Automated Data Collection with R: A Practical Guide to Web Scraping and Text Mining

6 AJAX

At this point in the book you are familiar with HTML's versatile markup vocabulary for structuring content (Chapter 2) and HTTP, the primary protocol for requesting information from web servers (Chapter 5). In combination, these two technologies not only provide the foundation for virtually all web services, but they also define a reliable infrastructure for disseminating information throughout the Web.

Despite their popularity, HTML/HTTP impose strict constraints on the way users access information. If you abstract from the examples we have previously introduced, you find that the HTML/HTTP infrastructure implies a rather static display of content in a page layout, which is retrieved through sequential, iterative requests initiated by the user. The inherent inflexibility of HTML/HTTP is most apparent in its inability to create more dynamic displays of information, such as we are used to from standard desktop applications. After receiving an HTML document from the server the visual appearance of the screen will not change since HTTP provides no mechanism to update a page after it has been downloaded. What impedes HTML/HTTP from providing content more dynamically is its lack of three critical elements:

1. a mechanism to register user behavior in the browser (and not just on the server);
2. a scripting engine to formulate responses to those events;
3. a more versatile data requesting mechanism for fetching information asynchronously.

Because HTML/HTTP is technically unable to ...