Architectural Overview

Dj edited this page Mar 16, 2015 · 1 revision

Architectural Overview and Design Principles

Focus on the User

The WebDriver APIs are focused on driving the browser from the user's point of view. For this reason, you won't find methods such as "fireEvent", and this is why "getText" attempts to return the text as a user would see it. This is also why it's important that installation of webdriver be as simple as possible.

Use a "Best Fit" Language

The key idea behind the various implementations of WebDriver is that each browser has a language that is most natural to use when attempting to drive it. All of the drivers are built with the idea that as much as possible should be done in this "best fit" language, while the implementation that the user sees is a thin wrapper around this. This may be represented as:

Browser "Best Fit" Language
Firefox Javascript in an XPCOM component
Internet Explorer C++ mainly using the IE Automation APIs, but occasionally using other features of Windows

In addition, some of the languages that WebDriver is offered in (notably Java) have something that supports simulating a browser. These are generally modeled using composition.

A Layered Design

In order to be useful, WebDriver must not make the user learn all the different implementation languages --- they don't care and they shouldn't have to. In order to be easy to write and maintain, as much logic as possible needs to be done in the "best fit" language of the browser. This naturally leads to a design where the API presented to the user is a thin wrapper around the core of each driver.

One obvious benefit to this design is that writing a language binding for WebDriver becomes a matter of bridging to the "best fit" language. Another benefit is that it becomes very easy to work on a single driver, given you understand the "best fit" language. For example, it's possible to work on the FirefoxDriver completely independently of the InternetExplorerDriver. Better, once a feature in a driver is working in one binding language (for example, Java), it should be easy to add that support to other binding languages.

More detailed information about each driver, how to write language bindings and its implementation can be found on their pages:

  • ChromeDriver
  • FirefoxDriverInternals
  • HtmlUnitDriverInternals
  • InternetExplorerDriverInternals

Reducing the Cost of Change

As even a cursory read of the architectural documents suggest, there are multiple, largely independent implementations of webdriver, as well as the original Selenium 1.x implementation. The goal for reducing the cost of change on the project is to share as much code as possible. Fortunately, a lot of the tasks we want to perform are "read only", querying the current state of the DOM. It makes sense to write these shared functions in Javascript.

In order to do this, we are working on AutomationAtoms (atoms). These take the form of a Javascript library that makes use of Google's Closure. There are two use cases for the atoms:

  1. A "monolithic driver", such as Selenium 1.x or the Firefox driver, where the bulk of the driver is written in JS.
  2. Augmenting an existing driver written in another language (eg: the InternetExplorerDriver), where we want to use tiny fragments of highly compressed JS in order to reduce execution and parsing time.

Closure comes with the Closure compiler, which allows us to take a Javascript library and compile it to minimized and optimized Javascript, as offering the ability to generate an unoptimized version. We shall be making use of this capability to support both use cases.