CF.RESERVE key capacity [bucketsize] [maxiterations] [expansion]

Available in: Redis Stack

Time complexity: O(1)

Create a Cuckoo Filter as key with a single sub-filter for the initial amount of capacity for items. Because of how Cuckoo Filters work, the filter is likely to declare itself full before capacity is reached and therefore fill rate will likely never reach 100%. The fill rate can be improved by using a larger bucketsize at the cost of a higher error rate. When the filter self-declare itself full, it will auto-expand by generating additional sub-filters at the cost of reduced performance and increased error rate. The new sub-filter is created with size of the previous sub-filter multiplied by expansion. Like bucket size, additional sub-filters grow the error rate linearly. The size of the new sub-filter is the size of the last sub-filter multiplied by expansion. The default value is 1.

The minimal false positive error rate is 2/255 ≈ 0.78% when bucket size of 1 is used. Larger buckets increase the error rate linearly (for example, a bucket size of 3 yields a 2.35% error rate) but improve the fill rate of the filter.

maxiterations dictates the number of attempts to find a slot for the incoming fingerprint. Once the filter gets full, high maxIterations value will slow down insertions. The default value is 20.

Unused capacity in prior sub-filters is automatically used when possible. The filter can grow up to 32 times.


  • key: The key under which the filter is found.
  • capacity: Estimated capacity for the filter. Capacity is rounded to the next 2^n number. The filter will likely not fill up to 100% of it's capacity. Make sure to reserve extra capacity if you want to avoid expansions.

Optional parameters:

  • bucketsize: Number of items in each bucket. A higher bucket size value improves the fill rate but also causes a higher error rate and slightly slower performance.
  • maxiterations: Number of attempts to swap items between buckets before declaring filter as full and creating an additional filter. A low value is better for performance and a higher number is better for filter fill rate.
  • expansion: When a new filter is created, its size is the size of the current filter multiplied by expansion. Expansion is rounded to the next 2^n number.


[] otherwise.


redis> CF.RESERVE cf 1000
redis> CF.RESERVE cf 1000
(error) ERR item exists