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In the first part of this series on duplex communication in Silverlight 4, we did a deep dive in one of the available options offered by the platform, namely the Http Polling Duplex binding. It’s advised that you read that article first, since this second part continues where we left of in the first part.

To summarize the first part quickly, we saw that Silverlight applications in some situations can really benefit from duplex communication. In scenarios where the client needs to be updated because of changes in the state of the server side (for example a change in a database, a file change…), the best solution is working duplex.

One of the new features in the WCF stack in Silverlight 4 is an improvement of our PollingDuplex binding, which allows duplex communication over HTTP. HTTP is a request/reply communication medium, so some tricks are needed to make it look like a duplex transport, where both the client and the server can send messages to each other.

It enables user to look at the http traffic and diagnoses issues. Natively it provides Raw, Hex, Xml and some other views that understand specific formats. For the format it doesn’t understand, it provides an Inspector plugin model that user can easily implement and hook it up to display http package in a more readable format.  In this post, I will show you a new inspector that understands .NET framing used by Polling Duplex WCF in Silverlight 4. It also understands raw binary Xml used by WCF RIA services and binary soap message used in general WCF services.

Imagine you have a long task to accomplish and need to get updates from the webserver that is running the activity. This task may be an import or parsing of a complex file.  Doing this activity with a normal page or with a webservice may incur in timeouts and obviously leave the user waiting the end of the task without a visible notification

Did you know it is now easier to debug the server side PollingDuplex in Silverlight 4? With the addition of tracing, you can now get a peek at what is happening internally within your service. Traces from PollingDuplex will include information about what failed in communicating with your clients and how it was handled.

Silverlight 4 RC that shipped recently at MIX 2010 supports a new mode of the HTTP polling duplex protocol with greatly improved performance compared to the version in Silverlight 3. This post compares the performance of the three mechanisms for asynchronous data push from the server to the client available in Silverlight 4 RC: the net.tcp protocol, and the two modes of the HTTP polling duplex protocol (SingleMessagePerPoll and MultipleMessagesPerPoll).

In this video posted by Yavor Georgiev you can learn how to enable scale out between multiple web role instances.

Effectively Invensys used a WCF service hosted in a Windows Azure web role to tunnel bi-directional HTTP traffic from the Windows Azure platform AppFabric service bus capability to Silverlight clients. Unfortunately this workaround is needed since the TCP-based relay binding that service bus offers is not supported in Silverlight at this point.

Once Invensys was using PollingDuplex in Azure, they needed to scale to a large number of clients, so they needed to enable scale out between multiple web role instances. This capability is not available out-of-the-box in Silverlight 3, so some hackery was required.

In this article Andrea Boschin demonstrates how to avoid flooding in a Polling Duplex Service.

Returning on the PollingDuplex argument, today I would like to illustrate a tecnique I used to avoid the flooding in my Polling Duplex service. The problem come from the fact that my server retain a list of connected clients and is unable to detect the disconnection in a short time. After a while the notification from the server to the disconnected client will timeout so the server handle the timeout and remove the client from its list. But if for some reason a client Register itself multiple times the server notification list will grow and this may become a problem causing the service to slow down its response time.