A
Web crawler is a computer program that browses the
World Wide WebThe World Wide Web is a system of interlinked hypertext documents accessed via the Internet...
in a methodical, automated manner or in an orderly fashion. Other terms for Web crawlers are
ants,
automatic indexers,
bots,
Web spiders,
Web robots, or—especially in the
FOAFFOAF is a machine-readable ontology describing persons, their activities and their relations to other people and objects. Anyone can use FOAF to describe him or herself...
community—
Web scutters.
This process is called
Web crawling or
spidering. Many sites, in particular
search enginesA web search engine is designed to search for information on the World Wide Web and FTP servers. The search results are generally presented in a list of results often referred to as SERPS, or "search engine results pages". The information may consist of web pages, images, information and other...
, use spidering as a means of providing up-to-date data. Web crawlers are mainly used to create a copy of all the visited pages for later processing by a search engine that will
indexSearch engine indexing collects, parses, and stores data to facilitate fast and accurate information retrieval. Index design incorporates interdisciplinary concepts from linguistics, cognitive psychology, mathematics, informatics, physics, and computer science...
the downloaded pages to provide fast searches. Crawlers can also be used for automating maintenance tasks on a Web site, such as checking links or validating
HTMLHyperText Markup Language is the predominant markup language for web pages. HTML elements are the basic building-blocks of webpages....
code. Also, crawlers can be used to gather specific types of information from Web pages,such as harvesting e-mail addresses (usually for sending spam).
A Web crawler is one type of
botInternet bots, also known as web robots, WWW robots or simply bots, are software applications that run automated tasks over the Internet. Typically, bots perform tasks that are both simple and structurally repetitive, at a much higher rate than would be possible for a human alone...
, or software agent. In general, it starts with a list of
URLsIn computing, a uniform resource locator or universal resource locator is a specific character string that constitutes a reference to an Internet resource....
to visit, called the
seeds. As the crawler visits these URLs, it identifies all the
hyperlinkIn computing, a hyperlink is a reference to data that the reader can directly follow, or that is followed automatically. A hyperlink points to a whole document or to a specific element within a document. Hypertext is text with hyperlinks...
s in the page 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.
The large volume implies that the crawler can only download a fraction of the Web pages within a given time, so it needs to prioritize its downloads. The high rate of change implies that the pages might have already been updated or even deleted.
The number of possible crawlable URLs being generated by server-side software has also made it difficult for web crawlers to avoid retrieving duplicate content. Endless combinations of HTTP GET (URL-based) parameters exist, of which only a small selection will actually return unique content. For example, a simple online photo gallery may offer three options to users, as specified through HTTP GET parameters in the URL. If there exist four ways to sort images, three choices of thumbnail size, two file formats, and an option to disable user-provided content, then the same set of content can be accessed with 48 different URLs, all of which may be linked on the site. This mathematical combination creates a problem for crawlers, as they must sort through endless combinations of relatively minor scripted changes in order to retrieve unique content.
As Edwards
et al. noted, "Given that the
bandwidthIn computer networking and computer science, bandwidth, network bandwidth, data bandwidth, or digital bandwidth is a measure of available or consumed data communication resources expressed in bits/second or multiples of it .Note that in textbooks on wireless communications, modem data transmission,...
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.
The behavior of a Web crawler is the outcome of a combination of policies:
- a selection policy that states which pages to download,
- a re-visit policy that states when to check for changes to the pages,
- a politeness policy that states how to avoid overloading Web sites, and
- 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 2005 study showed that large-scale search engines index no more than 40%-70% of the indexable Web; a previous study by Dr. Steve Lawrence and
Lee GilesC. Lee Giles is the David Reese Professor at the College of Information Sciences and Technology at the Pennsylvania State University. He is also Professor of Computer Science and Engineering, Professor of Supply Chain and Information Systems, and Director of the Intelligent Systems Research...
showed that no search engine indexed more than 16% of the Web in 1999. As a crawler always downloads just a fraction of the Web pages, it is highly desirable that the downloaded fraction contains 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 is the case of
vertical search enginesA vertical search engine, as distinct from a general web search engine, focuses on a specific segment of online content. The vertical content area may be based on topicality, media type, or genre of content. Common verticals include shopping, the automotive industry, legal information, medical...
restricted to a single
top-level domainA top-level domain is one of the domains at the highest level in the hierarchical Domain Name System of the Internet. The top-level domain names are installed in the root zone of the name space. For all domains in lower levels, it is the last part of the domain name, that is, the last label of a...
