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Web crawler, sometimes called a spider or spiderbot and often shortened to crawler, is an
Internet bot An Internet bot, web robot, robot, or simply bot, is a software application that runs automated tasks ( scripts) on the Internet, usually with the intent to imitate human activity, such as messaging, on a large scale. An Internet bot plays the ...
that systematically browses the
World Wide Web The World Wide Web (WWW or simply the Web) is an information system that enables Content (media), content sharing over the Internet through user-friendly ways meant to appeal to users beyond Information technology, IT specialists and hobbyis ...
and that is typically operated by search engines for the purpose of
Web indexing Web indexing, or Internet indexing, comprises methods for indexing the contents of a website or of the Internet as a whole. Individual websites or intranets may use a back-of-the-book index, while search engines usually use keywords and Metadata ( ...
(''web spidering''). Web
search engine A search engine is a software system that provides hyperlinks to web pages, and other relevant information on World Wide Web, the Web in response to a user's web query, query. The user enters a query in a web browser or a mobile app, and the sea ...
s and some other
website A website (also written as a web site) is any web page whose content is identified by a common domain name and is published on at least one web server. Websites are typically dedicated to a particular topic or purpose, such as news, educatio ...
s use Web crawling or spidering
software Software consists of computer programs that instruct the Execution (computing), execution of a computer. Software also includes design documents and specifications. The history of software is closely tied to the development of digital comput ...
to update their
web content Web content is the text, visual or audio content that is made available online and user encountered as part of the online usage and experience on websites. It may include text, images, sounds and audio, online videos, among other items place ...
or indices of other sites' web content. Web crawlers copy pages for processing by a search engine, which
indexes Index (: indexes or indices) may refer to: Arts, entertainment, and media Fictional entities * Index (A Certain Magical Index), Index (''A Certain Magical Index''), a character in the light novel series ''A Certain Magical Index'' * The Index, a ...
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 robots.txt is the filename used for implementing the Robots Exclusion Protocol, a standard used by websites to indicate to visiting web crawlers and other web robots which portions of the website they are allowed to visit. The standard, dev ...
file can request
bots The British Overseas Territories (BOTs) or alternatively referred to as the United Kingdom Overseas Territories (UKOTs) are the fourteen dependent territory, territories with a constitutional and historical link with the United Kingdom that, ...
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
hyperlink In computing, a hyperlink, or simply a link, is a digital reference providing direct access to Data (computing), data by a user (computing), user's point and click, clicking or touchscreen, tapping. A hyperlink points to a whole document or to ...
s and
HTML Hypertext Markup Language (HTML) is the standard markup language for documents designed to be displayed in a web browser. It defines the content and structure of web content. It is often assisted by technologies such as Cascading Style Sheets ( ...
code. They can also be used for
web scraping Web scraping, web harvesting, or web data extraction is data scraping used for data extraction, extracting data from websites. Web scraping software may directly access the World Wide Web using the Hypertext Transfer Protocol or a web browser. W ...
and
data-driven programming In computer programming, data-driven programming is a programming paradigm in which the program statements describe the data to be matched and the processing required rather than defining a sequence of steps to be taken. Standard examples of dat ...
.


