How to fire off and poll a windows service from asp.net page

Question

client wants an asp.net page that has a button to fire off a database update from an external source with hundreds of records. This process takes a long time. He also wants status update as the process runs, like "processing 10 out of 1000 records". In reading various articles, I'm thinking of putting the database update code in a windows service. I've never worked with windows services before and I can't find many tutorials on how to fire off a windows service and poll it from an asp.net page. My questions are is this the best way to handle this process? And, does anyone have any examples on how they've accomplished this?

Answer 1

There are a few ways to approach this.

You're right in that executing a long-running task within the Web's worker process doesn't usually end well: it ties up resources, the app pool can get recycled, etc. In most of my projects of any complexity, I usually end up with 4 pieces: the database, a DLL with my model, a "Worker" that is a Windows service, and an ASP.NET Web site.

The "Worker" is a Windows service that is always running and uses Quartz.net to execute scheduled tasks using the same model that the Web site uses. These can be all sorts of periodic tasks that seem to crop up when maintaining a Web site of any complexity: VacuumExpiredPickTicketsJob, BackupAndFtpDatabaseJob, SendBackorderReminderEmailsJob, etc.

Writing a Windows service is not difficult in C# (there is a built-in template in Visual Studio, but you pretty much inherit from ServiceBase and you're off to the races), and libraries like TopShelf make it even easier to deploy them.

What is left is triggering the update from the Web site and communicating the results back to the user. This can be as simple or as complicated as you want it to be. If this is something that has to scale up to lots of users, you might use something like MSMQ to queue up update commands to the Windows service, and the Windows service would respond to that queue. I get the impression that that is probably overkill here.

For a handful of users, you could override your service's OnCustomCommand(int command) method to be the trigger. Your Web site would then use ExecuteCommand() of the ServiceController class to get the process started. Your Web site and service would agree on the parameter value that means "do that update thing," let's say 142 (since it has to be a number between 128 and 255 for reasons of history).

As for communicating progress back to the client, it's probably easiest to just have the Web page use a timer and an AJAX call to poll for updated progress data. You can get fancy with new stuff like WebSockets (bleeding edge stuff as I write this) and long polling, but regular polling will simply work for something that doesn't need to scale.

Hope this helps!

Answer 2

In addition to Nicholas' thorough answer, another option is to deploy your back end processes as command line scripts, and schedule them to run through Window's built in task scheduler, which has improved quite a bit in Windows Server 2008+. Or you can use any other host of task scheduler applications.

I find the command line approach to be easier for MIS staff to understand and configure, and to migrate to new servers, versus standard Windows services.