, 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.
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-firstIn graph theory, breadth-first search is a graph search algorithm that begins at the root node and explores all the neighboring nodes...
,
backlinkBacklinks, also known as incoming links, inbound links, inlinks, and inward links, are incoming links to a website or web page...
-count and partial
PagerankPageRank is a link analysis algorithm, named after Larry Page and used by the Google Internet search engine, that assigns a numerical weighting to each element of a hyperlinked set of documents, such as the World Wide Web, with the purpose of "measuring" its relative importance within the set...
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 Ph.D. 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 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. Surprisingly, some visits that accumulate PageRank very quickly (most notably, breadth-first and the omniscent 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 the OPIC strategy and a strategy that uses the length of the per-site queues are better than
breadth-firstIn graph theory, breadth-first search is a graph search algorithm that begins at the root node and explores all the neighboring nodes...
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.
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 Menczer and by 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 Pinkerton in the first web crawler of the early days of the Web. Diligenti
et al. propose to use 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..
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 World Wide Web, a query string is the part of a Uniform Resource Locator that contains data to be passed to web applications such as CGI programs....
in them (are dynamically produced) in order to avoid
spider trapA spider trap is a set of web pages that may intentionally or unintentionally be used to cause a web crawler or search bot to make an infinite number of requests or cause a poorly constructed crawler to crash. Web crawlers are also called web spiders, from which the name is derived...
s 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 to simplify its URLs.
URL normalization
Crawlers usually perform some type of
URL normalizationURL normalization is the process by which URLs are modified and standardized in a consistent manner. The goal of the normalization process is to transform a URL into a normalized or canonical URL so it is possible to determine if two syntactically different URLs may be equivalent.Search engines...
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 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.
Many path-ascending crawlers are also known as
Web harvestingWeb harvesting is commonly used to describe Web scraping from a multitude of sites. It also refers to an implementation of a Web crawler that uses human expertise or machine guidance to direct the crawler to URLs which compose a specialized collection or set of knowledge...
software, because they're used to "harvest" or collect all the content — perhaps the collection of photos in a gallery — from a specific page or host.
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:
-
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 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 out-dated, 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. Note that 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. Needless to say, if a single crawler is performing multiple requests per second and/or downloading large files, a server would 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 protocolThe Robot Exclusion Standard, also known as the Robots Exclusion Protocol or robots.txt protocol, is a convention to prevent cooperating web crawlers and other web robots from accessing all or part of a website which is otherwise publicly viewable. Robots are often used by search engines to...
, 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 Ask Jeeves, MSNMSN is a collection of Internet sites and services provided by Microsoft. The Microsoft Network debuted as an online service and Internet service provider on August 24, 1995, to coincide with the release of the Windows 95 operating system.The range of services offered by MSN has changed since its...
and YahooYahoo! Inc. is an American multinational internet corporation headquartered in Sunnyvale, California, United States. The company is perhaps best known for its web portal, search engine , Yahoo! Directory, Yahoo! Mail, Yahoo! News, Yahoo! Groups, Yahoo! Answers, advertising, online mapping ,...
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 connections 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. This does not seem acceptable.
Cho uses 10 seconds as an interval for accesses, and the WIRE crawler uses 15 seconds as 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 10t 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. BrinSergey Mikhaylovich Brin is a Russian-born American computer scientist and internet entrepreneur who, with Larry Page, co-founded Google, one of the largest internet companies. , his personal wealth is estimated to be $16.7 billion....
and PageLawrence "Larry" Page is an American computer scientist and internet entrepreneur who, with Sergey Brin, is best known as the co-founder of Google. As of April 4, 2011, he is also the chief executive of Google, as announced on January 20, 2011...
note that: "... 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 parallelParallel computing is a form of computation in which many calculations are carried out simultaneously, operating on the principle that large problems can often be divided into smaller ones, which are then solved concurrently . There are several different forms of parallel computing: bit-level,...
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:
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 spammingIn computing, spamdexing is the deliberate manipulation of search engine indexes...
", which prevent major search engines from publishing their ranking algorithms.
Crawler identification
Web crawlers typically identify themselves to a Web server by using the User-agentIn computing, a user agent is a client application implementing a network protocol used in communications within a client–server distributed computing system...
field of an HTTP request. Web site administrators typically examine their Web serverWeb server can refer to either the hardware or the software that helps to deliver content that can be accessed through the Internet....
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 URLIn computing, a uniform resource locator or universal resource locator is a specific character string that constitutes a reference to an Internet resource....
where the Web site administrator may find out more information about the crawler. Spambots 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.
