Sunday, July 18, 2010

Selenium RC and ASP.NET MVC 2: Controller Invoked Twice

Admittedly MVC (as of writing I use ASP.NET MVC 2) has been designed from the ground up for automated testability (tutorial video about adding unit testing to an MVC application). For example you can test a controller without even launching the ASP.NET development web server. After all a controller is just another class in a .NET assembly.

However, at some point you may want to ensure that all the bits and pieces work together to provide the planned user experience. That is where acceptance tests enter the stage. I use Selenium for this, and a few days ago I hit an issue that turned out to be caused by Selenium server version 1.0.3. Here are the details.

The symptom that I observed was that a controller action was hit twice for a single Selenium.Open(…) command. First I thought that my test was wrong, so I stepped through it line by line. But no, there was only one open command for the URL in question. Next I checked my implementation, whether maybe accidentally I had created some code that implicitly would call or redirect to the same action. Again, this wasn’t the case as each time when I hit the break point on the action controller there was nothing in the call stack.

Then I used Fiddler (a web debugging proxy) for a while and yes, there were indeed a HEAD request and a GET request triggered by the Selenium.Open(…) command. And even more interesting, when I ran my complete test suite I found several cases where the GET request was preceded by a HEAD request for the same URL.

The concerning bit, however, was that I couldn’t find a way how to reproduce this with a browser that I operated manually. Only the automated acceptance tests through Selenium RC created this behavior.

For a moment I considered trying to use caching on the server side to avoid executing the action more than once. But then I decided to drill down to get more details. In global.asax.cs I added the following code (Of course you can use loggers other than log4net):

protected void Application_BeginRequest() {
   Log.InfoFormat("Request type is {0}.", Request.RequestType);
   Log.InfoFormat("Request URL is {0}.", Request.Url);

private static readonly log4net.ILog Log =

As a result I was able to track all requests. Of course you wouldn’t want to do this for production purposes. In this case I just wanted to have more information about what was going on. It turned out that Fiddler was right as I found several HEAD requests followed by a GET request.

After some research I came across a discussion about Selenium RC head requests and it turned out that this was a known issue in Selenium server version 1.0.3. As of writing this was fixed in trunk and I thought for a moment about building from trunk but then decided on a different path. And that solution worked as well: Instead of using version 1.0.3 I am now using Selenium Server version 2.0a5 plus the .NET driver from the 1.0.3 package.

So here is what you need to download:

  1. Selenium Remote Control 1.0.3 which includes the .NET driver. Don’t use the server from this download.
  2. Selenium Server Standalone 2.0a5. Use this jar file as your server. The command line at the Windows command prompt changes from “java –jar selenium-server.jar” to “java -jar selenium-server-standalone-2.0a5.jar”.

Then start the 2.0a5 server and run your tests. The HEAD/GET issue should be gone. In my case it was and I’m now back to extending my test suite finally making progress again.

My configuration: Visual Studio 2010, .NET 4.0, ASP.NET MVC 2, Vista Ultimate 32, various browsers (IE, Firefox, Chrome, Opera, Safari). The issue I describe here may be different than the one you observe. Consequentially it is possible that this suggested solution doesn’t work for you.


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