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COM & .NET Component Services by Juval Lowy

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Chapter 8. COM+ Queued Components

COM+ Queued Components is a service that allows a client to call object methods asynchronously. The client is blocked only for a short duration while COM+ processes the request, and the object executes the call at a later point. You can think of queued components as asynchronous COM+.

Under classic COM and DCOM, all method calls on your object are synchronous—the client is blocked while the object executes the call. Classic COM developers often had to develop a proprietary mechanism for asynchronously invoking calls on their objects. One recurring mechanism had the object spin off a worker thread to process the client request and immediately return control to the client. The object would later signal the client somehow when the call completed (if the client needed to know), and the client had to distinguish between method completions of multiple objects.

Such solutions coupled the clients to the objects and were inconsistent. Different vendors provided slightly different solutions, requiring different programming models on the client side at times.

The first release of MTS and Microsoft Message Queue (MSMQ) in 1998 provided another way to support asynchronous object method calls with COM. MSMQ is a message queuing service that allows you to post messages from one queue to another, potentially across machines.

Clients and objects could use MSMQ to facilitate COM asynchronous method calls. With MSMQ, the client posts a message to a designated queue ...

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