September 17, 2011

The writeup by Simon Stewart from the Architecture of Open Source Applications discusses how the architecture of Selenium v2 was guided by the lessons drawn from Selenium 1. The figures and discussions on the Selenium also mention the fact that a proxy-server is used to get past cross-domain security policies. But how does Selenium v1 work under the covers?

Here's the general steps of what happens when you go from launching a Selenium RC (v1) server. Suppose you wrote this Python script (let's call on your local Linux box and launched a Selenium RC server on a Windows 7 machine.

from selenium import selenium

selenium = selenium(“", 4444, "*iexplore", "")

1. The Selenium RC server starts and waits for an HTTP connection from a client. When the client connects and sends an instruction to initiate new Selenium instance, the Selenium RC server will launch the browser and configure the browser's proxy settings.

2. Selenium v1 JavaScript code is also loaded into the browser. This JavaScript code is what is used by Selenium for performing all its automated testing (i.e. used to query DOM elements, generate mouse clicks, etc.). You can click View Source on the Selenium RC tab to see what JavaScript code is loaded.

Note that there's also a runTest() JavaScript command executed when this page is loaded. When this happens, an Ajax connection is also initiated in selenium-remoterunner.js:

nextCommand: function () {this.xmlHttpForCommandsAndResults = XmlHttp.create();
sendToRC(postResult, urlParms, fnBind(this._HandleHttpResponse, this), this.xmlHttpForCommandsAndResults);

During the configuration of this proxy, all /selenium-server URL's are intercepted and routed to the RC server. By using an Ajax connection that uses the same URL as the one that is being tested, Selenium v1 can therefore avoid triggering browser cross-domain security policies and establish a connection back to the Selenium RC server to sending/receiving future commands. You'll notice how this browser proxy configuration works if you tried to add a /selenium-server to any URL (i.e. would normally return back a Page Not Found error).

3. The RC server then opens a URL connection specified by the client API with a /selenium-server/core/Blank.html?start=true. (Note that when creating a Selenium instance, a specific URL must also be provided.) If this connection was successful, it also helps to verify that the proxy configuration was setup properly.

4. What happens if this Ajax connection from the browser to the RC server times out? If the request timeouts (i.e. no command), another Ajax request is sent and the channel is re-established. This is performed within the selenium-executionloop.js, which initiates a new Ajax connection back to the Selenium server.

continueTest : function() {LOG.debug("currentTest.continueTest() - acquire the next command");                                            
        if (! this.aborted) {                                                                                          
            this.currentCommand = this.nextCommand();                                                                  

6. When this communication channel is setup, Selenium clients sending an v1 API command will be sent to the server, which in turns relays the communicate on this Ajax channel to send/receive commands. Selenium v1 relies on JavaScript to simulate all browser interactions, and the appropriate command is executed. Selenium v2 avoids this issue entirely by using native drivers that bind tightly to the operating system...but this approach introduces new issues and complexities, which will be discussed in this week's upcoming event.

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