How to use Selenium Grid to its full potential.
Selenium is a test automation tool that automates browsers. Selenium grid is built on the traditional set up of Selenium. It takes advantage of the following properties.
- The Selenium test, the application under test and the remote control/browser pair can communicate through HTTP, so they do not need to be collocated.
- The Selenium RC and the browser are never tied up to a specific application and hence, they can be easily shared through multiple applications and multiple projects.
The Selenium Grid transparently distributes tests across multiple physical and virtual locations so that they can be run in parallel. This speeds up the testing process, giving back quick and accurate feedback. Through Selenium Grid, the existing computer infrastructure can be leveraged. It enables running multiple tests in parallel, on multiple nodes/clients in a heterogenous environment of OS and browser support.
The Selenium Grid allows running of multiple parallel instances of WebDriver or Selenium RC. It makes all nodes appear as a single instance and so the tests do not need to understand the actual infrastructure. To run the grid the Selenium Server Standalone package includes
- The Hub
- The Web Driver
- The Selenium RC
The Selenium exposes an external interface similar to the traditional Remote Control. It acts like a central point which receives the entire test request and distributes it to the right nodes. It is responsible for
- Allocating a selenium RC to a specific test
- Limiting the number of concurrent test runs on each Remote Control
- Shielding the tests from the actual grid infrastructure.
Testing Practices with Selenium Grid
- Download the latest Selenium Server standalone JAR file. Here we are referring to version 2.25.0
- Use the following command to launch the Selenium Server. It will start the Selenium Server Hub role.
- java -jar selenium-server-standalone-2.25.0.jar -port 4444 -role hub -nodeTimeout 600
- On launching Selenium Standalone in the Hub role, it starts listening to nodes and Selenium testing. By default, the Hub provides 5 sessions on the server.
- For running the tests with Selenium Grid, the Selenium WebDriver tests use the class instances of RemoteWebDriver and DesiredCapabilites in order to define the browser, version and platform in which the tests will be executed on. Based on the preferences in the DesiredCapabilities, the Hub will redirect the test to the node matching the preferences. These preferences are set using the methods of DesiredCapabilities class. They are
- setBrowserName()
- setVersion()
- setPlatform()
In order to refine the number of available browsers, an additional browser parameter can be specified. This can also be used for specifying more than one browser as well.
The command will look like this:
java -jar selenium-server-standalone-2.42.0.jar -role node -browser browserName=safari -browser browserName=chrome -browser browserName=firefox -hub http://localhost:4444/grid/register
For setting up Selenium Grid to work with Internet Explorer or Chrome, additional configuration may be required for each. It is recommended to check out the browser driver documentation for this.
When using Selenium Grid, confusions can occur from time to time. Using Selenium Extras helps to clear these confusions and helps to fix issues which come up outside of the web browser. Some of its cleanup tasks are:
- After a complete test run it kills any remaining or crashed instances of the browser used.
- It modifies he browser settings on launch for a hassle free test execution.
- It periodically starts grid nodes to clear up memory leaks.
- It automatically records a video of the running test session at the OS level which helps to review the test failure more effectively.
Furthermore, restarting the whole OS after a certain amount of test runs allows to clear up any remaining crashed items. Restarting the entire machine after every 10 to 20 test runs gives a huge boost in stability.
Conclusion
The main advantage of Selenium Grid is to be able to run tests in parallel. It is to be noted that Selenium Grid does not by itself provide for parallelization. It can handle multiple connections, but the tester will need to find a way to set up and execute the tests in parallel. Some notable approaches include testing through a framework (TestNG in Java), testing through Continuous Integration or testing through third party library.
