82-575: [REDACTED] Look up crawler in Wiktionary, the free dictionary. Crawler may refer to: Web crawler , a computer program that gathers and categorizes information on the World Wide Web A first-instar nymph of a scale insect that has legs and walks around before it attaches itself and becomes stationary Crawler (BEAM) in robotics A type of crane on tracks "Crawlers" ( Into
164-534: 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 . A web crawler
246-550: A "one-to-many" link, an "extended link" or a "multi-tailed link") is a hyperlink which leads to multiple endpoints; the link is a set-valued function . Tim Berners-Lee saw the possibility of using hyperlinks to link any information to any other information over the Internet . Hyperlinks were therefore integral to the creation of the World Wide Web . Web pages are written in the hypertext mark-up language HTML . This
328-411: A "trail" of related information, and then scroll back and forth among pages in a trail as if they were on a single microfilm reel. In a series of books and articles published from 1964 through 1980, Nelson transposed Bush's concept of automated cross-referencing into the computer context, made it applicable to specific text strings rather than whole pages, generalized it from a local desk-sized machine to
410-513: A 100,000-pages synthetic graph with a power-law distribution of in-links. However, there was no comparison with other strategies nor experiments in the real Web. Boldi et al. used simulation on subsets of the Web of 40 million pages from the .it domain and 100 million pages from the WebBase crawl, testing breadth-first against depth-first, random ordering and an omniscient strategy. The comparison
492-429: A Web crawler has finished its crawl, many events could have happened, including creations, updates, and deletions. From the search engine's point of view, there is a cost associated with not detecting an event, and thus having an outdated copy of a resource. The most-used cost functions are freshness and age. Freshness : This is a binary measure that indicates whether the local copy is accurate or not. The freshness of
574-460: A consistent manner. There are several types of normalization that may be performed including conversion of URLs to lowercase, removal of "." and ".." segments, and adding trailing slashes to the non-empty path component. Some crawlers intend to download/upload as many resources as possible from a particular web site. So path-ascending crawler was introduced that would ascend to every path in each URL that it intends to crawl. For example, when given
656-416: A crawler is, because this is the first one they have seen." A parallel crawler is a crawler that runs multiple processes in parallel. The goal is to maximize the download rate while minimizing the overhead from parallelization and to avoid repeated downloads of the same page. To avoid downloading the same page more than once, the crawling system requires a policy for assigning the new URLs discovered during
738-450: A crawler may examine the URL and only request a resource if the URL ends with certain characters such as .html, .htm, .asp, .aspx, .php, .jsp, .jspx or a slash. This strategy may cause numerous HTML Web resources to be unintentionally skipped. Some crawlers may also avoid requesting any resources that have a "?" in them (are dynamically produced) in order to avoid spider traps that may cause
820-512: A document, e.g. a webpage , or other resource, or to a position in a webpage. The latter is achieved by means of an HTML element with a "name" or "id" attribute at that position of the HTML document. The URL of the position is the URL of the webpage with a fragment identifier – "# id attribute " – appended. When linking to PDF documents from an HTML page the " id attribute " can be replaced with syntax that references
902-608: A focused crawling depends mostly on the richness of links in the specific topic being searched, and a focused crawling usually relies on a general Web search engine for providing starting points. An example of the focused crawlers are academic crawlers, which crawls free-access academic related documents, such as the citeseerxbot , which is the crawler of CiteSeer search engine. Other academic search engines are Google Scholar and Microsoft Academic Search etc. Because most academic papers are published in PDF formats, such kind of crawler
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#1732772485642984-401: A given query. Web crawlers that attempt to download pages that are similar to each other are called focused crawler or topical crawlers . The concepts of topical and focused crawling were first introduced by Filippo Menczer and by Soumen Chakrabarti et al. The main problem in focused crawling is that in the context of a Web crawler, we would like to be able to predict the similarity of
1066-431: A graphical user interface, the appearance of a mouse cursor may change into a hand motif to indicate a link. In most graphical web browsers, links are displayed in underlined blue text when they have not been visited, but underlined purple text when they have. When the user activates the link (e.g., by clicking on it with the mouse ) the browser displays the link's target. If the target is not an HTML file, depending on
1148-421: A hyperlink may vary with the hypertext system and may sometimes depend on the link itself; for instance, on the World Wide Web most hyperlinks cause the target document to replace the document being displayed, but some are marked to cause the target document to open in a new window (or, perhaps, in a new tab ). Another possibility is transclusion , for which the link target is a document fragment that replaces
