Sets

A set holds unique binary members without a defined iteration order. Unlike a Bloom/Cuckoo filter, SISMEMBER answers exact membership and SCARD is the exact number of members. Choose sets when false positives are unacceptable and the member identities fit in RAM.

Membership and sampling

target/release/ffkv-cli -p 6382 SADD docs:set:active alice bob alice
target/release/ffkv-cli -p 6382 SCARD docs:set:active
target/release/ffkv-cli -p 6382 SISMEMBER docs:set:active alice
target/release/ffkv-cli -p 6382 SMISMEMBER docs:set:active alice carol
target/release/ffkv-cli -p 6382 SMEMBERS docs:set:active
target/release/ffkv-cli -p 6382 SRANDMEMBER docs:set:active
target/release/ffkv-cli -p 6382 SPOP docs:set:active 1

The initial cardinality is two, not three. SRANDMEMBER samples without deleting; SPOP removes. Do not expect stable sample values or SMEMBERS order. SREM removes named members; SMOVE atomically transfers one member between sets.

Algebra

target/release/ffkv-cli -p 6382 SADD docs:set:engineering alice bob
target/release/ffkv-cli -p 6382 SADD docs:set:oncall bob carol
target/release/ffkv-cli -p 6382 SINTER docs:set:engineering docs:set:oncall
target/release/ffkv-cli -p 6382 SUNIONSTORE docs:set:everyone docs:set:engineering docs:set:oncall
target/release/ffkv-cli -p 6382 SSCAN docs:set:everyone 0 COUNT 10

The intersection contains bob. SUNION/SINTER/SDIFF return results; STORE variants replace a destination. Source types are validated before replacement, including when a destination overlaps a source. Cross-key algebra is atomic across shards.

SSCAN returns an opaque cursor and a page. Continue with the returned cursor until zero; COUNT is a hint, not an exact page size. It is not a snapshot, so concurrent mutations can affect observations. Avoid whole-set responses for large collections.

Errors and storage

Operations on incompatible types return WRONGTYPE. Memory and response limits still apply even if every individual member is small; oversized destructive operations are rejected before removal rather than silently truncating results. Member identities remain in RAM in hybrid mode.

With persistence enabled, SPOP journals the actual selected members. Recovery does not draw a new random sample. Without a data directory, no set survives restart. A disconnected SPOP caller cannot assume nothing was removed; inspect state before retrying.

See SADD, SINTERSTORE, persistence, and Bloom filters for the memory/accuracy tradeoff.