Nomenclature

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


Overview

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 server A web server is computer software and underlying Computer hardware, hardware that accepts requests via Hypertext Transfer Protocol, HTTP (the network protocol created to distribute web content) or its secure variant HTTPS. A user agent, co ...
s that respond to those URLs, it identifies all the
hyperlink In computing, a hyperlink, or simply a link, is a digital reference providing direct access to Data (computing), data by a user (computing), user's point and click, clicking or touchscreen, tapping. A hyperlink points to a whole document or to ...
s 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
website A website (also written as a web site) is any web page whose content is identified by a common domain name and is published on at least one web server. Websites are typically dedicated to a particular topic or purpose, such as news, educatio ...
s (or
web archiving Web archiving is the process of collecting, preserving, and providing access to material from the World Wide Web. The aim is to ensure that information is preserved in an archival format for research and the public. Web archivists typically ...
), 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 page A web page (or webpage) is a World Wide Web, Web document that is accessed in a web browser. A website typically consists of many web pages hyperlink, linked together under a common domain name. The term "web page" is therefore a metaphor of pap ...
s. The
repository Repository may refer to: Archives and online databases * Content repository, a database with an associated set of data management tools, allowing application-independent access to the content * Disciplinary repository (or subject repository), an ...
only stores
HTML Hypertext Markup Language (HTML) is the standard markup language for documents designed to be displayed in a web browser. It defines the content and structure of web content. It is often assisted by technologies such as Cascading Style Sheets ( ...
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 HTTP (Hypertext Transfer Protocol) is an application layer protocol in the Internet protocol suite model for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide Web, wher ...
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 Thumbnails are reduced-size versions of pictures or videos, used to help in recognizing and organizing them, serving the same role for images as a normal text index does for words. In the age of digital images, visual search engines and image-o ...
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 Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandwidth'', or ''radio bandwidth'', a measure of the width of a frequency range * Bandwidth (computing), the rate of data transfer, bit rate or thr ...
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.


Crawling policy

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 website (also written as a web site) is any web page whose content is identified by a common domain name and is published on at least one web server. Websites are typically dedicated to a particular topic or purpose, such as news, education ...
. * a ''parallelization policy'' that states how to coordinate distributed web crawlers.


Selection policy

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 Search engines, including web search engines, selection-based search engines, metasearch engines, desktop search tools, and web portals and vertical market websites have a search facility for online databases. By content/topic Gene ...
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 A web page (or webpage) is a World Wide Web, Web document that is accessed in a web browser. A website typically consists of many web pages hyperlink, linked together under a common domain name. The term "web page" is therefore a metaphor of pap ...
, 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 In science and engineering, an intrinsic property is a property of a specified subject that exists itself or within the subject. An extrinsic property is not essential or inherent to the subject that is being characterized. For example, mass i ...
quality, its popularity in terms of links or visits, and even of its URL (the latter is the case of vertical search engines restricted to a single
top-level domain A top-level domain (TLD) is one of the domain name, domains at the highest level in the hierarchical Domain Name System of the Internet after the root domain. The top-level domain names are installed in the DNS root zone, root zone of the nam ...
, 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 the stanford.edu domain, in which a crawling simulation was done with different strategies. The ordering metrics tested were breadth-first,
backlink From the point of view of a given web resource (referent), a backlink is a regular hyperlink on another web resource (the referrer) that points to the referent. A ''web resource'' may be (for example) a website, web page, or web directory. A ba ...
count and partial
PageRank PageRank (PR) is an algorithm used by Google Search to rank web pages in their search engine results. It is named after both the term "web page" and co-founder Larry Page. PageRank is a way of measuring the importance of website pages. Accordin ...
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 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 the crawl originates." Abiteboul designed a crawling strategy based on an
algorithm In mathematics and computer science, an algorithm () is a finite sequence of Rigour#Mathematics, mathematically rigorous instructions, typically used to solve a class of specific Computational problem, problems or to perform a computation. Algo ...
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 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 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.Baeza-Yates, R.; Castillo, C.; Marin, M. and Rodriguez, A. (2005)
"Crawling a Country: Better Strategies than Breadth-First for Web Page Ordering."
In: ''Proceedings of the Industrial and Practical Experience track of the 14th conference on World Wide Web'', pages 864–872, Chiba, Japan. ACM Press.
They showed that both 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 a previously-crawled-Web graph using this new method. Using these seeds, a new crawl can be very effective.


Restricting followed links

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, 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 the crawler to download an infinite number of URLs from a Web site. This strategy is unreliable if the site uses
URL rewriting In web applications, a rewrite engine is a software component that performs rewriting on URLs (Uniform Resource Locators), modifying their appearance. This modification is called URL rewriting. It is a way of implementing URL mapping or routin ...
to simplify its URLs.