1230-602: A large tracked vehicle used by NASA to transport spacecraft Nightcrawler ( Lumbricus terrestris ), an annelid worm also called "common earthworm" and "dew worm" Topics referred to by the same term [REDACTED] This disambiguation page lists articles associated with the title Crawler . If an internal link led you here, you may wish to change the link to point directly to the intended article. Retrieved from " https://en.wikipedia.org/w/index.php?title=Crawler&oldid=1158137030 " Category : Disambiguation pages Hidden categories: Short description
1312-407: A new window to be created. It is especially common to see this type of link when one large website links to an external page. The intention in that case is to ensure that the person browsing is aware that there is no endorsement of the site being linked to by the site that was linked from. However, the attribute is sometimes overused and can sometimes cause many windows to be created even while browsing
1394-412: A number of challenges in system design, I/O and network efficiency, and robustness and manageability. Hyperlink In computing , a hyperlink , or simply a link , is a digital reference to data that the user can follow or be guided to by clicking or tapping . A hyperlink points to a whole document or to a specific element within a document. Hypertext is text with hyperlinks. The text that
1476-446: A page p in the repository at time t is defined as: Age : This is a measure that indicates how outdated the local copy is. The age of a page p in the repository, at time t is defined as: Coffman et al. worked with a definition of the objective of a Web crawler that is equivalent to freshness, but use a different wording: they propose that a crawler must minimize the fraction of time pages remain outdated. They also noted that
1558-541: A page number or another element of the PDF, for example, "# page=386 ". A web browser usually displays a hyperlink in some distinguishing way, e.g. in a different color , font or style , or with certain symbols following to visualize link target or document types. This is also called link decoration . The behavior and style of links can be specified using the Cascading Style Sheets (CSS) language. In
1640-409: A previously-crawled-Web graph using this new method. Using these seeds, a new crawl can be very effective. A crawler may only want to seek out HTML pages and avoid all other MIME types . In order to request only HTML resources, a crawler may make an HTTP HEAD request to determine a Web resource's MIME type before requesting the entire resource with a GET request. To avoid making numerous HEAD requests,
1722-473: A regular window , but in a special hover box , which disappears when the cursor is moved away (sometimes it disappears anyway after a few seconds, and reappears when the cursor is moved away and back). Mozilla Firefox , IE , Opera , and many other web browsers all show the URL. In addition, the URL is commonly shown in the status bar . Normally, a link opens in the current frame or window, but sites that use frames and multiple windows for navigation can add
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#17327724856421804-407: 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
1886-400: A seed URL of http://llama.org/hamster/monkey/page.html, it will attempt to crawl /hamster/monkey/, /hamster/, and /. Cothey found that a path-ascending crawler was very effective in finding isolated resources, or resources for which no inbound link would have been found in regular crawling. The importance of a page for a crawler can also be expressed as a function of the similarity of a page to
1968-513: A server can have a hard time keeping up with requests from multiple crawlers. As noted by Koster, the use of Web crawlers is useful for a number of tasks, but comes with a price for the general community. The costs of using Web crawlers include: A partial solution to these problems is the robots exclusion protocol , also known as the robots.txt protocol that is a standard for administrators to indicate which parts of their Web servers should not be accessed by crawlers. This standard does not include
2050-562: A single document (1966), and soon after for connecting between paragraphs within separate documents (1968), with NLS . Ben Shneiderman working with graduate student Dan Ostroff designed and implemented the highlighted link in the HyperTIES system in 1983. HyperTIES was used to produce the world's first electronic journal, the July 1988 Communications of the ACM , which was cited as the source for
2132-544: A single domain. Cho also wrote his PhD dissertation at Stanford on web crawling. Najork and Wiener performed an actual crawl on 328 million pages, using breadth-first ordering. They found that a breadth-first crawl captures pages with high Pagerank early in the crawl (but they did not compare this strategy against other strategies). The explanation given by the authors for this result is that "the most important pages have many links to them from numerous hosts, and those links will be found early, regardless of on which host or page
2214-542: A single site. Another special page name is "_top", which causes any frames in the current window to be cleared away so that browsing can continue in the full window. The term "link" was coined in 1965 (or possibly 1964) by Ted Nelson at the start of Project Xanadu . Nelson had been inspired by " As We May Think ", a popular 1945 essay by Vannevar Bush . In the essay, Bush described a microfilm-based machine (the Memex ) in which one could link any two pages of information into
