creating barcode vb.net Lesson 1: Defining Behavioral Contracts in Visual C#

Painting Data Matrix in Visual C# Lesson 1: Defining Behavioral Contracts

Lesson 1: Defining Behavioral Contracts
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message, it is processed asynchronously from that point on and the client is free to do other things. However, the way the WCF plumbing handles this means that the consumer actually blocks, even after it has sent its one-way message, until the dispatcher on the service side (the last in the chain of channels on the service side) has taken the message off its internal queue and dispatched it to a service instance in the form of a method call on that object. Thus, if the service happens to be backed up in processing messages, your one-way message will sit in its queue until it is processed, thereby blocking the caller until that point in time. This could even be done intentionally, to some extent, due to the throttling settings for the service. See 12, Concurrency for more about concurrency and throttling.
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The Duplex MEP is a two-way message channel whose usage might be applicable in either of these two situations: The consumer sends a message to the service to initiate some longer-running processing and then subsequently requires notification back from the service, confirming that the requested processing has been done. The consumer needs to be able to receive unsolicited messages from the service. Because there are two Service contracts to consider in a Duplex MEP scenario, namely the Service contract and the Callback contract, this MEP is declared by associating the Callback contract with the Service contract. This association is done through the CallbackContract property of the ServiceContractAttribute that is adorning the service that needs to be capable of calling back to a consumer. For an example of this, consider a simple Hello World application, this time where a consumer sends a one-way message to a greeting service, and at some point after that, the service calls back to the consumer and says, I have processed your greeting request and here it is; I am calling you back with it now. The following code shows not only the Service contract, the Callback contract, and how they are associated but also how the service implementation (that is, the service type) uses the OperationContext to ask for a reference to the Callback contract, thereby enabling the service to call back to the consumer. Here is the code:
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' VB <ServiceContract()> _ Public Interface IGreetingHandler <OperationContract(IsOneWay:=True)> _ Sub GreetingProduced(ByVal greeting As String) End Interface <ServiceContract(CallbackContract:= _ GetType(IGreetingHandler))> _ Public Interface IGreetingService
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Contracts
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<OperationContract(IsOneWay:=True)> _ Sub RequestGreeting(ByVal name As String) End Interface <ServiceBehavior(InstanceContextMode := _ InstanceContextMode.PerSession)> Public Class GreetingService Implements IGreetingService Public Sub RequestGreeting(ByVal name As String) _ Implements IGreetingService.RequestGreeting Console.WriteLine("In GreetingService.RequestGreeting") Dim callbackHandler As IGreetingHandler callbackHandler = OperationContext.Current.GetCallbackChannel( _ Of IGreetingHandler)() callbackHandler.GreetingProduced("Hello " + name) End Sub End Class // C# [ServiceContract] interface IGreetingHandler { [OperationContract(IsOneWay = true)] void GreetingProduced(string greeting); } [ServiceContract(CallbackContract = typeof(IGreetingHandler))] interface IGreetingService { [OperationContract(IsOneWay = true)] void RequestGreeting(string name); } [ServiceBehavior(InstanceContextMode = InstanceContextMode.PerSession)] class GreetingService : IGreetingService { public void RequestGreeting(string name) { Console.WriteLine("In Service.Greet"); IGreetingHandler callbackHandler = OperationContext.Current.GetCallbackChannel<IGreetingHandler>(); callbackHandler.GreetingProduced("Hello " + name); } }
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Lesson 1: Defining Behavioral Contracts
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Duplex Channels and Client-Side Proxies
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Several more steps are required on the client proxy side for duplex channels to be set up, and 4 discusses this in further detail. In the preceding code, don t worry too much about the instancing mode option declared within the ServiceBehaviorAttribute adorning the service type. For now, just understand that callback channels require a session to be established between the consumer and the service. You ll learn more about sessions, session management, and instancing in 10. As with the OneWay MEP, you must consider some points before deciding the Duplex MEP is right for your service operations. The following are just some of the points you should take into account before using the Duplex MEP: The Duplex MEP is problematic in the real world because the service needs a connection back to the client, something that is generally avoided for security reasons and often impossible because of intervening firewalls and Network Address Translation (NAT) problems. A Duplex MEP doesn t scale very well because, as mentioned previously, its usage depends upon the existence of a long-running session maintained between the consumer and the service, thereby taking you back to the client/server era and all the scaling problems that entailed. Service orientation works best when every single message exchange is an independent event and doesn t depend on any messages that went before it that is, when it is conversationally stateless. You lose interoperability. For instance, as Duplex MEP is implemented today with WCF callback channels, there is no chance at interoperability with clients on other platforms such as Java. Threading problems can easily occur when the operations on either end of the callback channel are not one-way. Note that in the example you looked at, all the operations involved were one-way.
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