It is important for Web crawlers to identify themselves so that Web site administrators can contact the owner if needed. In some cases, crawlers may be accidentally trapped in a crawler trapA spider trap is a set of web pages that may intentionally or unintentionally be used to cause a web crawler or search bot to make an infinite number of requests or cause a poorly constructed crawler to crash. Web crawlers are also called web spiders, from which the name is derived...
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 engineA web search engine is designed to search for information on the World Wide Web and FTP servers. The search results are generally presented in a list of results often referred to as SERPS, or "search engine results pages". The information may consist of web pages, images, information and other...
.
Examples
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:
- Yahoo! Slurp
Yahoo! Slurp is a web crawler from Yahoo! that obtains content for the Yahoo! Search engine. Slurp is based on search technology Yahoo! acquired when it purchased Inktomi.Slurp identifies itself by using the following User agent strings:...
is the name of the Yahoo Search crawler.
- Bingbot
bingbot is a web-crawling robot , deployed by Microsoft to supply Bing . It collects documents from the web to build a searchable index for the Bing . It replaced msnbot as the main Bing Crawler on October 2010.A typical user agent string for bingbot is " Mozilla/5.0 "...
is the name of Microsoft's BingBing is a web search engine from Microsoft.Bing may also refer to:* An onomatopœia of a bell sound* Bing cherry, a variety of cherry* Bing , Chinese flatbread* Bing , a German company that manufactured toys and kitchen utensils...
webcrawler. It replaced Msnbotmsnbot was a web-crawling robot , deployed by Microsoft to collect documents from the web to build a searchable index for the MSN Search engine. It went into beta in 2004, and had full public release in 2005. The month of October 2010 saw the official retirement of msnbot and its replacement by...
.
- FAST Crawler is a distributed crawler, used by Fast Search & Transfer
Fast Search & Transfer ASA is a Norwegian company based in Oslo. FAST focuses on data search technologies. It also has offices located in Germany, Italy, Sri Lanka, France, Japan, the United Kingdom, the United States, Brazil, Mexico and other countries around the world. The company was founded...
, and a general description of its architecture is available.
- Googlebot
Googlebot is the search bot software used by Google, which collects documents from the web to build a searchable index for the Google search engine....
is described in some detail, but the reference is only about an early version of its architecture, which was based in C++ and PythonPython is a general-purpose, high-level programming language whose design philosophy emphasizes code readability. Python claims to "[combine] remarkable power with very clear syntax", and its standard library is large and comprehensive...
. 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.
- Methabot is a scriptable web crawler written in C, released under the ISC license.
- PolyBot is a distributed crawler written in C++ and Python, which is composed of a "crawl manager", one or more "downloaders" and one or more "DNS resolvers". Collected URLs are added to a queue on disk, and processed later to search for seen URLs in batch mode. The politeness policy considers both third and second level domains (e.g.: www.example.com and www2.example.com are third level domains) because third level domains are usually hosted by the same Web server.
- RBSE was the first published web crawler. It was based on two programs: the first program, "spider" maintains a queue in a relational database, and the second program "mite", is a modified www ASCII
The American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...
browser that downloads the pages from the Web.
- WebCrawler
WebCrawler is a metasearch engine that blends the top search results from Google, Yahoo!, Bing Search , Ask.com, About.com, MIVA, LookSmart and other popular search engines. WebCrawler also provides users the option to search for images, audio, video, news, yellow pages and white pages...
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.
- World Wide Web Worm 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 text-search utility originally written for Unix. The name comes from the ed command g/re/p...
UnixUnix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
command.
- WebFountain
WebFountain is an Internet analytical engine implemented by IBM for the study of unstructured data on the World Wide Web. IBM describes WebFountain as:...
is a distributed, modular crawler similar to Mercator but written in C++. It features a "controller" machine that coordinates a series of "ant" machines. After repeatedly downloading pages, a change rate is inferred for each page and a non-linear programming method must be used to solve the equation system for maximizing freshness. The authors recommend to use this crawling order in the early stages of the crawl, and then switch to a uniform crawling order, in which all pages are being visited with the same frequency.
- WebRACE is a crawling and caching module implemented in Java, and used as a part of a more generic system called eRACE. The system receives requests from users for downloading web pages, so the crawler acts in part as a smart proxy server. The system also handles requests for "subscriptions" to Web pages that must be monitored: when the pages change, they must be downloaded by the crawler and the subscriber must be notified. The most outstanding feature of WebRACE is that, while most crawlers start with a set of "seed" URLs, WebRACE is continuously receiving new starting URLs to crawl from.
