Hi,
Usually I would look at writing a Windows Service to manage tasks that aren't suited to being hosted in a web application. These types of tasks are usually long running processes or scheduled tasks. Although this is normally the primary approach for these types of tasks, people have looked at ways of running these kinds of background processes in a web application by kicking off a number of threads in the Application_Start event exposed by Global.asax. The problem with this approach has always been that if your IIS worker process dies, then your background thread is killed too (effectively your 'Windows Service' is stopped until the next request is received).
ASP .NET 4.0 offers a solution to this problem. You can now set the StartMode to 'AlwaysRunning' as described in this blog post by Scott Gu. Somewhere in the comments on this post, someone asks a question about the viability of hosting Windows Service type tasks in IIS since the new feature ensures the worker process is always running. Scott mentioned that it would definitely support the scenario. Further to this, the recent introduction of AppFabric means that Microsoft themselves are providing simple hooks for hosting and monitoring WCF and WF services in a web application.
What does this mean for those of us that used to write Windows Services to support our web apps? Should we adopt this model? What are the pitfalls? As far as I can tell, there are a number of benefits to hosting 'Windows Service' processes in a web application, the most useful being the ease of deployment. Furthermore, we can actually start developing simple user interfaces to our services which provide information about what is happening at runtime.
If I had to go this route, I don't think that I would host my 'Windows Service' type functionality in the customer facing web application. I would probably develop a new web application project (much like I would in the Windows Service context) that would host my long running/scheduled task processes. I guess there are few reasons for this.
- Security. There may be a different security model for the UI displaying information about the running background processes. I would not want to expose this UI to anyone else but the ops team. Also, the web application may run as a different user which has an elevated set of permissions.
- Maintenance. It would be great to be able to deploy changes to the application hosting the background processes without impacting on user's using the front end website.
- Performance. Having the application separated from the main site processing user requests means that background threads will not diminish IIS's capability to handle the incoming request queue. Furthermore, the application processing the background tasks could be deployed to a separate server if required.
I would be really interested to hear your thoughts on this approach and whether I should be sticking with Windows Services. I am very tempted to try this new approach.