Since the announcement by IBM and Linden Lab that OpenSim can talk to Second Life, I've been thinking again about RESTful Virtual Reality.

I'm not the first, of course. Others have been motivated by the same goal: To bring the Web's scalability, linkability and interoperability into Virtual World platforms.

Ultimately, how to use the same techniques as the Web to link Virtual Worlds together into a single, massive 'Virtual Universe'.

Here's how I would architect the Universe Web...


The Universe Web needs Entities. (I'm going to be capitalising Significant Things a lot here to help the Web comparison - hope you don't mind). There's no need to make the Entity list too complicated since we're not doing online games yet - just a world like Second Life.

Here's a list of different kinds of Entity:

  • Places: buildings, streets, hills, lakes
  • Things: trees, books, birds, clocks
  • People


State, Links and animation

Entities aren't exactly the Objects of Object-Oriented programming: they have lots of public State. In a Virtual World, you can directly experience their colour, position, orientation, physical relation to each other, etc., thanks to the local render engine.

The basic structure of the Universe Web is made up of Linked Entities. A tree is Linked to its ground which is Linked to a building which is Linked to its occupants which are Linked to their clothing. Kneebones may even be Linked to thighbones.

Entities are often animated. Actually, Places are generally static, and People generally animate themselves. So, when talking in general, we should look at the animation of Things, like trees, books, birds and clocks.

There are two ways an Entity's animation can be guided: by Laws and by Rules.



Springiness is a handy Law. For example, tree branches, pages in a book and birds' wings and legs can all have springiness. We can declare a mid-air location in a bird's State and a springiness in its legs, then if the bird lands on a branch, the springiness can trigger the bird and branch waving gently to a halt.

We could even allow a single bird to become part of a flock Entity, where the flock only knows how many birds it has and the coordinates of its centre. Each bird will then be animated by global flock Laws.

The book can have a State of open or closed. If bookness was elevated to a global Law, then a book deciding that its State was now simply closed could trigger the elaborate, springy closing of its pages with a nice 'shlump' sound at the end.

The clock's current time may be set by the clock Entity itself, then the Law of time take over in between the clock updating its own time directly.

A Law can be invoked over an Entity's animation by simply adding the Law's name and parameters to its State. Such Laws can be handled by the physics engine in the same way surface texture is handled by the render engine: States such as 'red' need global meaning, Law applications such as 'springy' need global meaning.



When a clock reaches its alarm time, it rings. When its wizard master approaches, a magic book may glow and open. Birds fly away from danger (if a fox is too close, fly away from it). In between danger, birds and foxes look for food (if a bird is close, run towards it).

These are Rules. Rules are driven by the Observation of one Entity by another. The fox Observes the bird, the book Observes the wizard. Rules apply to Entity types: fox Rules, bird Rules, book Rules, etc. Rules can guide the animation of an Entity by evolving its State.

State evolution depends on the current State plus the States of the Observed Entities: Rules make an Entity change from one State to the next by taking the Entity's own State, Observing the States of the Entities around that it cares about, then deciding what the next, new State should be. If an Entity changes State, those Entities Observing it may themselves change State according to their own Rules, and so-on.

Links are part of an Entity's State, and Links may therefore change on the application of Rules. In other words, the virtual world may restructure when a Rule is run.

An Entity may either Observe another Entity, or Link to it, or both. An Entity that Links to another needn't Observe it. Trees don't bother to Observe the ground they grow from. Conversely, an Entity that Observes another needn't Link to it. A fox would only Link to a bird if she was lucky enough to catch it. Meanwhile, it is true that there's probably a chain of Links to everything that's Observed.

Unlike Laws, Rules can't or shouldn't be second-guessed, so have to be run 'at source' or 'inside the Entity', then the new State distributed to both the Observing Entities and to all the render engines.

But render engines are actually Observing People Entities..


People are First Class

In Virtual Worlds, we have People avatars: People are first class Entities. People can Observe and can change State, but it's a real user doing the animation, not a set of Rules. Observation and State change by a Person is implemented through the render engine.

Although self-animated, People can still make use of Laws. For example Laws can be invoked to allow gestures to be easier to make and to look better, and to allow People to 'run forward' rather than putting their 'left foot forward, ..'. Avatar skills become global Laws which can be triggered by an avatar's State animated by a user.


The Universe Wide Web

The Universe Web I've just described is a lot like the World Wide Web:

  • An Entity is like a Web Resource
  • The State of an Entity is like an HTML Web Representation of a Web Resource
  • A Link in an Entity's State is like a Web URL inside an HTML Web Representation
  • Laws are like Javascript, Flash, link hover, blink tags and animated GIFs
  • Rule sets are like PHP scripts or Java servers
  • A State Observation is like a Web Representation Transfer

A crucial difference between the Resource Web and the Entity Web is that, here it's another Entity doing the Observation. One Entity transfers its State to another.

It's as if the GET or POST has another Entity 'at the client end'. GET is like an Entity subscribing or Observing; POST is like an Entity publishing or notifying its State to another (an HTML form has a content type, too).

People are just examples of that Entity 'at the client end'. Instead of a user hidden behind a client browser, we have first-class People Entities doing 'GET's and 'POST's: Observing the Virtual World and acting (i.e., changing State) within it. Other examples are foxes and clocks.


Notation and Protocol

There are two open standards we need to have in progress while implementing the Universe Web: the notation and the protocol (our HTML/URI and HTTP).

Firstly, we have to define the notation for Entities and their Rules. Both the State of Entities and the Rules that animate them will be written in the same notation: a Rule talks directly about State. To me, basing that on JSON (rather than XML) is a clear choice. Add inter-JSON Entity Links (essentially UUIDs), then work out a simple JSON Rule language. We will then need to define JSON 'schemas' for the kinds of Entity we would model. These can be written in the Rule language itself.

Secondly, to define the protocol for Entities to Observe or subscribe to one another. It will be a 'symmetric, asynchronous HTTP' - a Peer-to-Peer, Publish-Subscribe protocol. UDP and Multicast stand out for this kind of interaction. A packet will subscribe, a packet will return current State, a subsequent packet will multicast State updates. We can re-use all the HTTP cache headers, only now we can push updates into caches and into render programs. Packets will inevitably end up tunnelled through TCP and HTTP, mind...

This lighter notation, which also breaks the data up into smaller pieces, and this multicast pushing of State change into caches, along with P2P-style operation, will mean the Universe Web scales even better than its parent.

Also, programming declaratively using Rules is both highly productive and further cleanly scalable through parallel processing.

Of course, it will be pretty trivial to allow inter-Web cross-over such as referring to PNG texture images via URL and pulling Universe Web Entities into a Javascript application that subscribes to them via Comet.


Building the Universe Web

So that's my Universe Web. It's basically the same as the World Wide Web and thus as scalable, linkable and interoperable. It comes with added symmetry: Entities, including People as avatars, exchange State in each direction.

It's a two-way data Web, not a one-way document Web.

I've started work on the Universe Web: the notation, the protocol and the implementation (in C, for portability across platforms, including mobile). Contact me if you want to help!