In addition to the specific crawler architectures listed above, there are general crawler architectures published by Cho
and Chakrabarti.
Open-source crawlers
- Aspseek
ASPseek is an open source internet search engine developed by SWsoft.It can index a few million URLs and search for words and phrases, use wildcards, and do a Boolean search...
is a crawler, indexer and a search engine written in C++ and licensed under the GPLThe GNU General Public License is the most widely used free software license, originally written by Richard Stallman for the GNU Project....
- crawler4j is a crawler written in Java and released under an Apache License
The Apache License is a copyfree free software license authored by the Apache Software Foundation . The Apache License requires preservation of the copyright notice and disclaimer....
. It can be configured in a few minutes and is suitable for educational purposes.
- Crawley Framework is a crawling framework that provides several types of crawlers. It's written in Python and released under the GPL
The GNU General Public License is the most widely used free software license, originally written by Richard Stallman for the GNU Project....
license.
- DataparkSearch
DataparkSearch is a search engine designed to organize search within a website, group of websites, intranet or local system.DataparkSearch is written in C. Distributed under the terms of the GNU General Public License, DataparkSearch is free software....
is a crawler and search engine released under the GNU General Public LicenseThe GNU General Public License is the most widely used free software license, originally written by Richard Stallman for the GNU Project....
.
- GNU Wget
GNU Wget is a computer program that retrieves content from web servers, and is part of the GNU Project. Its name is derived from World Wide Web and get...
is a command-line-operated crawler written in CC is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
and released under the GPLThe GNU General Public License is the most widely used free software license, originally written by Richard Stallman for the GNU Project....
. It is typically used to mirror Web and FTP sites.
- GRUB
Grub is an open source distributed search crawler platform.Users of Grub can download the peer-to-peer grubclient software and let it run during computer idle time. The client indexes the URLs and send them back to the main grub server in a highly compressed form...
is an open source distributed search crawler that Wikia SearchWikia Search was a short-lived free and open-source Web search engine launched by Wikia, a for-profit wiki-hosting company founded in late 2004 by Jimmy Wales and Angela Beesley....
uses to crawl the web.
- Heritrix
Heritrix is the Internet Archive’s web crawler, which was specially designed for web archiving. It is open-source and written in Java. The main interface is accessible using a web browser, and there is a command-line tool that can optionally be used to initiate crawls.Heritrix was developed...
is the Internet ArchiveThe Internet Archive is a non-profit digital library with the stated mission of "universal access to all knowledge". It offers permanent storage and access to collections of digitized materials, including websites, music, moving images, and nearly 3 million public domain books. The Internet Archive...
's archival-quality crawler, designed for archiving periodic snapshots of a large portion of the Web. It was written in JavaJava is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
.
- ht://Dig
ht://Dig is a free software indexing and searching system created in 1995 by Andrew Scherpbier while he was employed at San Diego State University. It can provide a search engine for a single website....
includes a Web crawler in its indexing engine.
- HTTrack
HTTrack is a free and open source Web crawler and offline browser, developed by Xavier Roche and licensed under the GNU General Public License....
uses a Web crawler to create a mirror of a web site for off-line viewing. It is written in CC is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
and released under the GPLThe GNU General Public License is the most widely used free software license, originally written by Richard Stallman for the GNU Project....
.
- ICDL Crawler
ICDL crawling is an open distributed web crawling technology based on Website Parse Template .- What is Website Parse Template? :Website Parse Template is an XML based open format which provides HTML structure description of Web pages. The WPT format allows web crawlers to generate Semantic Web’s...
is a cross-platformIn computing, cross-platform, or multi-platform, is an attribute conferred to computer software or computing methods and concepts that are implemented and inter-operate on multiple computer platforms...
web crawler written in C++C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
and intended to crawl Web sites based on Web-site Parse TemplatesWebsite Parse Template is an XML-based open format which provides HTML structure description of website pages. WPT format allows web crawlers to generate Semantic Web’s RDFs for web pages...
using computer's free CPU resources only.
- mnoGoSearch
mnoGoSearch is an open source search engine for Unix-like computer systems written in C. It is distributed under the GNU General Public License and designed to organize search within a website, group of websites, intranet or local system....
is a crawler, indexer and a search engine written in C and licensed under the GPLThe GNU General Public License is the most widely used free software license, originally written by Richard Stallman for the GNU Project....
