| DCOM Explained by Rosemary Rock-Evans Digital Press ISBN: 1555582168 Pub Date: 09/01/98

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### DCOM and Other Types of Middleware

There are hundreds of middleware products on the market. Perhaps the best known category of middleware products are the database connectivity products provided by companies such as Intersolv, Information Builders, Shadow, Oracle, and Sybase with their ODBC (Open Database Connectivity) drivers and gateways.

Other types of middleware include MOM (Message-Oriented Middleware), ORBs (Object Request Brokers), RPC (Remote Procedure Call), and DTPM (Distributed Transaction Processing Middleware). Other well-known middleware products include Tuxedo and TOP END (DTPMs); MQSeries, BEAmessageQ, VCOM, and PIPES (MOM); Orbix, DSOM, VisiBroker, and XShell (ORBs).

So how can one classify DCOM—is it an ORB, a DTPM, a MOM product, an RPC, or a database connectivity product? The answer is that DCOM is unique because it is a complete hybrid of all these types.

There are five main strands to Microsoft’s middleware service offerings—its database connectivity services, its MOM services, its transaction processing services, its links to the mainframe (CICS and IMS), and its RPC services. The MS RPC provides the foundation for all the other services; although the developer sees only the component based interface, he or she has no need to program to the RPC mechanism.

Thus, DCOM provides distributed messaging services, object request broker services, distributed transaction services, and data connectivity services all layered over its own RPC mechanism. Microsoft, therefore, does not provide a set of completely disparate and unconnected set of products, but one integrated solution with multiple capability. The user does not have to choose a product or more than one product to support applications, nor does a user have to learn different product approaches or philosophies. He or she uses one set of services, which are already available if he or she is using Windows NT.

Messaging capability is provided by the Messaging services in DCOM, transaction services are provided by the Transaction Server in DCOM, database connectivity is provided by the database connectivity services, and so on. The developer could use several services together, accessing the queues in the Messaging server and data in various data sources and putting all of these under transaction control.

DCOM is thus a special product and unique on the market. Microsoft’s aim is to provide the developer with all the services needed to develop a distributed application from one single source and using one common interface—that used in COM.

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