In my previous post I discussed the merits of the COMET technique for pushing data to the browser over standard web protocols.
The basic idea behind COMET is that the browser keeps an HTTP connection open and the server uses it to send data back. This contrasts with HTTP’s Request/Response paradigm where the browser is specifically requesting data when it needs it. There are two main types of COMET – Streaming and Long-Polling. For Streaming COMET, the browser opens a single connection and keeps it open indefinitely, receiving events from the server when they are ready to be sent and immediately processing them. With Long-Polling, a connection is opened and is kept open until the server has data to send to the browser (or data is sent immediately if waiting). Once the data is received, a new connection is immediately opened which again blocks until data is ready to be received, and so on.
Over the last several years a protocol for COMET, known as Bayeux, has been formalised by the dojo foundation. The protocol specifies the wire format for messaging between client and server as well as the transport mechanisms involved, and supports both streaming and long-polling over standard HTTP transports. It’s essentially a specification for a pub/sub message bus, where clients can publish messages to channels and subscribe to messages on those channels. A number of client libraries have support for Bayeux style comet messaging, most notably the Dojo Toolkit.
There are also a number of server frameworks for COMET out there, including one from Dojo themselves – cometd - which implements Bayeux and integrates with their client framework. There are also commercial COMET servers out there, such as LightStreamer and Liberator, which actually offer .NET integration and support for integrating with enterprise messaging systems such as Tibco RV. However, searching around I have been unable to find any that run within ASP.NET (well definitely not free ones anyway) and I think it’s a very important thing to support within ASP.NET - custom servers are fine, but you can’t exactly run them on shared hosting providers!
One of the main challenges to developing a COMET server which runs within IIS is the threading model of ASP.NET (I believe Apache has the same issues, but not sure). With ASP.NET, each request is serviced by a single thread from the thread pool. This basically rules out being able to support Streaming COMET, as we would have to keep a thread aside for each client, and there a limited number of threads in the thread pool. So this leaves the Long-Polling option. This is wholly possible within ASP.NET as it has support for asynchronous processing, meaning we can put the thread back on the thread pool until we actually have data to send back to the client.
And this is what I am working on now. The project is called AspComet and is available at the AspComet Google Code Website via SVN. It’s pretty basic at the moment and it just supports a small test case, however it’s built against the Bayeux spec and therefore can be used with any client library which supports Bayeux (I am testing with dojo of course).
In the next post, I will describe a bit about how it works and walk through a sample simple chat application built using it.