URL normalization

Crawlers usually perform some type of
URL normalization URI normalization is the process by which Uniform Resource Identifier, URIs are modified and standardized in a consistent manner. The goal of the normalization process is to transform a URI into a normalized URI so it is possible to determine i ...
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 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.


Path-ascending crawling

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 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.


Focused crawling

The importance of a page for a crawler can also be expressed as a function of the similarity of a page to 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 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 PinkertonPinkerton, B. (1994)
Finding what people want: Experiences with the WebCrawler
In Proceedings of the First World Wide Web Conference, Geneva, Switzerland.
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 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.


=Academic focused crawler

= 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 CiteSeerX (formerly called CiteSeer) is a public search engine and digital library for scientific and academic papers, primarily in the fields of computer and information science. CiteSeer's goal is to improve the dissemination and access of a ...
X search engine. Other academic search engines are
Google Scholar Google Scholar is a freely accessible web search engine that indexes the full text or metadata of Academic publishing, scholarly literature across an array of publishing formats and disciplines. Released in Beta release, beta in November 2004, th ...
and
Microsoft Academic Search Microsoft Academic Search (MAS) was a research project and academic search engine retired in 2012. It relaunched in 2016 as Microsoft Academic, which in turn was shut down in 2022. The content of the latter was allegedly incorporated into The L ...
etc. Because most academic papers are published in
PDF Portable document format (PDF), standardized as ISO 32000, is a file format developed by Adobe Inc., Adobe in 1992 to present documents, including text formatting and images, in a manner independent of application software, computer hardware, ...
formats, such kind of crawler is particularly interested in crawling PDF,
PostScript PostScript (PS) is a page description language and dynamically typed, stack-based programming language. It is most commonly used in the electronic publishing and desktop publishing realm, but as a Turing complete programming language, it c ...
files,
Microsoft Word Microsoft Word is a word processor program, word processing program developed by Microsoft. It was first released on October 25, 1983, under the name Multi-Tool Word for Xenix systems. Subsequent versions were later written for several other platf ...
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 Middleware is a type of computer software program that provides services to software applications beyond those available from the operating system. It can be described as "software glue". Middleware makes it easier for software developers to imple ...
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 is challenging and can add a significant overhead to the crawling process, so this is performed as a post crawling process using
machine learning Machine learning (ML) is a field of study in artificial intelligence concerned with the development and study of Computational statistics, statistical algorithms that can learn from data and generalise to unseen data, and thus perform Task ( ...
or
regular expression A regular expression (shortened as regex or regexp), sometimes referred to as rational expression, is a sequence of characters that specifies a match pattern in text. Usually such patterns are used by string-searching algorithms for "find" ...
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 is important in boosting the efficiencies of these web crawlers. Other academic crawlers may download plain text and
HTML Hypertext Markup Language (HTML) is the standard markup language for documents designed to be displayed in a web browser. It defines the content and structure of web content. It is often assisted by technologies such as Cascading Style Sheets ( ...
files, that contains
metadata Metadata (or metainformation) is "data that provides information about other data", but not the content of the data itself, such as the text of a message or the image itself. There are many distinct types of metadata, including: * Descriptive ...
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.


=Semantic focused crawler

= 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 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 In machine learning, support vector machines (SVMs, also support vector networks) are supervised learning, supervised Maximum-margin hyperplane, max-margin models with associated learning algorithms that analyze data for Statistical classification ...
to update the content of ontological concepts when crawling Web pages.


Re-visit policy

The Web has a very dynamic nature, and crawling a fraction of the Web can take weeks or months. By the time 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 a page ''p'' in the repository at time ''t'' is defined as: : F_p(t) = \begin 1 & ~p~~t\\ 0 & \end 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: : A_p(t) = \begin 0 & ~p~~t\\ t - ~p & \end 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 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 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: * Uniform policy: This involves re-visiting all pages in the collection with the same frequency, regardless of their rates of change. * Proportional policy: This involves re-visiting more often the pages that change more frequently. The visiting frequency is directly proportional to the (estimated) change frequency. In both cases, 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 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 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 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 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.


