Erlang

Two CouchDB databases got the same replications in the same order. One ended with a document deleted, the other with it live, and the only difference was a validation function that accepts everything. This is the full path from a theory of convergence, to a model of CouchDB’s replication, to a prediction committed before any test, to the one scenario the model got wrong, to the bug behind it and the fix that merged into CouchDB.

Go, Node.js, Java virtual threads, Erlang, Rust, Python, Kotlin: each language’s concurrency model is a different engineering trade-off against the same physics. This article builds the framework for understanding all of them, starting from the OS scheduler and working upward.