This article gives an overview of the architecture of Urbit applications. I spent a weekend trying out Urbit, reading documentation, and digging through the source code. I'm always on the lookout for the next milestone system that will change the internet and computing landscape. In particular, I find decentralized and peer-to-peer systems interesting because I have a sense that the internet is not quite right. It could be better if only someone could figure out how to do that and make it mainstream.
Urbit is an operating system and network designed to give users control by running applications on personal servers instead of on centralized servers operated by the application creators. This means data is stored on personal servers and is not immediately accessible to application creators. Both the Urbit operating system and network run on top of existing computing infrastructure. It's not a baremetal operating system (it runs under Linux, macOS, and Windows) or a new Layer 3 network protocol (it uses UDP). If you want to know more there is an overview of Urbit here.
The operating function
The Urbit kernel, Arvo, is a single-function operating system in the sense of purely functional programming. The operating system function takes the previous state and input events and produces the next state and output events. This means that the state of the system can be saved after each invocation. If there is a power failure or the system otherwise stops execution it's easy to resume it later from the last state.
Urbit has a virtual machine and runtime that supports this programming environment. The low-level language is called Nock and the higher-level language is called Hoon. I haven't investigated them in detail, but they appear to support deterministic purely functional programming with I/O and other side-effects kept outside via monads and passing around inputs like the current time.
Applications
Applications, also called agents, follow the purely functional model where they produce the next state as their result. Agents expose their services in three ways:
- Peek, a read-only query that fetches data without changing state.
- Poke, a command and response similar to a Remote Procedure Call (RPC).
- Subscriptions, a stream of updates that may be delivered over time until the subscription is closed.
For example, an application that keeps a counter can define a poke interface for incrementing the counter and a peek interface for querying its value. A subscription can be used to receive an update whenever the counter changes.
Urbit supports peeks, pokes, and subscriptions over the network. This is how applications running on different personal servers can communicate. If we want to replicate a remote counter we can subscribe to it and then poke our local counter every time an update is received. This replication model leads to the store/hook/view architecture, a way of splitting applications into components that support local state, remote replication, and a user interface. In our counter example the store would be the counter, the hook would be the code that replicates remote counters, and the view would provide any logic needed for the user interface to control the counter.
Interacting with the outside world
User interfaces for applications are typically implemented in Landscape, a web-based user interface for interacting with Urbit from your browser. The user interface can be a modern JavaScript application that communicates with the agent running inside Urbit via the HTTP JSON API. This API supports peeks, pokes, and subscriptions. In other words, the application's backend is implemented as an Urbit agent while the frontend is a regular client-side web application.
Of course there are also APIs for data storage, networking, HTTP, etc. For example, the weather widget in Landscape fetches the weather from a web service using an HTTP request.
Urbit also supports peer discovery so you can resolve the funny IDs like ~bitbet-bolbel and establish connections to remote Urbit instances. The IDs are allocated hierarchically and ultimately registered on the Ethereum blockchain.
Criticisms
Keep in mind I only spent a weekend investigating Urbit so I don't understand the full system and could be wrong about what I have described. Also, I've spent a lot of time and am therefore invested in Linux and conventional programming environments. Feedback from the Urbit community is welcome, just send me an email or message me on IRC or Matrix.
The application and network model is intended for personal servers. I don't think people want personal servers. It's been tried before by Sandstorm, FreedomBox, and various projects without mainstream success. I think a more interesting model for the powerful devices we run today is one without any "server" at all. Instead of having an always-on server that is hosted somewhere, apps should be able to replicate and sync directly between a laptop and a phone. Having the option to run a personal server for always-on services like chat rooms or file hosting is nice, but many things don't need this. I wish Urbit was less focussed on personal servers and more on apps that replicate and sync directly between each other.
Urbit is obfuscated by the most extreme not invented here (NIH) syndrome I have ever seen. I tried to keep the terminology at a minimum in this article, so it might not be obvious unless you dive into the documentation or source code yourself. Not only is most of it a reinvention of existing stuff but it also uses new terminology for everything. It was disappointing to find that what first appeared like an alien system that might hold interesting discoveries was just a quirky reimplementation of existing concepts.
It will be difficult for Urbit to catch on as a platform since it has no common terminology with existing programming environments. If you want to write an app for Urbit using the Hoon programming language you'll have to wade through a lot of NIH at every level of the stack (programming language, operating system, APIs). There is an argument that reinventing everything allows the whole system to be small and self-contained, but in practice that's not true since Landscape apps are JavaScript web applications. They drag in the entire conventional computing environment that Urbit was supposed to replace. I wonder if the same kind of system can be built on top of a browser plus Deno with WebRTC for the server side, reusing existing technology that is actively developed by teams much larger than Urbit. It seems like a waste because Urbit doesn't really innovate in VMs, programming languages, etc yet the NIH stuff needs to be maintained.
Finally, for a system that is very much exposed to the network, I didn't see a strong discipline or much support for writing secure applications. The degree of network transparency that Urbit agents have also means that they present an attack surface. I would have expected the documentation and APIs/tooling to steer developers in a direction where it's hard to make mistakes. My impression is that a lot of the attack surface in agents is hand coded and security issues could become commonplace when Urbit gains more apps written by a wider developer community.
Despite this criticism I really enjoyed playing with Urbit. It is a cool rabbit hole to go down.
Conclusion
Urbit applications boil down to a relatively familiar interface similar to what can be done with gRPC: command/response, querying data, and subscriptions. The Urbit network allows applications to talk to each other directly in a peer-to-peer fashion. Users run apps on personal servers instead of centralized servers operated by the application creators (like Twitter, Facebook, etc). If Urbit can attract enough early adopters then it could become an interesting operating system and ecosystem that overcomes some of the issues of today's centralized internet. If you're wondering, I think it's worth spending a weekend exploring Urbit!