Politeness 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, 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: * network resources, as crawlers require considerable bandwidth and operate with a high degree of parallelism during a long period of time; * server overload, especially if the frequency of accesses to a given server is too high; * poorly written crawlers, which can crash servers or routers, or which download pages they cannot handle; and * personal crawlers that, if deployed by too many users, can disrupt networks and Web servers. A partial solution to these problems is the
robots exclusion protocol robots.txt is the filename used for implementing the Robots Exclusion Protocol, a standard used by websites to indicate to visiting web crawlers and other web robots which portions of the website they are allowed to visit. The standard, dev ...
, 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 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 Google LLC (, ) is an American multinational corporation and technology company focusing on online advertising, search engine technology, cloud computing, computer software, quantum computing, e-commerce, consumer electronics, and artificial ...
,
Ask Jeeves Ask.com (known originally as Ask Jeeves) is an answer engine, e-magazine, and former web search engine, operated by Ask Media Group. It was conceptualized and developed in 1996 by Garrett Gruener and David Warthen (based in Berkeley, Califo ...
, MSN and
Yahoo! Search Yahoo! Search is a search engine owned and operated by Yahoo!, using Microsoft Bing to power results. Originally, "Yahoo! Search" referred to a Yahoo!-provided interface that sent Web search query, queries to a searchable index of pages supple ...
are able to use an extra "Crawl-delay:" parameter in the
robots.txt robots.txt is the filename used for implementing the Robots Exclusion Protocol, a standard used by websites to indicate to visiting web crawlers and other web robots which portions of the website they are allowed to visit. The standard, dev ...
file to indicate the number of seconds to delay between requests. The first proposed interval between successive pageloads 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 the default.Baeza-Yates, R. and Castillo, C. (2002)
Balancing volume, quality and freshness in Web crawling
In Soft Computing Systems – Design, Management and Applications, pages 565–572, Santiago, Chile. IOS Press Amsterdam.
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 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 Sergey Mikhailovich Brin (; born August 21, 1973) is an American computer scientist and businessman who co-founded Google with Larry Page. He was the president of Google's parent company, Alphabet Inc., until stepping down from the role on D ...
and
Larry Page Lawrence Edward Page (born March 26, 1973) is an American businessman, computer engineer and computer scientist best known for co-founding Google with Sergey Brin. Page was chief executive officer of Google from 1997 until August 2001 when ...
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 a crawler is, because this is the first one they have seen."


Parallelization policy

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 the crawling process, as the same URL can be found by two different crawling processes.


Architectures

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:Shkapenyuk, V. and Suel, T. (2002)
Design and implementation of a high performance distributed web crawler
In Proceedings of the 18th International Conference on Data Engineering (ICDE), pages 357-368, San Jose, California. IEEE CS Press.
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.


Security

While most of the website owners are keen to have their pages indexed as broadly as possible to have strong presence in
search engines Search engines, including web search engines, selection-based search engines, metasearch engines, desktop search tools, and web portals and vertical market websites have a search facility for online databases. By content/topic Gene ...
, web crawling can also have
unintended consequences In the social sciences, unintended consequences (sometimes unanticipated consequences or unforeseen consequences, more colloquially called knock-on effects) are outcomes of a purposeful action that are not intended or foreseen. The term was po ...
and lead to a
compromise To compromise is to make a deal between different parties where each party gives up part of their demand. In arguments, compromise means finding agreement through communication, through a mutual acceptance of terms—often involving variations fr ...
or
data breach A data breach, also known as data leakage, is "the unauthorized exposure, disclosure, or loss of personal information". Attackers have a variety of motives, from financial gain to political activism, political repression, and espionage. There ...
if a search engine indexes resources that should not be publicly available, or pages revealing potentially vulnerable versions of software. Apart from standard web application security recommendations website owners can reduce their exposure to opportunistic hacking by only allowing search engines to index the public parts of their websites (with
robots.txt robots.txt is the filename used for implementing the Robots Exclusion Protocol, a standard used by websites to indicate to visiting web crawlers and other web robots which portions of the website they are allowed to visit. The standard, dev ...
) and explicitly blocking them from indexing transactional parts (login pages, private pages, etc.).


