CF.RESERVE key capacity [BUCKETSIZE bucketsize]
  [MAXITERATIONS maxiterations] [EXPANSION expansion]
Available in:
Redis Stack / Bloom 1.0.0
Time complexity:

Creates an empty cuckoo filter with a single sub-filter for the initial specified capacity.

According to the cuckoo filter behavior, the filter is likely to declare itself full before capacity is reached; therefore, the fill rate will likely never reach 100 percent. 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 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.

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

Required arguments


is key name for the the cuckoo filter to be created.


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 arguments

BUCKETSIZE 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.

bucketsize is an integer between 1 and 255. The default value is 2.

MAXITERATIONS 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.

maxiterations is an integer between 1 and 65535. The default value is 20.

EXPANSION expansion

When a new filter is created, its size is the size of the current filter multiplied by expansion.

expansion is an integer between 0 and 32768. The default value is 1.

Expansion is rounded to the next 2^n number.

Return value

Returns one of these replies:

  • Simple string reply - OK if filter created successfully
  • [] on error (invalid arguments, key already exists, etc.)


redis> CF.RESERVE cf 1000

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


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