Selenium is many things but at its core, it is a toolset for web browser automation that uses the best techniques available to remotely control browser instances and emulate a user’s interaction with the browser.
Although used primarily for front-end testing of websites, Selenium is at its core a browser user agent library. The interfaces are ubiquitous to their application, which encourages composition with other libraries to suit your purpose.
One of the project’s guiding principles is to support a common interface for all (major) browser technologies. Web browsers are incredibly complex, highly engineered applications, performing their operations in completely different ways but which frequently look the same while doing so. Even though the text is rendered in the same fonts, the images are displayed in the same place and the links take you to the same destination. What is happening underneath is as different as night and day. Selenium “abstracts” these differences, hiding their details and intricacies from the person writing the code. This allows you to write several lines of code to perform a complicated workflow, but these same lines will execute on Firefox, Internet Explorer, Chrome, and all other supported browsers.
Selenium’s minimalist design approach gives it the versatility to be included as a component in bigger applications. The surrounding infrastructure provided under the Selenium umbrella gives you the tools to put together your grid of browsers so tests can be run on different browsers and multiple operating systems across a range of machines.
Imagine a bank of computers in your server room or data center all firing up browsers at the same time hitting your site’s links, forms, and tables—testing your application 24 hours a day. Through the simple programming interface provided for the most common languages, these tests will run tirelessly in parallel, reporting back to you when errors occur.
It is an aim to help make this a reality for you, by providing users with tools and documentation to not only control browsers but to make it easy to scale and deploy such grids.
Many of the most important companies in the world have adopted Selenium for their browser-based testing, often replacing years-long efforts involving other proprietary tools. As it has grown in popularity, so have its requirements and challenges multiplied.
As the web becomes more complicated and new technologies are added to websites, it’s the mission of this project to keep up with them where possible. Being an open source project, this support is provided through the generous donation of time from many volunteers, every one of which has a “day job”.
Another mission of the project is to encourage more volunteers to partake in this effort, and build a strong community so that the project can continue to keep up with emerging technologies and remain a dominant platform for functional test automation.
Since then, Selenium has grown and matured a lot, into a tool widely used by many—if not most—of the largest organisations around the world. Selenium has gone from a homebrewed test automation toolkit developed at Thoughtworks for a niché audience and a specific use case, to the world’s de facto browser automation library.
Just as Selenium RC made use of the tools of the trade available at the time, Selenium WebDriver drives that tradition on by taking the browser interaction part to the browser vendor’s home turf and asking them to take responsibility of the backend, browser-facing implementations. Recently this work has evolved into a W3C standardisation process where the goal is to turn the WebDriver component in Selenium into the du jour remote control library for user agents.