Crawler identification

Web crawlers typically identify themselves to a Web server by using the
User-agent In computing, the User-Agent header is an HTTP header intended to identify the user agent responsible for making a given HTTP request. Whereas the character sequence User-Agent comprises the name of the header itself, the header value that a giv ...
field of an
HTTP HTTP (Hypertext Transfer Protocol) is an application layer protocol in the Internet protocol suite model for distributed, collaborative, hypermedia information systems. HTTP is the foundation of data communication for the World Wide Web, wher ...
request. Web site administrators typically examine their
Web server A web server is computer software and underlying Computer hardware, hardware that accepts requests via Hypertext Transfer Protocol, HTTP (the network protocol created to distribute web content) or its secure variant HTTPS. A user agent, co ...
s' log and use the user agent field to determine which crawlers have visited the web server and how often. The user agent field may include a
URL A uniform resource locator (URL), colloquially known as an address on the Web, is a reference to a resource that specifies its location on a computer network and a mechanism for retrieving it. A URL is a specific type of Uniform Resource Identi ...
where the Web site administrator may find out more information about the crawler. Examining Web server log is tedious task, and therefore some administrators use tools to identify, track and verify Web crawlers.
Spambots A spambot is a computer program designed to assist in the sending of Spam (electronic), spam. Spambots usually create accounts and send spam messages with them. Web hosts and website operators have responded by banning spammers, leading to an ongo ...
and other malicious Web crawlers are unlikely to place identifying information in the user agent field, or they may mask their identity as a browser or other well-known crawler. Web site administrators prefer Web crawlers to identify themselves so that they can contact the owner if needed. In some cases, crawlers may be accidentally trapped in a crawler trap or they may be overloading a Web server with requests, and the owner needs to stop the crawler. Identification is also useful for administrators that are interested in knowing when they may expect their Web pages to be indexed by a particular
search engine A search engine is a software system that provides hyperlinks to web pages, and other relevant information on World Wide Web, the Web in response to a user's web query, query. The user enters a query in a web browser or a mobile app, and the sea ...
.


Crawling the deep web

A vast amount of web pages lie in the deep or invisible web. These pages are typically only accessible by submitting queries to a database, and regular crawlers are unable to find these pages if there are no links that point to them. Google's
Sitemaps Sitemaps is a protocol in XML format meant for a webmaster to inform search engines about URLs on a website that are available for web crawling. It allows webmasters to include additional information about each URL: when it was last updated, h ...
protocol and mod oai are intended to allow discovery of these deep-Web resources. Deep web crawling also multiplies the number of web links to be crawled. Some crawlers only take some of the URLs in <a href="URL"> form. In some cases, such as the
Googlebot Googlebot is the web crawler software used by Google that collects documents from the web to build a searchable index for the Google Search engine. This name is actually used to refer to two different types of web crawlers: a desktop crawler (to ...
, Web crawling is done on all text contained inside the hypertext content, tags, or text. Strategic approaches may be taken to target deep Web content. With a technique called
screen scraping Data scraping is a technique where a computer program extracts data from human-readable output coming from another program. Description Normally, data transfer between programs is accomplished using data structures suited for automated processin ...
, specialized software may be customized to automatically and repeatedly query a given Web form with the intention of aggregating the resulting data. Such software can be used to span multiple Web forms across multiple Websites. Data extracted from the results of one Web form submission can be taken and applied as input to another Web form thus establishing continuity across the Deep Web in a way not possible with traditional web crawlers. Pages built on
AJAX Ajax may refer to: Greek mythology and tragedy * Ajax the Great, a Greek mythological hero, son of King Telamon and Periboea * Ajax the Lesser, a Greek mythological hero, son of Oileus, the king of Locris * Ajax (play), ''Ajax'' (play), by the an ...
are among those causing problems to web crawlers.
Google Google LLC (, ) is an American multinational corporation and technology company focusing on online advertising, search engine technology, cloud computing, computer software, quantum computing, e-commerce, consumer electronics, and artificial ...
has proposed a format of AJAX calls that their bot can recognize and index.


