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The debate of performance versus maintainability always exists on a continuum. In this environment, way off to the side of this continuum that favors performance at the expense of maintainability is DCOM. Oddly enough, exporting DCOM proxies and invoking them via an interop assembly from the client is still the best-performing option for remote procedure calls. A DCOM package can be exported from COM+ using the Export option on the context menu of a COM+ application, as shown in Figure 8-4. Notice we ve selected the Application proxy radio button. This causes the wizard to create an installable package of COM+ proxies, which are wrapper types that expose the interface of the configured components on the client, but marshal the actual method call across the network to the Component Services server using the DCOM RPC mechanism.
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CHAPTER 8 HOSTING AND COMMUNICATIONS
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Figure 8-4. The COM+ Application Export Wizard This option is only available on server applications, because when you re using DCOM you re relying on DLLHost.exe to act as your host. Remember that a library application is created in the process of the caller. Since you re not using ASP .NET as the host, there is no server-side process within which to create the library application types. DLLHost.exe provides the dedicated process, listening for requests coming in from the network. Managed clients need to access these proxy types via a COM interop assembly, as they are exported as COM libraries. It s hard to believe this performs twice as fast as the fastest Remoting configuration, considering you re introducing two layers of interoperability between managed and unmanaged code: one on the client to go from managed code to the COM proxy, and another on the server to move from the COM-based Component Services call stack to the managed call stack of our Serviced Components. This performance comes at a high price for maintainability. DCOM is notoriously difficult to configure correctly, especially if there are firewalls or complex security requirements in place. We won t be digging into the details here. This technology has been around for a decade, so there are volumes of information and references available for its usage. Hosting in ASP .NET should be considered as a first option if it will meet the performance demands of your environment. The DCOM option should really only be on the table for rich client applications; with a web application, it almost always makes more sense to host COM+ in-process on the web server. Possible exceptions to this would show up in environments with a lot of web servers running a lot of different applications. In these cases, it may be possible, for example, that you want the data access layer isolated on a single server to pool connections to the database. You should still consider ASMX for exposing this data access layer via SOAP before using DCOM. Remoting may even be a better option. It s not as easy to configure as ASMX, but it s still significantly simpler than DCOM. When performance is the primary concern, however, DCOM is still about twice as fast as Remoting for crossing machine boundaries.
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CHAPTER 8 HOSTING AND COMMUNICATIONS
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Message Queuing
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Message Queuing (MSMQ) is Microsoft s messaging solution. Access to queues is exposed in the Framework via types in the System.Messaging namespace. This is one of the two main ways in .NET to leverage MSMQ. The other is to use Enterprise Services, which provides a layer of abstraction on top of queues and their messages by automatically transforming method calls into messages when they re configured as Queued Components (QC) (see 7 for details on Queued Components). There are many tangible benefits to a message queuing infrastructure. Client functionality is not tied to server availability. Since message queuing is done asynchronously from the perspective of the client, if the server is unavailable or under a heavy load, the client is not required to wait for the server to become available before continuing processing. This is very handy for dealing with times of high load on the server. Requests for services are serialized in the message queue, and clients continue to work without waiting for their requests to be processed. You also get guaranteed delivery of your service request. Since the message is persisted to disk when it arrives at the server, even if the server is unavailable, the message will be processed when it comes back online. The downside is that there can be no response returned from the service request. This is intrinsic in the design of any asynchronous messaging infrastructure, and it seriously limits the number of operations that are viable for service requests. In some situations the client/ server paradigm can be flipped around, and the client can have its own queues exposed to receive messages back from the server (see Figure 8-5). We ll show you an example of this later in the chapter.
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