Technical overview

    Urbit is a clean-slate software stack designed to implement an encrypted P2P network of general-purpose personal servers. Each server on this network is a deterministic computer called an 'urbit' that runs on a Unix-based virtual machine.

    The current Urbit stack includes (among other things):

    • Arvo: the functional operating system of each urbit, written in Hoon.
    • Azimuth: the Urbit identity layer, built on the Ethereum blockchain.
    • Hoon: a strictly typed functional programming language whose standard library includes a Hoon-to-Nock compiler.
    • Nock: a low-level combinator language whose formal specification fits readably on a t-shirt.
    • Vere: a Nock interpreter and Unix-based virtual machine that mediates between each urbit and the Unix software layer.

    Anatomy of a personal server

    Your urbit is a deterministic computer in the sense that its state is a pure function of its event history. Every event in this history is a transaction; your urbit's state is effectively an ACID database.

    Because each urbit is deterministic we can describe its role appropriately in purely functional terms: it maps an input event and the old urbit state to a list of output actions and the subsequent state. This is the Urbit transition function.

    <input event, old state> -> <output actions, new state>
    

    For example, one input event could be a keystroke from the terminal, say [enter] after having already typed (add 2 2); and an output action could be to print in the terminal window the resulting value of a computation performed when the user hit [enter], in this case 4. The input event is stored in the urbit's event history.

    Events always start from outside of your urbit, whether they're local to the computer running the urbit (e.g., a keystroke in the terminal) or they originate elsewhere (e.g., a packet received from another urbit). When an event is processed, various parts of the urbit state can be modified before the resulting list of output actions is returned.

    Can output actions from your urbit cause side-effects in the outside world? The answer had better be "yes," because a personal server without side effects isn't useful for much. In another sense the answer had better be "no," or else there is a risk of losing functional purity; your urbit cannot guarantee that the side effects in question actually occur. What's the solution?

    Each urbit is sandboxed in a virtual machine, Vere, which runs on Unix. Code running in your urbit cannot make Unix system calls or otherwise directly affect the underlying platform. Strictly speaking, internal urbit code can only change internal urbit state; it has no way of sending events outside of its runtime environment. Functional purity is preserved.

    In practical terms, however, you don't want your urbit to be an impotent brain in a vat. That's why Vere also serves as the intermediary between your urbit and Unix. Vere observes the list of output events, and when external action is called for makes the appropriate system calls itself. When external events relevant to your urbit occur in the Unix layer, Vere encodes and delivers them as input events.

    Arvo

    Arvo is a purely functional, non-preemptive OS, written in Hoon, that serves as the event manager of your urbit. It can upgrade itself from over the network without downtime. The Arvo kernel proper is quite simple -- it's only about 600 lines of code, excluding its various modules.

    The Urbit transition function is implemented in Arvo. Upon being 'poked' by Vere with the pair of <input event, state>, Arvo directs the event to the appropriate OS module. The result of each Vere 'poke' is a pair of <output events, new state>. Events are typed, and each has an explicit call-stack structure indicating the event's source module in Arvo.

    Arvo modules are also called 'vanes'. Arvo's vanes are:

    • Ames: defines and implements Urbit's encrypted P2P network protocol, as well as Urbit's identity protocol.
    • Behn: manages timer events for other vanes.
    • Clay: global, version-controlled, and referentially-transparent file system.
    • Dill: terminal driver.
    • Eyre: HTTP web client and server.
    • Ford: typed functional build system.
    • Gall: application sandbox and manager.
      • Dojo (app): Shell and Hoon REPL.
      • Talk (app): Chat client.

    Hoon

    Hoon is a strictly typed functional programming language that compiles itself to Nock and is designed to support higher-order functional programming without requiring knowledge of category theory or other advanced mathematics. Haskell is fun but it isn't for everybody.

    Hoon aspires to a concrete, imperative feel. To discourage the creation of write-only code, Hoon forbids user-level macros and uses ASCII digraphs instead of keywords. The type system infers only forward and does not use unification, but is not much weaker than Haskell's. The compiler and inference engine is about 3000 lines of Hoon.

    Nock

    Nock is a low-level homoiconic combinator language. It's so simple that its specification fits on a t-shirt. In some ways Nock resembles a nano-Lisp but its ambitions are more narrow. Most Lisps are one-layer: a practical language is to be created by extending a theoretically simple interpreter. The abstraction is simple and the implementation is practical. Unfortunately it's far more difficult to enforce both simplicity and practicality in an actual Lisp codebase. Hoon and Nock are two layers: Hoon, the practical layer, compiles itself to Nock, the simple layer. Your urbit runs in Vere, which includes a Nock interpreter, so it can upgrade Hoon over the network without downtime.

    The Nock data model is quite simple. Every piece of data is a 'noun'. A noun is an atom or a cell. An atom is any unsigned integer. A cell is an ordered pair of nouns. Nouns are acyclic and expose no pointer equality test.

    Vere

    Vere is the Nock runtime environment and Urbit VM. It's written in C, runs on Unix, and is the intermediate layer between your urbit and Unix. As noted earlier, Unix system calls are made by Vere, not Arvo; Vere must also encode and deliver relevant external events to Arvo. Vere is also responsible for implementing jets and maintaining the persistent state of each urbit.

    In principle, Vere keeps a comprehensive log of every event from the time you initially booted your urbit. What happens if the physical machine loses power and your urbit's state is 'lost' from memory? When your urbit restarts it will replay its entire event history and totally recover its latest state from scratch.

    In practice, event logs become large and unwieldy over time. Periodically a snapshot of the permanent state is taken and the logs are pruned. You're still able to rebuild your state in case of power outage, down to the last keystroke.

    Vere is not essential to the Urbit stack; one can imagine using Urbit on a hypervisor, or even bare metal. One member of the community is even working on an independent implementation of Urbit using Graal/Truffle on the JVM.

    The Urbit stack (compiler, standard library, kernel, modules, and applications, but excluding Vere) is about 30,000 lines of Hoon. Urbit is patent-free and MIT licensed.

    Azimuth

    Azimuth is a general-purpose public-key infrastructure (PKI) on the Ethereum blockchain, used as a decentralized ledger for Urbit identities that we call points. Having a point is necessary to use the Urbit network, which makes it important to have a neutral ledger to determine who owns what.

    Azimuth is not, however, part of the Urbit stack. Azimuth is a parallel system that can be used as a generalized identity system for other projects. Azimuth "touches" the Urbit ecosystem when a point is used to boot a virtual computer on the Urbit network for the first time. When that happens, the point considered linked to Azimuth and the point’s full powers are available for use. Once a point is linked, it cannot be unlinked.

    A metaphor might make the relationship between these two systems to understand: Azimuth is the bank vault that stores the deed to your house. The Urbit network is the neighborhood that you live in.