Visual vs programmatic crawlers

There are a number of "visual web scraper/crawler" products available on the web which will crawl pages and structure data into columns and rows based on the users requirements. One of the main difference between a classic and a visual crawler is the level of programming ability required to set up a crawler. The latest generation of "visual scrapers" remove the majority of the programming skill needed to be able to program and start a crawl to scrape web data. The visual scraping/crawling method relies on the user "teaching" a piece of crawler technology, which then follows patterns in semi-structured data sources. The dominant method for teaching a visual crawler is by highlighting data in a browser and training columns and rows. While the technology is not new, for example it was the basis of Needlebase which has been bought by Google (as part of a larger acquisition of ITA Labs), there is continued growth and investment in this area by investors and end-users.


List of web crawlers

The following is a list of published crawler architectures for general-purpose crawlers (excluding focused web crawlers), with a brief description that includes the names given to the different components and outstanding features:


Historical web crawlers


WolfBot
was a massively multi threaded crawler built in 2001 by Mani Singh a Civil Engineering graduate from the University of California at Davis. *
World Wide Web Worm The World Wide Web Worm (WWWW) was one of the earliest search engines for the World Wide Web (WWW). It was developed in September 1993 by Oliver McBryan at the University of Colorado as a research project. It is claimed by some to be the first se ...
was a crawler used to build a simple index of document titles and URLs. The index could be searched by using the
grep grep is a command-line utility for searching plaintext datasets for lines that match a regular expression. Its name comes from the ed command g/re/p (global regular expression search and print), which has the same effect. grep was originally de ...
Unix Unix (, ; trademarked as UNIX) is a family of multitasking, multi-user computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, a ...
command. * Yahoo! Slurp was the name of the
Yahoo! Yahoo (, styled yahoo''!'' in its logo) is an American web portal that provides the search engine Yahoo Search and related services including My Yahoo, Yahoo Mail, Yahoo News, Yahoo Finance, Yahoo Sports, y!entertainment, yahoo!life, and its a ...
Search crawler until Yahoo! contracted with
Microsoft Microsoft Corporation is an American multinational corporation and technology company, technology conglomerate headquartered in Redmond, Washington. Founded in 1975, the company became influential in the History of personal computers#The ear ...
to use Bingbot instead.


