Web Application Testing with Selenium

At first glance, it looks like Selenium RC supports enough parallelism that any additional distributed processing capability would not be needed. After all, a single Selenium RC server allows you to open a number of parallel sessions (that is, drive a number of browsers at the same time) and a single Selenium RC client. In addition to being able to work with multiple concurrent sessions of one server, it can communicate with multiple servers at the same time.

However, in practice, running more than six browsers on the same Selenium RC server is not advisable due to performance issues. Additionally, managing a large number of Selenium RC servers is a major headache and does not scale very well. This is where Selenium Grid can help.

Selenium Grid introduces another component to the Selenium architecture—Selenium Hub, which manages a pool of available Selenium Remote Control entities and is responsible for the following:

As far as your RC client programming is concerned, the move from Selenium RC to Grid requires minimal code changes. All you have to do is to change the infamous browser string parameter. For instance, change "*firefox" to something like "Firefox on Windows" or "Safari on Mac".

Selenium Hub's configuration is a bit more complex. First, you have to modify the grid_configuration.yml file. Let's say you want to use two RC instances—one with Firefox on Linux and another with Internet Explorer on Windows. In that case, your configuration file will look like this:

hub:
   port: 4444
   environments:
       - name:    "Firefox on Linux"
         browser: "*firefox"
       - name:    "IE on Windows"
         browser: "*iexplore"

After that, you should use ant to launch the Selenium Hub by running ant launch-hub on the hub machine. The RC instances are created by running the following commands, one on a Linux machine and one on a Windows machine.

On the Linux machine:

ant -Denvironment="Firefox on Linux" \
    -DhubURL=http://<hub-IP-address>:4444 \
        launch-remote-control

On the Windows machine:

ant -Denvironment="IE on Windows" \
    -DhubURL=http://<hub-IP-address>:4444 \
        launch-remote-control

After that, you can code your RC client to use any of the above RC server instances via the hub.

Comprehensive description of the Selenium API is beyond the scope of this article, but the list below demonstrates what the framework is capable of:

For more details, check the Perl API link at the end of the article. API documentation for other languages is available as well. If you find that the API is lacking somewhere, you always can extend it by executing your own JavaScript functions using $sel->get_eval($script).