Enabling the Queued Component in .NET

Encoder QR Code in .NET Enabling the Queued Component

Enabling the Queued Component
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Two settings on the Queuing tab of the application s Properties window are crucial for enabling QCs, as shown in Figure 31-19: the Queued setting must be activated for the application to be reachable via MSMQ queues, and the Listen setting must be activated for the listener object to be instantiated on the server. You can enable the former but leave the latter disabled to avoid server components being activated only during cer tain time periods (for example, on weekends or at night). If you re working with COM+ 1.5, you can also set the maximum number of concurrent players that is, the maximum number of threads that will process incoming messages.
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31:
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Serviced Components
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Figure 31-19
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The Queuing tab of the application s Properties window
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When you click OK to apply your changes, COM+ creates all the MSMQ queues that are needed to process incoming messages, as shown in Figure 31-20.
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Figure 31-20
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The queues created for a COM+ application that can contain queued components
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When creating a .NET serviced component, you can enable queuing and set all rele vant values by means of an ApplicationQueuing attribute at the assembly level:
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This syntax lets you control all values. <Assembly: ApplicationQueuing(Enabled:=True, QueueListenerEnabled:=True, _ MaxListenerThreads:=10)> This syntax enables queuing but disables the listener object and uses zero for MaxListenerThreads (the default, no limit is enforced). <Assembly: ApplicationQueuing()>
Part VII:
Advanced Topics
Defining the Queued Interface
Clients can call asynchronously only the methods that belong to an interface for which queuing is enabled, and therefore you must define an interface that contains such methods:
Public Interface IMoveMoney Sub MoveMoney(ByVal senderID As Integer, ByVal receiverID As Integer, _ ByVal amount As Decimal) End Interface
Notice that the interface must be visible through COM Interop, and therefore it must be public and not marked with a ComVisible(False) attribute. Next you use the InterfaceQueuing attribute on the class that contains the methods to be invoked asynchronously:
<InterfaceQueuing(Interface:="IMoveMoney ), EventTrackingEnabled() > _ Public Class AsyncMoneyMover Inherits ServicedComponent Implements IMoveMoney Sub MoveMoney(ByVal senderID As Integer, ByVal receiverID As Integer, _ ByVal amount As Decimal) Implements IMoveMoney.MoveMoney UpdateAccount(senderID, -amount) UpdateAccount(receiverID, amount) End Sub End Class
You can also mark an interface as queued from the MMC snap-in, on the Queuing tab of the interface s Properties window. Notice that you can even mark individual meth ods of the interface as queued. All the methods in the interface undergo the limitations I explained previously that is, they must be Subs and take only ByVal arguments. If one or more methods don t comply with these limitations, you get an error when the component is registered in the COM+ catalog. When you ve completed the definition of the queued component, you should compile the project, install the assembly in the GAC, and register it with the Regsvcs tool.
Using the Queued Component
Clients don t instantiate queued components as they do regular COM+ components. Instead, they use the Marshal.BindToMoniker shared method with the following syn tax:
Create an instance of the component, and cast to the queued interface. Dim mover As BankComponent.IMoveMoney = DirectCast(Marshal.BindToMoniker( _ queue:/new:BankComponent.AsyncMoneyMover ), BankComponent.IMoveMoney ) Invoke the method(s) asynchronously. mover.MoveMoney(1, 2, 12000) mover.MoveMoney(1, 3, 7000) Activate the QC by forcing the release of the component. Marshal.ReleaseComObject(mover) mover = Nothing
31:
Serviced Components
The string that you pass to the BindToMoniker method can include optional values that let you decide the computer on which the QC is instantiated. For example, the follow ing string creates the QC on a computer whose name is MyServer:
queue:ComputerName=MyServer/new:BankComponent.AsyncMoneyMover
See the MSDN documentation for additional arguments you can pass in this string.
MSMQ Messages
Queued components hide the complexity of asynchronous programming by offering a high-level view of MSMQ and its messages. If you need to exert more control over your application, however, you can manipulate MSMQ messages directly.
Creating a Queue
You can create an MSMQ queue either manually from the Computer Manager MMC snap-in or via code. In the first case, you right-click on the Private Queues node, choose the New command, and assign the new queue a name. (See the left portion of Figure 31-21.) After creating the queue, you can set a few more attributes in its Prop erties window, most notably its label, as shown in the right portion of Figure 31-21.
Figure 31-21
Creating a new private queue (on the left) and setting its properties (on the right)
To access MSMQ queues programmatically, you must add a reference to the Sys tem.Messaging.Dll library and insert an Imports statement in the System.Messaging namespace. The most important class in this namespace is MessageQueue, which exposes shared methods for creating a queue, checking whether a queue exists, and enumerating all the public and private queues on a machine. This class also exposes instances of read-only properties that return information about a specific queue, such as FormatName, Id, Transactional, and CreateTime:
Part VII:
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