From technology to style

intro, components, case study, crossing boundaries, styles, platform, summary, Q/A, literature

We distinguish between three different architectural styles:

the distributed objects style
the (dynamically) downloadable code style
the mobile objects style

This distinction is arbitrary, in the sense that other distinctions are conceivable. However, the distinction above is well motivated by the technology matrix introduced in section Technology, as reflected in the feature-based description given below.

	distributed objects	downloadable code	mobile objects
Component	object	object/class	agent
Connector	ORB	various	methods
Creation	server	client	any
Location	server	client	any
Client	fixed	extensible	extensible
Server	extensible	fixed	extensible

slide: Feature classification

The distributed objects style comprises software architectures which consist of software components providing services to client applications. Each object is located at a single, fixed place. Objects on different machines are connected by an ORB (Object Request Broker). Example technologies supporting this architectural style are CORBA and DCOM.

The second architectural style is the (dynamically) downloadable code style. Classes may be downloaded, to be used on client machines for instantiating objects, which will run on the client machine. Example technologies supporting this style are Java applets, JavaBeans, and ActiveX controls.

Finally, in the mobile objects style, objects may migrate from host to host, carrying both functionality and data when they move. Consequently, mobile objects may communicate with the local objects of the host they currently reside on. Mobile objects are a means to implement agents which wander through a network, collecting information, negotiating with other agents, periodically reporting back results to the user who launched the agents. Technologies supporting the mobile object style are agent ORBs such as Voyager.

Features

In slide Features, an overview is given of the characteristic features of each style. Clearly, the styles differ in what are considered as constituent parts (components and connectors), location issues (which determine where objects are created and where they are located during their lifetime), and functionality issues (that is whether either the client or server is functionally extensible).

We may regard the location issues as the prime discriminators of the architectural styles discussed. Adopting the distributed objects architectural style, new objects can be added at the server-side, where they will stay for the remainder of their lifetime. In contrast, adopting the downloadable code style, objects may be created at the client-side, from classes obtained from the server. Most flexible is the mobile objects style, which allows for objects to reside on either server or client machines.

The location properties directly affect the way that the system is extensible with new functionality. Clearly, the mobile code style offers the maximum of flexibility and functional extensibility. Nevertheless, as we will discuss shortly, there are tradeoffs involved. The maximum in flexibility and extensibility does not necessarily offer the optimal solution!