Hashing-function agnostic Cuckoo filters for Redis

Cuckoo filters are a probabilistic data structure that allows you to test for membership of an element in a set without having to hold the whole set in memory.

This is done at the cost of having a probability of getting a false positive response, which, in other words, means that they can only answer "Definitely no" or "Probably yes". The false positive probability is roughly inversely related to how much memory you are willing to allocate to the filter.

The most iconic data structure used for this kind of task are Bloom filters but Cuckoo filters boast both better practical performance and efficiency, and, more importantly, the ability of deleting elements from the filter.

Bloom filters only support insertion of new items. Some extensions of Bloom filters have the ability of deleting items but they achieve so at the expense of precision or memory usage, resulting in a far worse tradeoff compared to what Cuckoo filters offer.

For more information consult the project's wiki.

Cuckoo filters offer a very interesting division of labour between server and clients.

Since Cuckoo filters rely on a single hashing of the original item you want to insert, it is possible to off-load that part of the computation to the client. In practical terms it means that instead of sending the whole item to Redis, the clients send {hash, fingerprint} of the original item.

• You need to push trough the cable a constant amount of data per item instead of N bytes (Redis is a remote service afterall, you're going through a UNIX socket at the very least).
• To perform well, Cuckoo filters rely on a good choice of fingerprint for each item and it should not be left to the library.
• The hash function can be decided by you, meaning that this module is hashing-function agnostic.

The last point is the most important one. It allows you to be more flexible in case you need to reason about item hashes across different clients potentially written in different languages.

Additionally, different hashing function families specialize on different use cases that might interest you or not. For example some work best for small data (< 7 bytes), some the opposite. Some focus more on performance at the expense of more collisions, while some others behave better than the rest on peculiar platforms.

This blogpost shows a few benchmarks of different hashing function families.

Considering all of that, the choice of {hashing func, fingerprinting func} has to be up to you.

For the internal partial hashing that has to happen when reallocating a fingerprint server-side, this implementation uses FNV1a which is robust and fast for 1 byte inputs (the size of a fingerprint).

Thanks to how Cuckoo filters work, that choice is completely transparent to the clients.

If you know how Cuckoo filters work and are interested in knowing the implementation details of this module, please consult the project's wiki.

1. Download a precompiled binary from the Release section of this repo or compile with make all (linux and osx supported)

2. Put the redis-cuckoofilter.so module in a folder readable by your Redis installation

3. To try out the module you can send MODULE LOAD /path/to/redis-cuckoofilter.so using redis-cli or a client of your choice

4. Once you save on disk a key containing a Cuckoo filter you will need to add loadmodule /path/to/redis-cuckoofilter.so to your redis.conf, otherwise Redis will not load complaining that it doesn't know how to read some data from the .rdb file.

redis-cli> CF.INIT test 64K
(integer) 65536
 
redis-cli> CF.ADD test 5366164415461427448 97
OK

redis-cli> CF.CHECK test 5366164415461427448 97
(integer) 1

redis-cli> CF.REM test 5366164415461427448 97
OK 

redis-cli> CF.CHECK test 5366164415461427448 97
(integer) 0

You can find the complete command list on the project's wiki.

Consult the project's wiki.

You can find how to run tests on the project's wiki.

• Cuckoo filters for multisets: currently you can add a maximum of 2 * bucketsize copies of the same element before getting a too full error. Making a filter that adds a counter for each bucketslot would create a filter specifically designed for handling multisets.

• Dynamic Cuckoo filters: resize instead of failing with too full

MIT License

Copyright (c) 2017 Loris Cro

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.