COM+ Services The initial release of COM+ (that is, COM+ 1.0), which comes with Windows 2000 provides the following services to applications: Automatic transactions This allows a set of objects to perform individual operations. Each object votes for the success or failure of the entire group of operations. If all objects vote for success, the entire transaction is committed; otherwise, the entire transaction is rolled back. Object pooling With object pooling, COM+ creates objects and keeps them in a pool, where they are ready to be used when the next client makes a request. This improves the performance of a server application that hosts objects that are frequently used but expensive to create. Just-in-time (JIT) activation The objective of JIT activation is to minimize the amount of time an object lives and consumes resources on the server. With JIT activation, the client can hold a reference to an object on the server for a long time, but the server creates the object only when the client calls a method on the object. After the method call is completed, the object is freed and its memory is reclaimed. JIT activation enables applications to scale up as the number of users increases. Role-based security In the role-based security model, access to parts of an application are granted or denied based on the role to which the callers belong. A role defines which members of a Windows domain are allowed to work with specific components, methods, or interfaces. Queued components The queued components service enables you to create components that can execute asynchronously or in disconnected mode. Queued components ensure availability of a system even when one or more subsystems are temporarily unavailable. Consider a scenario in which salespeople take their laptop computers to the field and enter orders on the go. Because they are in disconnected mode, these orders can be queued up in a message queue. When salespeople connect back to the network, the orders can be retrieved from the message queue and processed by the order-processing components on the server. Loosely coupled events Loosely coupled events enable an object (publisher) to publish an event. Other objects (subscribers) can subscribe to an event. COM+ does not require publishers or subscribers to know about each other. Therefore, loosely coupled events greatly simplify the programming model for distributed applications.
COM+ 1.5 (a part of Windows XP and Windows Server 2003) adds two additional features: Capability to run a COM+ application as a Windows service COM+ 1.5 enables you to configure a COM+ application to run as a Windows service. Capability to run a COM+ application as a Web service COM+ 1.5 can expose any COM+ component as an XML Web service as long as the component complies with Web services design guidelines. COM+ installs the Web service with IIS and generates the proper Web service configuration and information files.
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