visual basic 6.0 barcode generator Fundamentals of a Web Crawler in Java

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Fundamentals of a Web Crawler
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Despite the numerous applications for Web crawlers, at the core they are all fundamentally the same. Following is the process by which Web crawlers work:
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1. Download the Web page. 2. Parse through the downloaded page and retrieve all the links. 3. For each link retrieved, repeat the process.
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6: Crawling the Web with Java
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Now let s look at each step of the process in more detail. In the first step, a Web crawler takes a URL and downloads the page from the Internet at the given URL. Oftentimes the downloaded page is saved to a file on disk or put in a database. Saving the page allows the crawler or other software to go back later and manipulate the page, be it for indexing words (as in the case with a search engine) or for archiving the page for use by an automated archiver. In the second step, a Web crawler parses through the downloaded page and retrieves the links to other pages. Each link in the page is defined with an HTML anchor tag similar to the one shown here: <A HREF="http://www.host.com/directory/file.html">Link</A> After the crawler has retrieved the links from the page, each link is added to a list of links to be crawled. The third step of Web crawling repeats the process. All crawlers work in a recursive or loop fashion, but there are two different ways to handle it. Links can be crawled in a depth-first or breadth-first manner. Depth-first crawling follows each possible path to its conclusion before another path is tried. It works by finding the first link on the first page. It then crawls the page associated with that link, finding the first link on the new page, and so on, until the end of the path has been reached. The process continues until all the branches of all the links have been exhausted. Breadth-first crawling checks each link on a page before proceeding to the next page. Thus, it crawls each link on the first page and then crawls each link on the first page s first link, and so on, until each level of links has been exhausted. Choosing whether to use depthor breadth-first crawling often depends on the crawling application and its needs. Search Crawler uses breadth-first crawling, but you can change this behavior if you like. Although Web crawling seems quite simple at first glance, there s actually a lot that goes into creating a full-fledged Web crawling application. For example, Web crawlers need to adhere to the Robot protocol, as explained in the following section. Web crawlers also have to handle many exception scenarios such as Web server errors, redirects, and so on.
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Adhering to the Robot Protocol
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As you can imagine, crawling a Web site can put an enormous strain on a Web server s resources as a myriad of requests are made back to back. Typically, a few pages are downloaded at a time from a Web site, not hundreds or thousands in succession. Web sites also often have restricted areas that crawlers should not crawl. To address these concerns, many Web sites adopted the Robot protocol, which establishes guidelines that crawlers should follow. Over time, the protocol has become the unwritten law of the Internet for Web crawlers. The Robot protocol specifies that Web sites wishing to restrict certain areas or pages from crawling have a file called robots.txt placed at the root of the Web site. Ethical crawlers will
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reference the robot file and determine which parts of the site are disallowed for crawling. The disallowed areas will then be skipped by the ethical crawlers. Following is an example robots.txt file and an explanation of its format:
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# robots.txt for http://somehost.com/ User-agent: * Disallow: /cgi-bin/ Disallow: /registration Disallow: /login
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