How Postgres Lets Readers and Writers Coexist: MVCC Built in TypeScript

A developer tutorial published on DEV Community explains Multi-Version Concurrency Control (MVCC), the mechanism used by databases like PostgreSQL, MySQL, Oracle, and SQLite to allow concurrent reads and writes without blocking. The core principle of MVCC is that data is never overwritten or deleted in place — instead, the database only appends new versions of rows, each stamped with transaction IDs marking their creation and deletion. This append-only design means two simultaneous sessions can see different states of the same data without locking each other out. The article demonstrates this by showing how one session can delete a million rows while another session still reads them all, with neither session waiting on the other. The tutorial walks through building a simplified MVCC engine in approximately 100 lines of TypeScript, mirroring the xmin and xmax version-tracking columns found in real Postgres internals.