2296-425: 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
2378-465: A special "target" attribute to specify where the link loads. If no window exists with that name, a new window is created with the ID, which can be used to refer to the window later in the browsing session. Creation of new windows is probably the most common use of the "target" attribute. To prevent accidental reuse of a window, the special window names "_blank" and "_new" are usually available, and always cause
2460-425: A suggestion for the interval of visits to the same server, even though this interval is the most effective way of avoiding server overload. Recently commercial search engines like Google , Ask Jeeves , MSN and Yahoo! Search are able to use an extra "Crawl-delay:" parameter in the robots.txt file to indicate the number of seconds to delay between requests. The first proposed interval between successive pageloads
2542-424: A theoretical proprietary worldwide computer network, and advocated the creation of such a network. Though Nelson's Xanadu Corporation was eventually funded by Autodesk in the 1980s, it never created this proprietary public-access network. Meanwhile, working independently, a team led by Douglas Engelbart (with Jeff Rulifson as chief programmer ) was the first to implement the hyperlink concept for scrolling within
Crawler - Misplaced Pages Continue
2624-437: A typical web browser, this would display as the underlined word "Example" in blue, which when clicked would take the user to the example.com website. This contributes to a clean, easy to read text or document. By default, browsers will usually display hyperlinks as such: When the cursor hovers over a link, depending on the browser and graphical user interface, some informative text about the link can be shown, popping up, not in
2706-579: Is a distinction between the mere act of linking to someone else's website, and linking to content that is illegal (e.g., gambling illegal in the US) or infringing (e.g., illegal MP3 copies). Several courts have found that merely linking to someone else's website, even if by bypassing commercial advertising, is not copyright or trademark infringement, regardless of how much someone else might object. Linking to illegal or infringing content can be sufficiently problematic to give rise to legal liability. Compare for
2788-623: Is added. In certain jurisdictions , it is or has been held that hyperlinks are not merely references or citations , but are devices for copying web pages. In the Netherlands, Karin Spaink was initially convicted in this way of copyright infringement by linking, although this ruling was overturned in 2003. The courts that advocate this view see the mere publication of a hyperlink that connects to illegal material to be an illegal act in itself, regardless of whether referencing illegal material
2870-475: Is also known as a spider , an ant , an automatic indexer , or (in the FOAF software context) a Web scutter . 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
2952-417: Is challenging and can add a significant overhead to the crawling process, so this is performed as a post crawling process using machine learning or regular expression algorithms. These academic documents are usually obtained from home pages of faculties and students or from publication page of research institutes. Because academic documents make up only a small fraction of all web pages, a good seed selection
3034-591: Is different from Wikidata All article disambiguation pages All disambiguation pages Web crawler 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
3116-463: Is illegal. In 2004, Josephine Ho was acquitted of 'hyperlinks that corrupt traditional values' in Taiwan . In 2000, British Telecom sued Prodigy , claiming that Prodigy infringed its patent ( U.S. patent 4,873,662 ) on web hyperlinks. After litigation , a court found for Prodigy, ruling that British Telecom 's patent did not cover web hyperlinks. In United States jurisprudence , there
3198-510: Is important in boosting the efficiencies of these web crawlers. Other academic crawlers may download plain text and HTML files, that contains metadata of academic papers, such as titles, papers, and abstracts. This increases the overall number of papers, but a significant fraction may not provide free PDF downloads. Another type of focused crawlers is semantic focused crawler, which makes use of domain ontologies to represent topical maps and link Web pages with relevant ontological concepts for
3280-432: Is linked from is known as anchor text . A software system that is used for viewing and creating hypertext is a hypertext system , and to create a hyperlink is to hyperlink (or simply to link ). A user following hyperlinks is said to navigate or browse the hypertext. The document containing a hyperlink is known as its source document. For example, in content from Misplaced Pages or Google Search , many words and terms in
3362-463: Is neither the uniform policy nor the proportional policy. The optimal method for keeping average freshness high includes ignoring the pages that change too often, and the optimal for keeping average age low is to use access frequencies that monotonically (and sub-linearly) increase with the rate of change of each page. In both cases, the optimal is closer to the uniform policy than to the proportional policy: as Coffman et al. note, "in order to minimize
Crawler - Misplaced Pages Continue
