Keeping Connections Open in IIS

My web service needs to periodically broadcast messages to clients. The service is an Internet-facing application hosted inside of IIS. What's the best way to do this?

The big limitation in this scenario is that your clients might be behind a firewall and non-addressable. There are basically two architecture camps for broadcasting messages to clients over the Internet. The push architecture camp has the clients maintain a continuous connection to the server and pushes data out at each update. The pull architecture camp has the clients periodically poll the server to see if there's any new data. Both of these architectures are widely used and they trade latency versus resources off of each other. There are a few other architectures that work locally as well but aren't as useful over the Internet, such as multicasting and callbacks. I'm just going to pick one of these and talk about using a push architecture. The basic way to build a push architecture is to have clients connect to the server and then the server holds the connection open indefinitely to send messages.

If your service is hosted in IIS version 6 or below, then you don't have a lot of choice about the network protocol. Pushing data from the server to client is difficult with HTTP because the protocol is built on top of the request-reply model. A typical way of using HTTP to push is to make an empty request and send back a response using the chunked transfer encoding. Chunked transfers allow an HTTP server to send the response in pieces without having to specify the total length of the response up front. Normally, the client knows the message is done when the connection is closed or the pre-announced content length is reached. Neither of those options work in this case. Instead, the server needs to define some framing protocol so that the client can tell when the individual messages are done. The easiest way to get chunked HTTP transfers in WCF is to use the HTTP transport with streaming enabled.

If your service is hosted in IIS 7 (the Vista/Longhorn version of IIS), then you are able to pick other network protocols for your service, such as TCP. TCP is inherently duplex so it works across the Internet for server-initiated transfer of messages without having to connect back through a firewall. Since TCP is duplex, you can write a duplex contract that gives you a very nice programming model for sending the broadcasts. This is a lot less work on your part than the equivalent setup needed with HTTP.

Next time: Configuring IIS for Transport Security

Comments

• Anonymous
  October 20, 2006
  How do I configure my service to listen on all of the host names for my machine? How do I configure my

• Anonymous
  October 26, 2006
  The comment has been removed

• Anonymous
  September 24, 2007
  The comment has been removed