The count-min sketch is a probabilistic data structure that estimates how many times an event occurred without storing each individual instance. Using a small matrix and several hash functions, systems like Redis and Apache Flink count frequencies in massive streams with just a few kilobytes of memory.
The actor model is a concurrency paradigm where each actor processes messages in isolation, without shared memory. Erlang popularized it in telecommunications, and today it powers Akka, Elixir/OTP, and Microsoft Orleans. This guide explains how it works, when to use it, and how to write your first supervised actor.
MVCC is the mechanism that lets a SELECT query never wait for an UPDATE in progress: PostgreSQL, MySQL, and even SQLite use it to separate readers from writers. This guide explains how it works under the hood, with code examples, SQL isolation levels, and the most common mistakes when working with concurrent transactions.
