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4 ” Introduction A ppropriate U se of W eb Scraping Some data pro viders may only offer a web site while others may offer APIs that do not offer equivalent data in a conducive format or at all. Some web services may involve an extensive authentication process or be unavailable for public consumption. U nreliability of web service endpoints compared to web sites may also make them unfeasible to use. It is in situations like these that web scraping becomes a desirable alternative. It is common practice to avoid making changes that break backward-compatibility with existing applications that use web service APIs, or to version them to allo w application vendors time to transition to new revisions. As such, web services are significantly less prone to be altered than the markup structure of pages on a web site, especially without advance warning. This is especially true of sites that change frequently, which can drastically affect the stability of applications that emplo y web scraping. In a nutshell, web scraping should be used as a last resort when no other feasible options for acquiring the needed data are available. Legality of W eb Scraping The answer to this question is a bit extensive and veers off into “legal land.” As such, it is included in Appendix A to avoid detracting from the primary purpose of the book. Topics Co ver ed You ’re ob viously reading chapter 1 no w , which pro vides a brief introduction to web scraping, answers common questions, and leads into the meat of the book. • Chapter 2 deals with relevant details of the HTTP protocol, as HTTP clients are used in the process of document retrieval. This includes ho w requests and responses are structured and various headers that are used in each to implement features such as cookies, HTTP authentication, redirection, and more.
Introduction ” 5 • Chapters 3-7 co ver specific PHP HTTP client libraries and their features, usage, and advantages and disadvantages of each. • Chapter 8 goes into developing a custom client library and common concerns when using any library including prevention of throttling, access randomization, agent scheduling, and side effects of client-side scripts. • Chapter 9 details use of the tidy extension for correcting issues with retrieved markup prior to using other extensions to analyze it. • Chapters 10-12 review various XML extensions for PHP, compare and contrast the two classes of XML parsers, and pro vide a brief introduction to XP ath. • Chapter 13 is a study of CSS selectors, comparisons between them and XP ath expressions, and information on available libraries for using them to query markup documents. • Chapter 14 explores regular expressions using the PCRE extension, which can be useful in validating scraped data to ensure the stability of the web scraping application. • Chapter 15 outlines several general high-level strategies and best practices for designing and developing your web scraping applications.
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Chapter 13 CSS Selector Libraries T
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