3444-443: Is not allowed without permission. Contentious in particular are deep links , which do not point to a site's home page or other entry point designated by the site owner, but to content elsewhere, allowing the user to bypass the site's own designated flow, and inline links , which incorporate the content in question into the pages of the linking site, making it seem part of the linking site's own content unless an explicit attribution
3526-410: Is particularly interested in crawling PDF, PostScript files, Microsoft Word including their zipped formats. Because of this, general open-source crawlers, such as Heritrix , must be customized to filter out other MIME types , or a middleware is used to extract these documents out and import them to the focused crawl database and repository. Identifying whether these documents are academic or not
3608-422: Is the case of vertical search engines restricted to a single top-level domain , or search engines restricted to a fixed Web site). Designing a good selection policy has an added difficulty: it must work with partial information, as the complete set of Web pages is not known during crawling. Junghoo Cho et al. made the first study on policies for crawling scheduling. Their data set was a 180,000-pages crawl from
3690-500: Is to be maintained." A crawler must carefully choose at each step which pages to visit next. The behavior of a Web crawler is the outcome of a combination of policies: 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
3772-741: Is what a hyperlink to the home page of the W3C organization could look like in HTML code: This HTML code consists of several tags : Webgraph is a graph , formed from web pages as vertices and hyperlinks, as directed edges. The W3C recommendation called XLink describes hyperlinks that offer a far greater degree of functionality than those offered in HTML. These extended links can be multidirectional , remove linking from, within, and between XML documents. It can also describe simple links , which are unidirectional and therefore offer no more functionality than hyperlinks in HTML. Permalinks are URLs that are intended to remain unchanged for many years into
3854-511: Is why a link to a fragment is called an anchor link (that is, a link to an anchor). For example, in XML , the element <anchor id="name" />" provides anchoring capability (as long as the DTD or schema defines it), and in wiki markup , {{anchor|name}} is a typical example of implementing it. In word processor apps, anchors can be inserted where desired and may be called bookmarks . In URLs ,
3936-448: Is worth noticing that even when being very polite, and taking all the safeguards to avoid overloading Web servers, some complaints from Web server administrators are received. Sergey Brin and Larry Page noted in 1998, "... running a crawler which connects to more than half a million servers ... generates a fair amount of e-mail and phone calls. Because of the vast number of people coming on line, there are always those who do not know what
4018-457: The stanford.edu domain, in which a crawling simulation was done with different strategies. The ordering metrics tested were breadth-first , backlink count and partial PageRank calculations. One of the conclusions was that if the crawler wants to download pages with high Pagerank early during the crawling process, then the partial Pagerank strategy is the better, followed by breadth-first and backlink-count. However, these results are for just
4100-406: 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
4182-509: The file type and on the browser and its plugins , another program may be activated to open the file. The HTML code contains some or all of the five main characteristics of a link: It uses the HTML element "a" with the attribute "href" (HREF is an abbreviation for "Hypertext REFerence" ) and optionally also the attributes "title", "target", and " class " or "id": To embed a link into a web page, blogpost, or comment, it may take this form: In
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#17327724856424264-417: 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
4346-639: The Dark ) , an episode of the second season of Into the Dark The Crawler , an episode of the cartoon Extreme Ghostbusters Crawler (album) , an album by IDLES Crawler (band) , a British rock band Crawlers (band) , a British rock band A fictional creature in the video game Fable III A fictional creature in the movie The Descent See also [ edit ] Bottom crawler , an underwater exploration and recovery vehicle Crawl (disambiguation) Crawler-transporter ,
4428-545: The OPIC strategy and a strategy that uses the length of the per-site queues are better than breadth-first crawling, and that it is also very effective to use a previous crawl, when it is available, to guide the current one. Daneshpajouh et al. designed a community based algorithm for discovering good seeds. Their method crawls web pages with high PageRank from different communities in less iteration in comparison with crawl starting from random seeds. One can extract good seed from
4510-425: The Web are worth the same"), something that is not a realistic scenario, so further information about the Web page quality should be included to achieve a better crawling policy. Crawlers can retrieve data much quicker and in greater depth than human searchers, so they can have a crippling impact on the performance of a site. If a single crawler is performing multiple requests per second and/or downloading large files,
