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A set is an unordered collection of unique string members. Unlike lists, a set will never hold two identical values — attempting to add a duplicate is silently ignored. Sets excel at tracking membership: unique visitors, active session IDs, feature flags enabled for an account, or any group where you need fast O(1) existence checks and don’t care about order.

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SADD

Adds one or more members to the set stored at key. Members that already belong to the set are ignored. If key does not exist, a new set is created. Syntax: SADD key member [member ...]
Returns: Integer — the number of members that were newly added (duplicates are not counted).

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SREM

Removes one or more members from the set stored at key. Members that are not in the set are silently ignored. Syntax: SREM key member [member ...]
Returns: Integer — the number of members that were actually removed.

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SMEMBERS

Returns all members of the set stored at key. Because sets are unordered, the order of members in the response is not guaranteed. Syntax: SMEMBERS key
Returns: Array of bulk strings containing every member. Returns an empty array if the key does not exist.
The order of elements returned by SMEMBERS is not deterministic. If you need a stable order, sort the result in your application layer.

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SISMEMBER

Tests whether member belongs to the set stored at key. Syntax: SISMEMBER key member
Returns: 1 if the member exists in the set, 0 if it does not (or if the key does not exist).

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SCARD

Returns the cardinality (number of members) of the set stored at key. Syntax: SCARD key
Returns: Integer — the number of members, or 0 if the key does not exist.

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Practical example: tracking unique visitors

Sets are a natural choice for daily unique-visitor counting. Each user ID is recorded at most once per day, and you can query the exact count at any time:
Combine EXPIRE with your visitor sets to automatically clean up old day-buckets. For example: EXPIRE visitors:2024-01-01 604800 will remove the key after seven days.