.1:6379> SREM article:1:tags "caching"
(integer) 1
127.0.0.1:6379> SREM article:1:tags "nosql"
(integer) 0

SADD article:1:tags "redis" "database" "caching"
SADD article:1:tags "redis"
SREM article:1:tags "caching"
SREM article:1:tags "nosql"

Copy commands ⧉

SMEMBERS and SCARD — inspecting a Set

SMEMBERS key returns all members (order is not guaranteed). SCARD key returns the cardinality (number of members) without fetching them all.

127.0.0.1:6379> SADD user:42:interests "music" "hiking" "redis" "coffee"
(integer) 4
127.0.0.1:6379> SMEMBERS user:42:interests
1) "coffee"
2) "redis"
3) "music"
4) "hiking"
127.0.0.1:6379> SCARD user:42:interests
(integer) 4

SADD user:42:interests "music" "hiking" "redis" "coffee"
SMEMBERS user:42:interests
SCARD user:42:interests

Copy commands ⧉

SISMEMBER — membership check

SISMEMBER key member returns 1 if the member exists, 0 if not. This is an O(1) lookup regardless of Set size.

127.0.0.1:6379> SADD seen:urls "https://redis.io" "https://astro.build"
(integer) 2
127.0.0.1:6379> SISMEMBER seen:urls "https://redis.io"
(integer) 1
127.0.0.1:6379> SISMEMBER seen:urls "https://example.com"
(integer) 0

SADD seen:urls "https://redis.io" "https://astro.build"
SISMEMBER seen:urls "https://redis.io"
SISMEMBER seen:urls "https://example.com"

Copy commands ⧉

SRANDMEMBER and SPOP — random access

SRANDMEMBER key [count] returns random members without removing them. SPOP key [count] removes and returns random members. Both are useful for sampling, prizes, or random assignments.

127.0.0.1:6379> SADD prize:pool "alpha" "beta" "gamma" "delta" "epsilon"
(integer) 5
127.0.0.1:6379> SRANDMEMBER prize:pool 2
1) "delta"
2) "gamma"
127.0.0.1:6379> SCARD prize:pool
(integer) 5
127.0.0.1:6379> SPOP prize:pool 1
1) "alpha"
127.0.0.1:6379> SCARD prize:pool
(integer) 4

SADD prize:pool "alpha" "beta" "gamma" "delta" "epsilon"
SRANDMEMBER prize:pool 2
SCARD prize:pool
SPOP prize:pool 1
SCARD prize:pool

Copy commands ⧉

🟥 Sets guarantee uniqueness — great for tags and seen checks

When you SADD a value that already exists, Redis silently ignores it and returns 0. This means you can safely call SADD as an upsert with no prior existence check. Use this property for “seen” deduplication (visited URLs, processed event IDs) and tag management — adding the same tag twice is always safe.

What does SADD return when you add a member that already exists?

1 0 An error The member itself

Which command checks membership in O(1) without fetching all members?

SMEMBERS SCARD SISMEMBER SRANDMEMBER

What is the difference between SRANDMEMBER and SPOP?

SRANDMEMBER removes the member; SPOP does not SPOP removes the member; SRANDMEMBER does not Both remove the member Neither returns a value

What does SCARD return?

A random member All members of the set The number of members in the set Whether the set is empty

Submit

Mark as complete