4592-491: The Web crawler. The objective of the crawler is to keep the average freshness of pages in its collection as high as possible, or to keep the average age of pages as low as possible. These objectives are not equivalent: in the first case, the crawler is just concerned with how many pages are outdated, while in the second case, the crawler is concerned with how old the local copies of pages are. Two simple re-visiting policies were studied by Cho and Garcia-Molina: In both cases,
4674-468: 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 requires a metric of importance for prioritizing Web pages. The importance of a page is a function of its intrinsic quality, its popularity in terms of links or visits, and even of its URL (the latter
4756-471: The crawl originates." Abiteboul designed a crawling strategy based on an algorithm called OPIC (On-line Page Importance Computation). In OPIC, each page is given an initial sum of "cash" that is distributed equally among the pages it points to. It is similar to a PageRank computation, but it is faster and is only done in one step. An OPIC-driven crawler downloads first the pages in the crawling frontier with higher amounts of "cash". Experiments were carried in
4838-399: The crawler to download an infinite number of URLs from a Web site. This strategy is unreliable if the site uses URL rewriting to simplify its URLs. Crawlers usually perform some type of URL normalization in order to avoid crawling the same resource more than once. The term URL normalization , also called URL canonicalization , refers to the process of modifying and standardizing a URL in
4920-498: The crawling process, as the same URL can be found by two different crawling processes. 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
5002-584: The default. The MercatorWeb crawler follows an adaptive politeness policy: if it took t seconds to download a document from a given server, the crawler waits for 10 t seconds before downloading the next page. Dill et al. use 1 second. For those using Web crawlers for research purposes, a more detailed cost-benefit analysis is needed and ethical considerations should be taken into account when deciding where to crawl and how fast to crawl. Anecdotal evidence from access logs shows that access intervals from known crawlers vary between 20 seconds and 3–4 minutes. It
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#17327724856425084-603: The expected obsolescence time, the accesses to any particular page should be kept as evenly spaced as possible". Explicit formulas for the re-visit policy are not attainable in general, but they are obtained numerically, as they depend on the distribution of page changes. Cho and Garcia-Molina show that the exponential distribution is a good fit for describing page changes, while Ipeirotis et al. show how to use statistical tools to discover parameters that affect this distribution. The re-visiting policies considered here regard all pages as homogeneous in terms of quality ("all pages on
5166-441: The expense of less frequently updating pages, and (2) the freshness of rapidly changing pages lasts for shorter period than that of less frequently changing pages. In other words, a proportional policy allocates more resources to crawling frequently updating pages, but experiences less overall freshness time from them. To improve freshness, the crawler should penalize the elements that change too often. The optimal re-visiting policy
5248-485: The full linked content is not needed, as is the case when rearranging a page layout . An anchor hyperlink (anchor link) is a link bound to a portion of a document, which is often called a fragment . The fragment is generally a portion of text or a heading, though not necessarily. For instance, it may also be a hot area in an image ( image map in HTML ), a designated, often irregular part of an image. Fragments are marked with anchors (in any of various ways), which
5330-621: The future, yielding hyperlinks that are less susceptible to link rot . Permalinks are often rendered simply, that is, as friendly URLs, so as to be easy for people to type and remember. Permalinks are used in order to point and redirect readers to the same Web page , blog post or any online digital media. The scientific literature is a place where link persistence is crucial to the public knowledge. A 2013 study in BMC Bioinformatics analyzed 15,000 links in abstracts from Thomson Reuters' Web of Science citation index, founding that
5412-438: The hash character (#) precedes the name of the anchor for the fragment. One way to define a hot area in an image is by a list of coordinates that indicate its boundaries. For example, a political map of Africa may have each country hyperlinked to further information about that country. A separate invisible hot area interface allows for swapping skins or labels within the linked hot areas without repetitive embedding of links in
5494-456: The link anchor within the source document. Not only persons browsing the document may follow hyperlinks. These hyperlinks may also be followed automatically by programs. A program that traverses the hypertext, following each hyperlink and gathering all the retrieved documents is known as a Web spider or crawler . An inline link displays remote content without the need for embedding the content. The remote content may be accessed with or without
5576-537: The link concept in Tim Berners-Lee 's Spring 1989 manifesto for the Web. In 1988, Ben Shneiderman and Greg Kearsley used HyperTIES to publish "Hypertext Hands-On!", the world's first electronic book. Released in 1987 for the Apple Macintosh , the database program HyperCard allowed for hyperlinking between various pages within a document, as well as to other documents and separate applications on