In-house web crawlers

* Applebot is
Apple An apple is a round, edible fruit produced by an apple tree (''Malus'' spp.). Fruit trees of the orchard or domestic apple (''Malus domestica''), the most widely grown in the genus, are agriculture, cultivated worldwide. The tree originated ...
's web crawler. It supports
Siri Siri ( , backronym: Speech Interpretation and Recognition Interface) is a digital assistant purchased, developed, and popularized by Apple Inc., which is included in the iOS, iPadOS, watchOS, macOS, Apple TV, audioOS, and visionOS operating sys ...
and other products. * Bingbot is the name of Microsoft's
Bing Bing most often refers to: * Bing Crosby (1903–1977), American singer * Microsoft Bing, a web search engine Bing may also refer to: Food and drink * Bing (bread), a Chinese flatbread * Bing (soft drink), a UK brand * Bing cherry, a varie ...
webcrawler. It replaced '' Msnbot''. * Baiduspider is
Baidu Baidu, Inc. ( ; ) is a Chinese multinational technology company specializing in Internet services and artificial intelligence. It holds a dominant position in China's search engine market (via Baidu Search), and provides a wide variety of o ...
's web crawler. * DuckDuckBot is DuckDuckGo's web crawler. *
Googlebot Googlebot is the web crawler software used by Google that collects documents from the web to build a searchable index for the Google Search engine. This name is actually used to refer to two different types of web crawlers: a desktop crawler (to ...
is described in some detail, but the reference is only about an early version of its architecture, which was written in C++ and Python. The crawler was integrated with the indexing process, because text parsing was done for full-text indexing and also for URL extraction. There is a URL server that sends lists of URLs to be fetched by several crawling processes. During parsing, the URLs found were passed to a URL server that checked if the URL have been previously seen. If not, the URL was added to the queue of the URL server. *
WebCrawler WebCrawler is a search engine, and one of the oldest surviving search engines on the web today. For many years, it operated as a metasearch engine. WebCrawler was the first web search engine to provide full text search. History Brian Pinker ...
was used to build the first publicly available full-text index of a subset of the Web. It was based on lib-WWW to download pages, and another program to parse and order URLs for breadth-first exploration of the Web graph. It also included a real-time crawler that followed links based on the similarity of the anchor text with the provided query. * WebFountain is a distributed, modular crawler similar to Mercator but written in C++. *
Xenon Xenon is a chemical element; it has symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
is a web crawler used by government tax authorities to detect fraud.


Commercial web crawlers

The following web crawlers are available, for a price:: * Diffbot - programmatic general web crawler, available as an
API An application programming interface (API) is a connection between computers or between computer programs. It is a type of software interface, offering a service to other pieces of software. A document or standard that describes how to build ...
* SortSite - crawler for analyzing websites, available for
Windows Windows is a Product lining, product line of Proprietary software, proprietary graphical user interface, graphical operating systems developed and marketed by Microsoft. It is grouped into families and subfamilies that cater to particular sec ...
and
Mac OS Mac operating systems were developed by Apple Inc. in a succession of two major series. In 1984, Apple debuted the operating system that is now known as the classic Mac OS with its release of the original Macintosh System Software. The system ...
* Swiftbot -
Swiftype Swiftype is a search and index company based in San Francisco, California, that provides search software for organizations, websites, and computer programs. Notable customers include AT&T, Dr. Pepper, Hubspot and TechCrunch. History Swiftype w ...
's web crawler, available as
software as a service Software as a service (SaaS ) is a cloud computing service model where the provider offers use of application software to a client and manages all needed physical and software resources. SaaS is usually accessed via a web application. Unlike o ...
* Aleph Search - web crawler allowing massive collection with high scalability