(Linux machines only)
- Nutch
Nutch is an effort to build an open source web search engine based on Lucene Java for the search and index component.- Features :Nutch is coded entirely in the Java programming language, but data is written in language-independent formats...
is a crawler written in Java and released under an Apache LicenseThe Apache License is a copyfree free software license authored by the Apache Software Foundation . The Apache License requires preservation of the copyright notice and disclaimer....
. It can be used in conjunction with the LuceneApache Lucene is a free/open source information retrieval software library, originally created in Java by Doug Cutting. It is supported by the Apache Software Foundation and is released under the Apache Software License....
text-indexing package.
- Open Search Server is a search engine and web crawler software release under the GPL
The GNU General Public License is the most widely used free software license, originally written by Richard Stallman for the GNU Project....
.
- Pavuk
Pavuk is a GPL opensource web mirror software, with both command line and X Window GUI. Win32 ports are also available....
is a command-line Web mirror tool with optional X11 GUI crawler and released under the GPLThe GNU General Public License is the most widely used free software license, originally written by Richard Stallman for the GNU Project....
. It has bunch of advanced features compared to wget and httrack, e.g., regular expression based filtering and file creation rules.
- the tkWWW Robot, a crawler based on the tkWWW
tkWWW was an early web browser/WYSIWYG HTML editor written by Joseph Wang at the MIT as part of the Project Athena and the Globewide Network Academy project. The browser was based on the Tcl language and the tk toolkit extension but did not achieve broad user acceptance or market share although it...
web browser (licensed under GPLThe GNU General Public License is the most widely used free software license, originally written by Richard Stallman for the GNU Project....
).
- YaCy
YaCy is a free distributed search engine, built on principles of peer-to-peer networks. Its core is a computer program written in Java distributed on several hundred computers, , so-called YaCy-peers...
, a free distributed search engine, built on principles of peer-to-peer networks (licensed under GPLThe GNU General Public License is the most widely used free software license, originally written by Richard Stallman for the GNU Project....
).
Crawling the Deep Web
A vast amount of Web pages lie in the deep or invisible WebThe Deep Web refers to World Wide Web content that is not part of the Surface Web, which is indexed by standard search engines....
. 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 Sitemap Protocol and mod oaimod_oai is an Apache module that allows web crawlers to efficiently discover new, modified, and deleted web resources from a web server by using OAI-PMH, a protocol which is widely used in the digital libraries community...
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 <a href="URL"-shaped URLs. In some cases, such as the GooglebotGooglebot is the search bot software used by Google, which collects documents from the web to build a searchable index for the Google search engine....
, Web crawling is done on all text contained inside the hypertext content, tags, or text.
Crawling Web 2.0 Applications
See also
- Distributed web crawling
Distributed web crawling is a distributed computing technique whereby Internet search engines employ many computers to index the Internet via web crawling. Such systems may allow for users to voluntarily offer their own computing and bandwidth resources towards crawling web pages...
- Focused crawler
A focused crawler or topical crawler is a web crawler that attempts to download only web pages that are relevant to a pre-defined topic or set of topics. Topical crawling generally assumes that only the topic is given, while focused crawling also assumes that some labeled examples of relevant and...
- Search Engine Indexing – the step after crawling
- Spambot
A spambot is an automated computer program designed to assist in the sending of spam. Spambots usually create fake accounts and send spam using them, although it would be obvious that a spambot is sending it...
- Spider trap
A spider trap is a set of web pages that may intentionally or unintentionally be used to cause a web crawler or search bot to make an infinite number of requests or cause a poorly constructed crawler to crash. Web crawlers are also called web spiders, from which the name is derived...
- Spidering Hacks
Spidering Hacks: 100 Industrial-Strength Tips and Tools is a book of tips about spidering software, software which traverses data on the internet cataloging the information it finds. The book was written by Kevin Hemenway and Tara Calishain . The book was published by O'Reilly in October 2003...
– an O'Reilly book focused on spider-like programming
- Web archiving
Web archiving is the process of collecting portions of the World Wide Web and ensuring the collection is preserved in an archive, such as an archive site, for future researchers, historians, and the public. Due to the massive size of the Web, web archivists typically employ web crawlers for...
- Website mirroring software
- Website Parse Template
Website Parse Template is an XML-based open format which provides HTML structure description of website pages. WPT format allows web crawlers to generate Semantic Web’s RDFs for web pages...
- Web scraping
Web scraping is a computer software technique of extracting information from websites...
- 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...
Further reading