5658-412: The median lifespan of Web pages was 9.3 years, and just 62% were archived. The median lifespan of a Web page constitutes high-degree variable, but its order of magnitude usually is of some months. A link from one domain to another is said to be outbound from its source anchor and inbound to its target. The most common destination anchor is a URL used in the World Wide Web . This can refer to
5740-412: The problem of Web crawling can be modeled as a multiple-queue, single-server polling system, on which the Web crawler is the server and the Web sites are the queues. Page modifications are the arrival of the customers, and switch-over times are the interval between page accesses to a single Web site. Under this model, mean waiting time for a customer in the polling system is equivalent to the average age for
5822-469: The repeated crawling order of pages can be done either in a random or a fixed order. Cho and Garcia-Molina proved the surprising result that, in terms of average freshness, the uniform policy outperforms the proportional policy in both a simulated Web and a real Web crawl. Intuitively, the reasoning is that, as web crawlers have a limit to how many pages they can crawl in a given time frame, (1) they will allocate too many new crawls to rapidly changing pages at
5904-479: The same computer. In 1990, Windows Help , which was introduced with Microsoft Windows 3.0 , had widespread use of hyperlinks to link different pages in a single help file together; in addition, it had a visually different kind of hyperlink that caused a popup help message to appear when clicked, usually to give definitions of terms introduced on the help page. The first widely used open protocol that included hyperlinks from any Internet site to any other Internet site
5986-399: The selection and categorization purposes. In addition, ontologies can be automatically updated in the crawling process. Dong et al. introduced such an ontology-learning-based crawler using a support-vector machine to update the content of ontological concepts when crawling Web pages. The Web has a very dynamic nature, and crawling a fraction of the Web can take weeks or months. By the time
6068-439: 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
6150-432: The text are hyperlinked to definitions of those terms. Hyperlinks are often used to implement reference mechanisms such as tables of contents, footnotes , bibliographies , indexes , and glossaries . In some hypertext, hyperlinks can be bidirectional: they can be followed in two directions, so both ends act as anchors and as targets. More complex arrangements exist, such as many-to-many links. The effect of following
6232-421: The text of a given page to the query before actually downloading the page. A possible predictor is the anchor text of links; this was the approach taken by Pinkerton in the first web crawler of the early days of the Web. Diligenti et al. propose using the complete content of the pages already visited to infer the similarity between the driving query and the pages that have not been visited yet. The performance of
6314-449: The user following the link. An inline link may display a modified version of the content; for instance, instead of an image, a thumbnail , low resolution preview , cropped section, or magnified section may be shown. The full content is then usually available on demand, as is the case with print publishing software – e.g., with an external link . This allows for smaller file sizes and quicker response to changes when
6396-497: The various skin elements. Text hyperlink. Hyperlink is embedded into a word or a phrase and makes this text clickable. Image hyperlink. Hyperlink is embedded into an image and makes this image clickable. Bookmark hyperlink. Hyperlink is embedded into a text or an image and takes visitors to another part of a web page. E-mail hyperlink. Hyperlink is embedded into e-mail address and allows visitors to send an e-mail message to this e-mail address. A fat link (also known as
6478-527: 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
6560-465: Was 60 seconds. However, if pages were downloaded at this rate from a website with more than 100,000 pages over a perfect connection with zero latency and infinite bandwidth, it would take more than 2 months to download only that entire Web site; also, only a fraction of the resources from that Web server would be used. Cho uses 10 seconds as an interval for accesses, and the WIRE crawler uses 15 seconds as
6642-430: Was based on how well PageRank computed on a partial crawl approximates the true PageRank value. Some visits that accumulate PageRank very quickly (most notably, breadth-first and the omniscient visit) provide very poor progressive approximations. Baeza-Yates et al. used simulation on two subsets of the Web of 3 million pages from the .gr and .cl domain, testing several crawling strategies. They showed that both
6724-581: Was the Gopher protocol from 1991. It was soon eclipsed by HTML after the 1993 release of the Mosaic browser (which could handle Gopher links as well as HTML links). HTML's advantage was the ability to mix graphics, text, and hyperlinks, unlike Gopher, which just had menu-structured text and hyperlinks. While hyperlinking among webpages is an intrinsic feature of the web , some websites object to being linked by other websites; some have claimed that linking to them
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