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137 lines
5.2 KiB
Plaintext
---
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title: Introduction
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sort: 1
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next: true
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---
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# Introduction
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Urbit is a clean-slate system software stack defined as a
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deterministic computer. An encrypted P2P network, `%ames`, runs
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on a functional operating system, Arvo, written in a strict,
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typed functional language, Hoon, which compiles itself to a
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combinator interpreter, Nock, whose spec gzips to 340 bytes.
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What is Urbit for? Most directly, Urbit is designed as a
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personal cloud server for self-hosted web apps. It also uses
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HTTP APIs to manage data stuck in traditional web applications.
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More broadly, Urbit's network tackles identity and security
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problems which the Internet can't easily address. Programming
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for a deterministic single-level store is also a different
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experience from Unix programming, regardless of language.
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## Architectural overview
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A deterministic computer? Urbit's state is a pure function of
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its event history. In practice it uses a memory checkpoint and
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an append-only log. Every event is a transaction; Urbit is an
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ACID database and a single-level store. Urbit runs on Unix now,
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but it's easy to imagine on a hypervisor or even bare metal.
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A purely functional OS? Urbit is pure -- no code inside it can
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make system calls or otherwise affect the underlying platform.
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Instead, the top-level event function defines an I/O protocol.
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It maps an input event and the current state to a list of output
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actions and the subsequent state. In Hoon:
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$+([event state] [(list action) state])
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### Nock
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Nock is a sort of nano-Lisp without syntax, symbols or lambdas.
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Most Lisps are one-layer: they create a practical language by
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extending a theoretically simple interpreter. The abstraction is
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simple and the implementation is practical; there is no actual
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codebase both simple and practical. Hoon and Nock are two
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layers: Hoon compiles itself to pure Nock. Since Urbit is
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defined in Nock, not Hoon, we can upgrade Hoon over the air.
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The Nock data model is especially trivial. A *noun* is an atom
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or a cell. An atom is any unsigned integer. A cell is an
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ordered pair of nouns. Nouns are acyclic and expose no pointer
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equality test.
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### Hoon
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Hoon is a strict combinator language that avoids mathematical
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theory and notation. It aims at a mechanical, imperative feel.
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Hoon uses ASCII digraphs instead of keywords; there are no
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user-level macros. The type system infers only forward and does
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not use unification, but is not much weaker than Haskell's. The
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compiler and inference engine is about 2000 lines of Hoon.
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### Arvo
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Arvo is an event-driven OS written in Hoon. It can upgrade
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itself and everything inside it over the network. The Arvo
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kernel proper is 500 lines of Hoon, which implements a typed
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event system with explicit call-stack structure. Arvo ships
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it ships with modules that provide P2P networking (`%ames`), a
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revision-control system (`%clay`), a web client/server (`%eyre`),
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a functional build system (`%ford`), and an application engine
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`(%gall)`.
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### `%ames`
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`%ames`, the Urbit network, is an encrypted P2P protocol over
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UDP. Its address space is semi-decentralized; 64-bit addresses
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are hierarchically distributed, 128-bit addresses are
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self-created. Addresses (or *plots*) are rendered in a phonemic
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syntax for memorability. The scarcity of short plots helps
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control spam and other Sybil attacks. The short plot hierarchy
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is also reused as a supernode routing system for NAT traversal.
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### Apps
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Urbit ships with two default applications: a REPL or shell
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`:dojo`, and a distributed user-level message-bus `:talk`.
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`:talk` under the hood resembles NNTP; to the user, it looks like
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a self-hosted Slack or persistent IRC.
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The full Urbit stack (compiler, standard library, kernel,
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modules, and applications) is about 25,000 lines of Hoon.
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Urbit is patent-free and MIT licensed.
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## Status
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Anyone can run the Urbit VM, of course. But the `%ames` network
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is officially invitation-only. Not that we're antisocial -- just
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that we're under construction.
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Right now, Urbit's only practical use is to (a) build Urbit and
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(b) talk about Urbit. Its performance is lamentable. Its
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documentation is inadequate. Its keys are test keys. Its
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planets explode on a regular basis. We reserve the right to
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reboot ("flag-day") the whole network.
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However, Urbit is at least out of research mode and focused more
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or less exclusively on optimization and bug-fixing. So at least,
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whatever you learn will stay true. And bleeding edges are fun.
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## Getting involved
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If you're interested in following Urbit, you can:
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- Read our documentation at
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[urbit.org](http://urbit.org/docs)
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- Subscribe to our newsletter at [urbit.org](http://urbit.org).
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- Check out the
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[urbit-dev](https://groups.google.com/forum/#!forum/urbit-dev)
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mailing list.
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- Follow [@urbit_](https://twitter.com/urbit\_) on Twitter.
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- Hit us up by email, <span class="mono">urbit@urbit.org</span>.
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We're nice!
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## Code of conduct
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Everyone involved in the Urbit project needs to understand and
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respect our code of conduct, which is: "don't be rude."
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## Pronunciation and etymology
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Urbit is always pronounced "herb it," never "your bit." Not that
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it's not your bit! But "herb it" just sounds better.
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The origin of the name is just the Latin *urbi*, meaning city.
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