I had been working on building an in-memory LFU cache (least frequently used cache), and now that it is done, I thought of writing about the building blocks of an LFU cache. This article shares the building blocks of an LFU cache along with the ideas from two research papers: BP-Wrapper and TinyLFU.

Bitcask is an embeddable key/value storage engine that is defined as a "Log-Structured Hash Table" in the paper that introduced it. The model of Bitcask is simple: all the key/value pairs are written to append-only files and an in-memory data structure contains a mapping between each key and the position of the value in the data file.

LSM-tree (Log structured merge tree) is a data structure typically used when dealing with write-heavy workloads. LSM-tree optimizes the write-path by performing sequential writes to disk. WiscKey is a persistent LSM-tree-based key-value store that separates keys from values to minimize read and write amplification. The design of WiscKey is highly SSD optimized, leveraging both the sequential and random performance characteristics of the device.

A Bloom filter is a probabilistic data structure used to test whether an element is a set member. A bloom filter can query against large amounts of data and return either "possibly in the set" or "definitely not in the set".

Count-min sketch (CM sketch) is a probabilistic data structure used to estimate the frequency of events in a data stream.

What would it really mean to write code without automated tests, to deliver software without automated tests. Why would someone even think of writing code today and adding tests later?