Open-source crawlers

* Apache Nutch is a highly extensible and scalable web crawler written in Java and released under an
Apache License The Apache License is a permissive free software license written by the Apache Software Foundation (ASF). It allows users to use the software for any purpose, to distribute it, to modify it, and to distribute modified versions of the software ...
. It is based on
Apache Hadoop Apache Hadoop () is a collection of open-source software utilities for reliable, scalable, distributed computing. It provides a software framework for distributed storage and processing of big data using the MapReduce programming model. Hadoop wa ...
and can be used with Apache Solr or
Elasticsearch Elasticsearch is a Search engine (computing), search engine based on Apache Lucene, a free and open-source search engine. It provides a distributed, Multitenancy, multitenant-capable full-text search engine with an HTTP web interface and schema ...
. * Grub was an open source distributed search crawler that
Wikia Search Wikia Search was a short-lived free and open-source web search engine launched by Wikia, a for-profit wiki-hosting company founded by Jimmy Wales and Angela Beesley. Wikia Search followed other experiments by Wikia into search engine technolog ...
used to crawl the web. * Heritrix is the
Internet Archive The Internet Archive is an American 501(c)(3) organization, non-profit organization founded in 1996 by Brewster Kahle that runs a digital library website, archive.org. It provides free access to collections of digitized media including web ...
's archival-quality crawler, designed for archiving periodic snapshots of a large portion of the Web. It was written in
Java Java is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea (a part of Pacific Ocean) to the north. With a population of 156.9 million people (including Madura) in mid 2024, proje ...
. * ht://Dig includes a Web crawler in its indexing engine. * HTTrack uses a Web crawler to create a mirror of a web site for off-line viewing. It is written in C and released under the GPL. * Norconex Web Crawler is a highly extensible Web Crawler written in
Java Java is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea (a part of Pacific Ocean) to the north. With a population of 156.9 million people (including Madura) in mid 2024, proje ...
and released under an
Apache License The Apache License is a permissive free software license written by the Apache Software Foundation (ASF). It allows users to use the software for any purpose, to distribute it, to modify it, and to distribute modified versions of the software ...
. It can be used with many repositories such as Apache Solr,
Elasticsearch Elasticsearch is a Search engine (computing), search engine based on Apache Lucene, a free and open-source search engine. It provides a distributed, Multitenancy, multitenant-capable full-text search engine with an HTTP web interface and schema ...
, Microsoft Azure Cognitive Search, Amazon CloudSearch and more. * mnoGoSearch is a crawler, indexer and a search engine written in C and licensed under the GPL (*NIX machines only) * Open Search Server is a search engine and web crawler software release under the GPL. * Scrapy, an open source webcrawler framework, written in python (licensed under
BSD The Berkeley Software Distribution (BSD), also known as Berkeley Unix or BSD Unix, is a discontinued Unix operating system developed and distributed by the Computer Systems Research Group (CSRG) at the University of California, Berkeley, beginni ...
). * Seeks, a free distributed search engine (licensed under AGPL). * StormCrawler, a collection of resources for building low-latency, scalable web crawlers on Apache Storm (Apache License). * tkWWW Robot, a crawler based on the tkWWW web browser (licensed under GPL). * GNU Wget is a
command-line A command-line interface (CLI) is a means of interacting with software via commands each formatted as a line of text. Command-line interfaces emerged in the mid-1960s, on computer terminals, as an interactive and more user-friendly alternativ ...
-operated crawler written in C and released under the GPL. It is typically used to mirror Web and FTP sites. * YaCy, a free distributed search engine, built on principles of peer-to-peer networks (licensed under GPL).


See also

*
Automatic indexing Automatic indexing is the computerized process of scanning large volumes of documents against a controlled vocabulary, taxonomy, thesaurus or ontology and using those controlled terms to quickly and effectively index large electronic document depo ...
* Gnutella crawler *
Web archiving Web archiving is the process of collecting, preserving, and providing access to material from the World Wide Web. The aim is to ensure that information is preserved in an archival format for research and the public. Web archivists typically ...
*
Webgraph The webgraph describes the directed links between pages of the World Wide Web. A graph, in general, consists of several vertices, some pairs connected by edges. In a directed graph, edges are directed lines or arcs. The webgraph is a directed gra ...
* Website mirroring software *
Search Engine Scraping Search engine scraping is the process of harvesting URLs, descriptions, or other information from search engines. This is a specific form of screen scraping or web scraping dedicated to search engines only. Most commonly larger search engine opt ...
*
Web scraping Web scraping, web harvesting, or web data extraction is data scraping used for data extraction, extracting data from websites. Web scraping software may directly access the World Wide Web using the Hypertext Transfer Protocol or a web browser. W ...


References


Further reading

* Cho, Junghoo
"Web Crawling Project"
UCLA Computer Science Department.

from Wiley
WIVET
is a benchmarking project by
OWASP The Open Worldwide Application Security Project (formerly Open Web Application Security Project) (OWASP) is an online community that produces freely available articles, methodologies, documentation, tools, and technologies in the fields of Io ...
, which aims to measure if a web crawler can identify all the hyperlinks in a target website. * Shestakov, Denis
"Current Challenges in Web Crawling"
an
"Intelligent Web Crawling"
slides for tutorials given at ICWE'13 and WI-IAT'13. {{DEFAULTSORT:Web crawler Search engine software Internet search algorithms 12121212....>