Irmin is an OCaml library for building distributed databases with the same design principles as Git. Existing Git users will find many familiar features: branching/merging, immutable causal history for all changes, and the ability to restore to any previous state. It has been extensively used by major software projects over the past few years such as Docker for Mac/Windows, and noticeably through DataKit, which powers hundreds of thousands monthly builds on the opam-repository CI contributors may be familiar with.