Web services and persistent client connections.

I don't know how common this is, but this is a technique I've used on two projects recently that I found useful.  In both situations, I was writing a client application that needed to talk to a server, where the client was possibly behind a firewall, and the connection needed to be persistent.

Consider designing an instant messaging application, that you wanted to work over HTTP.  Web Services give you an easy way to connect and send messages, and protocols built on top of web services like security and authentication lets you handle those issues without having to re-invent the wheel.

But a requirements of this sort of application is that the server be able to deliver messages to the client asynchronously, and web services don't really support that.  The client is behind a firewall, so the server can't just make a service call, and there's no way in the web services world to connect and then wait for multiple response messages (or at least, not that I could see..).

So what I ended up doing is having the client connect via HTTP using a WebRequest sending the client's authentication information and a unique ID, and then keeping that connection open indefinitely.

When the client connects to establish the backchannel, the Global.Session_Start event inside the web service application creates a ClientConnection object containing the user's identity, an AutoResetEvent and a StringCollection, and adds this object to a list of ConnectedClients stored in application state.  Then the Page_Load locates the ClientConnection and does a WaitOne() on the event, basically going to sleep until the server has something to send to this client.

When the server does have something for the client, it locates the client in the ConnectedClients list, generates the message to send (all my messages are strings), adds it to the StringCollection, and then sets the event.  The Page_Load method wakes up, sends any queued messages, does a Response.Flush() to get the server to flush any buffered data (send it to the client immediately) and goes back to sleep.

Meanwhile on the client side, it's reading from the WebResponse that was created when it connected, so it's receiving each string as the server sends it. 

I've never seen this technique used elsewhere, but I find it useful for these reasons:

  • Management of connection state is pretty much taken care of by the web server - a session is created when the user connects, and it's destroyed when the user disconnects. 
  • Your web services calls are being handled by the same process that has the persistent connections back to all the connected clients.  No IPC required.  This is cool.
  • You get to leverage SSL in the web server for your back channel.
  • It's easy to create a monitoring webpage that shows the state of the server including all the connected clients, since the data is right there in the web app.

There are some drawbacks.

  • IIS doesn't check for the client being disconnected except when you send data.  I'm still looking for a good solution to this problem; one thing that works is to wake your process up every now and then and send a single byte to the client.  This will also help with anything along the way (like a firewall) that's noticing that an HTTP connection has been up for a very long time with no data travelling across it, so maybe it's not so bad.
  • Blocking in Page_Load means there's a thread per client.  ASP.NET has limits on the number of worker threads it will create - I've raid the limit quite a bit for my own app, but this is obviously not great.. I'm looking for a way around this as well.

Anyway I find this technique useful but the drawbacks are pretty severe - especially the thread per client.  If I can't find a way around that, then I'll probably end up trying to launch a thread in my Global.Application_Start that creates a listen connection and then use that for my back channel (since I like the idea of no IPC, just thread synchronization inside the server process for enumerating clients and sending data to them).