The Context-User-Item Object Model

The Elko Context Server supports applications build around the Context-User-Item object model.

A Context is a rendezvous and coordination point for communications among multiple parties. A context is hosted on a Context Server. Within a context may be found users and items.

A User represents a human being sitting at a client application, typically implemented as Javascript code in a web browser. At any given time, a user is in some context. Multiple users may be in the same context together, enabling them to engage in multiparty interactions of various kinds.

An Item is an object that embodies some aspect of the application. An item is always in some container, which may be a context, a user, or another item.

Contexts, users and items are all examples of unums, world objects in a distributed instantiation object pattern. Each of these objects has a representation, called a presence, that exists in the Context Server and in each of the clients. Each unum has a reference string (or ref for short) by which it may be named in JSON messages. When a client sends a message over a connection to a server and the server receives it, the message is delivered to the server presence of the object named by the message as its target. Similarly, a server may send a message over a connection to a client, addressing the message to the client presence of some object. Additionally, code running in the server may send a single message to all client presences of some object; the server message system will fan out that message to all the clients that hold presences of that object, using those clients' respective connections. This will be explained in more detail in the section on server programming below.

Additional objects may exist only on the server or on some particular client. These objects may be addressed as the targets of messages individually. For example, when a client first connects to the server, the only server object that is immediately in scope for that client is the communications session itself, which may be addressed as "session". In particular, the client may send a message to the session object requesting that the client's associated user be granted admission to some particular context.

Most of the logic that is specific to particular applications is embodied in objects known as Mods (short, variously, for "module" or "modification", depending on your perspective). A mod is simply a bundle of behavior and state that is attached to some particular context, user, or item. The mod augments the object to which it is attached with additional message protocols specific to whatever functionality it is that the mod provides. Mods are most frequently attached to items, since the main thing that distiguishes one item from another (aside from its identity) is what mods it has that customize its behavior.