web crawler and its types - Search
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    GeeksForGeeks
    https://www.geeksforgeeks.org › what-is-a-webcrawler-and-where-is-it-used

    What is a Webcrawler and where is it used? - GeeksforGeeks

    What are some examples of web crawler tools?
    Web crawler tools can be desktop- or cloud-based. Some examples of web crawlers used for search engine indexing include the following: Amazonbot is the Amazon web crawler. Bingbot is Microsoft's search engine crawler for Bing. DuckDuckBot is the crawler for the search engine DuckDuckGo. Googlebot is the crawler for Google's search engine.
    techtarget.com
    Web crawler - Wikipedia

    The behavior of a Web crawler is the outcome of a combination of policies:
    • a selection policy which states the pages to download,
    • a re-visit policy which states when to check for changes to the pages,
    • a politeness policy that states how to avoid overloading websites.
    • a parallelization policy that states how to coordinate distributed web crawlers.
    Given the current size of the Web, even large search engines cover o…

    The behavior of a Web crawler is the outcome of a combination of policies:
    • a selection policy which states the pages to download,
    • a re-visit policy which states when to check for changes to the pages,
    • a politeness policy that states how to avoid overloading websites.
    • a parallelization policy that states how to coordinate distributed web crawlers.
    Given the current size of the Web, even large search engines cover only a portion of the publicly available part. A 2009 study showed even large-scale search engines index no more than 40–70% of the indexable Web; a previous study by Steve Lawrence and Lee Giles showed that no search engine indexed more than 16% of the Web in 1999. As a crawler always downloads just a fraction of the Web pages, it is highly desirable for the downloaded fraction to contain the most relevant pages and not just a random sample of the Web.

    This …

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    Wikipedia

    A Web crawler, sometimes called a spider or spiderbot and often shortened to crawler, is an Internet bot that systematically browses the World Wide Web and that is typically operated by search engines for the purpose of Web indexing (web spidering).

    Web search engines and some other websites use Web crawling or spidering software to update their web content or indices of other sites' web content. Web crawlers copy pages for processing by a search engine, which indexes the downloaded pages so that users can search more efficiently.

    Crawlers consume resources on visited systems and often visit sites unprompted. Issues of schedule, load, and "politeness" come into play when large collections of pages are accessed. Mechanisms exist for public sites not wishing to be crawled to make this known to the crawling agent. For example, including a robots.txt file can request bots to index only parts of a website, or nothing at all.

    The number of Internet pages is extremely large; even the largest crawlers fall short of making a complete index. For this reason, search engines struggled to give relevant search results in the early years of the World Wide Web, before 2000. Today, relevant results are given almost instantly.

    Crawlers can validate hyperlinks and HTML code. They can also be used for web scraping and data-driven programming.

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    A web crawler is also known as a spider, an ant, an automatic indexer, or (in the FOAF software context) a Web scutter.

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    A Web crawler starts with a list of URLs to visit. Those first URLs are called the seeds. As the crawler visits these URLs, by communicating with web servers that respond to those URLs, it identifies all the hyperlinks in the retrieved web pages and adds them to the list of URLs to visit, called the crawl frontier. URLs from the frontier are recursively visited according to a set of policies. If the crawler is performing archiving of websites (or web archiving), it copies and saves the information as it goes. The archives are usually stored in such a way they can be viewed, read and navigated as if they were on the live web, but are preserved as 'snapshots'.

    The archive is known as the repository and is designed to store and manage the collection of web pages. The repository only stores HTML pages and these pages are stored as distinct files. A repository is similar to any other system that stores data, like a modern-day database. The only difference is that a repository does not need all the functionality offered by a database system. The repository stores the most recent version of the web page retrieved by the crawler.

    The large volume implies the crawler can only download a limited number of the Web pages within a given time, so it needs to prioritize its downloads. The high rate of change can imply the pages might have already been updated or even deleted.

    The number of possible URLs crawled being generated by server-side software has also made it difficult for web crawlers to avoid retrieving duplicate content. Endless combinations of HTTP GET (URL-based) parameters exist, of which only a small selection will actually return unique content. For example, a simple online photo gallery may offer three options to users, as specified through HTTP GET parameters in the URL. If there exist four ways to sort images, three choices of thumbnail size, two file formats, and an option to disable user-provided content, then the same set of content can be accessed with 48 different URLs, all of which may be linked on the site. This mathematical combination creates a problem for crawlers, as they must sort through endless combinations of relatively minor scripted changes in order to retrieve unique content.

    As Edwards et al. noted, "Given that the bandwidth for conducting crawls is neither infinite nor free, it is becoming essential to crawl the Web in not only a scalable, but efficient way, if some reasonable measure of quality or freshness is to be maintained." A crawler must carefully choose at each step which pages to visit next.

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    A crawler must not only have a good crawling strategy, as noted in the previous sections, but it should also have a highly optimized architecture.

    Shkapenyuk and Suel noted that:

    While it is fairly easy to build a slow crawler that downloads a few pages per second for a short period of time, building a high-performance system that can download hundreds of millions of pages over several weeks presents a number of challenges in system design, I/O and network efficiency, and robustness and manageability.

    Web crawlers are a central part of search engines, and details on their algorithms and architecture are kept as business secrets. When crawler designs are published, there is often an important lack of detail that prevents others from reproducing the work. There are also emerging concerns about "search engine spamming", which prevent major search engines from publishing their ranking algorithms.

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    Wikipedia text under CC-BY-SA license

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