Urbit's lifecycle function is a nano-interpreter called Nock. A typed functional language, Hoon, compiles itself to Nock. An event-driven OS, Arvo, is written in Hoon. Everything above Nock can upgrade itself over Urbit's own overlay packet network, Ames. Ames is live and stable with test keys.
Our Urbit interpreter runs on any Unix machine. The Urbit server is a single-level store -- both database and application engine. Every Urbit event is a transaction. Urbit is semantically frozen and cannot call out to Unix.