Categories Programming

Continued from Promises vs. Products

At the start of a pro sports event, the players come onto the field while their personal statistics come up on the stadium's scoreboard. Let's go through this process with the players who contend on the field of distributed components.

Lined up and ready to play are the forces of Microsoft's DCOM, a component model that some have called "COM with a longer wire." This appellation refers to Microsoft's original Component Object Model, conceived as a way for the pieces of an application on a single PC to communicate through standard interface protocols.

It is COM that lets an embedded Excel spreadsheet, for example, summon up the Excel menus when that spreadsheet object is edited in-place within a Word document or a PowerPoint presentation chart. DCOM allows COM entities to communicate across the universe of a network, as easily as they do within the single constellation of tasks on a Windows PC.

The catch, however, is that DCOM plays a running game: A DCOM object exists only as long as at least one process maintains a reference to that object. This model is not entirely suited to widely separated network nodes running many different tasks over communication links that may not be perfectly reliable.

Microsoft's COM+, currently existing only in the form of a technology preview, adds a runtime environment that loosens DCOM's tight coupling between specific events and their receivers. Managed by the COM+ runtime, more than one receiver could await a particular event, with each receiver performing different actions in response to a single triggering condition.

Meanwhile, in a huddle and eager to take possession of the ball is the all-star team of CORBA. The members of that team may not train together, but they share a common playbook and they're used to the long game. CORBA, unlike DCOM, was conceived for geographically dispersed networks handling many different tasks on many different time scales. A CORBA object retains its identity even if swapped out from memory to persistent storage, such as a disk file. It is possible to test the equivalence of two different references to what may be a single CORBA object: An object's identity persists, whether or not that object is currently the focus of any active process.

CORBA 3.0 improves the integration between Java modules and native-code components, and it vies with Microsoft Transaction Server in promising transparent integration of transaction protocols into objects' interactions.

Next: Components in Play

Published as Enterprise Computing in the 4/20/99 issue of PC Magazine.