WCF Reliable Messaging Demo

I recently delivered an 'Introducing WCF' presentation to one of my clients and wanted to find and create interesting demonstrations that didn't make WCF look totally boring next to the WPF stuff I had lined up for the session directly afterwards.

I came across Shy Cohen's cool demonstration of reliable messaging that breaks an image into chunks and sends the chunks to a WCF service which rebuilds the image. However, he also created a new channel for the client that would randomly drop messages so the image would arrive incomplete. Naturally, you could then turn on reliable messaging and the image would arrive just fine despite

This was perfect, visual and a great demonstration of reliable messaging. However, I thought it could be pushed a bit harder and include a demonstration of the reliable ordering of messages.

This is what it looks like and, yes, of course I used WPF :).

As you can see, the some of the messages have failed to be delivered (2 out of 25) and some of those that have arrived have done so in the wrong order. Now, if we turn on reliable messaging in both the client and server's app.config file...

<reliableSession ordered="false"/>

Now, all the images have arrived but in a bit of a jumble. So let's turn on ordered messages too...

<reliableSession ordered="true"/>

Hooray! Our messages have arrived intact and in the correct order.

How it works

It actually took me a while to work out how I would achieve the jumbling of messages. After all, messages were unlikely to get jumbled up over the wire when both the client and server are on the same machine. Clearly, I was going to have to intervene.

Since my sends are synchronous (blocking call until the individual message has been delivered) I use a ThreadPool thread for each send and then put a random sleep somewhere in the send to jumble up the messages. Only problem is that the ThreadPool doesn't guarantee that WorkItems will be processed in the order they were delivered so there was every chance that my messages would arrive into the WS-ReliableMessaging mechanisms in the wrong order to start with.

I needed to ensure that messages made it through the majority of the channel and binding stack in the correct order, but then got jumbled at the very last minute with my random sleep (up to 250ms). Once I realised this it was easy enough to adjust the custom binding Shy used in his demo and add a little bit of synchronisation between MessageInterceptor and the calling code (admittedly not best practice but then how often do you wanna jumble messages?).

Why not check out the code for yourself: Download the source.

Please bear in mind that the jumbling and dropping of messages is random so sometimes they all arrive just fine and in the correct order.