Published on January 4, 2008
Web Search: Web Search Spidering Spiders (Robots/Bots/Crawlers): Spiders (Robots/Bots/Crawlers) Start with a comprehensive set of root URL’s from which to start the search. Follow all links on these pages recursively to find additional pages. Index all novel found pages in an inverted index as they are encountered. May allow users to directly submit pages to be indexed (and crawled from). Search Strategies: Search Strategies Breadth-first Search Search Strategies (cont): Search Strategies (cont) Depth-first Search Search Strategy Trade-Off’s: Search Strategy Trade-Off’s Breadth-first explores uniformly outward from the root page but requires memory of all nodes on the previous level (exponential in depth). Standard spidering method. Depth-first requires memory of only depth times branching-factor (linear in depth) but gets “lost” pursuing a single thread. Both strategies implementable using a queue of links (URL’s). Avoiding Page Duplication: Avoiding Page Duplication Must detect when revisiting a page that has already been spidered (web is a graph not a tree). Must efficiently index visited pages to allow rapid recognition test. Tree indexing (e.g. trie) Hashtable Index page using URL as a key. Must canonicalize URL’s (e.g. delete ending “/”) Not detect duplicated or mirrored pages. Index page using textual content as a key. Requires first downloading page. Spidering Algorithm: Spidering Algorithm Initialize queue (Q) with initial set of known URL’s. Until Q empty or page or time limit exhausted: Pop URL, L, from front of Q. If L is not to an HTML page (.gif, .jpeg, .ps, .pdf, .ppt…) continue loop. If already visited L, continue loop. Download page, P, for L. If cannot download P (e.g. 404 error, robot excluded) continue loop. Index P (e.g. add to inverted index or store cached copy). Parse P to obtain list of new links N. Append N to the end of Q. Queueing Strategy: Queueing Strategy How new links added to the queue determines search strategy. FIFO (append to end of Q) gives breadth-first search. LIFO (add to front of Q) gives depth-first search. Heuristically ordering the Q gives a “focused crawler” that directs its search towards “interesting” pages. Restricting Spidering: Restricting Spidering Restrict spider to a particular site. Remove links to other sites from Q. Restrict spider to a particular directory. Remove links not in the specified directory. Obey page-owner restrictions (robot exclusion). Link Extraction: Link Extraction Must find all links in a page and extract URLs. <a href=“http://www.cs.utexas.edu/users/mooney/ir-course”> <frame src=“site-index.html”> Must complete relative URL’s using current page URL: <a href=“proj3”> to http://www.cs.utexas.edu/users/mooney/ir-course/proj3 <a href=“../cs343/syllabus.html”> to http://www.cs.utexas.edu/users/mooney/cs343/syllabus.html URL Syntax: URL Syntax A URL has the following syntax: <scheme>://<authority><path>?<query>#<fragment> An authority has the syntax: <host>:<port-number> A query passes variable values from an HTML form and has the syntax: <variable>=<value>&<variable>=<value>… A fragment is also called a reference or a ref and is a pointer within the document to a point specified by an anchor tag of the form: <A NAME=“<fragment>”> Java Spider: Java Spider Spidering code in ir.webutils package. Generic spider in Spider class. Does breadth-first crawl from a start URL and saves copy of each page in a local directory. This directory can then be indexed and searched using VSR InvertedIndex. Main method parameters: -u <start-URL> -d <save-directory> -c <page-count-limit> Java Spider (cont.): Java Spider (cont.) Robot Exclusion can be invoked to prevent crawling restricted sites/pages. -safe Specialized classes also restrict search: SiteSpider: Restrict to initial URL host. DirectorySpider: Restrict to below initial URL directory. Spider Java Classes: Spider Java Classes Link Canonicalization: Link Canonicalization Equivalent variations of ending directory normalized by removing ending slash. http://www.cs.utexas.edu/users/mooney/ http://www.cs.utexas.edu/users/mooney Internal page fragments (ref’s) removed: http://www.cs.utexas.edu/users/mooney/welcome.html#courses http://www.cs.utexas.edu/users/mooney/welcome.html Link Extraction in Java: Link Extraction in Java Java Swing contains an HTML parser. Parser uses “call-back” methods. Pass parser an object that has these methods: HandleText(char text, int position) HandleStartTag(HTML.Tag tag, MutableAttributeSet attributes, int position) HandleEndTag(HTML.Tag tag, int position) HandleSimpleTag (HTML.Tag tag, MutableAttributeSet attributes, int position) When parser encounters a tag or intervening text, it calls the appropriate method of this object. Link Extraction in Java (cont.): Link Extraction in Java (cont.) In HandleStartTag, if it is an “A” tag, take the HREF attribute value as an initial URL. Complete the URL using the base URL: new URL(URL baseURL, String relativeURL) Fails if baseURL ends in a directory name but this is not indicated by a final “/” Append a “/” to baseURL if it does not end in a file name with an extension (and therefore presumably is a directory). Cached File with Base URL : Cached File with Base URL Store copy of page in a local directory for eventual indexing for retrieval. BASE tag in the header section of an HTML file changes the base URL for all relative pointers: <BASE HREF=“<base-URL>”> This is specifically included in HTML for use in documents that were moved from their original location. Java Spider Trace: Java Spider Trace As a simple demo, SiteSpider was used to collect 100 pages starting at: UT CS Faculty Page See trace at: http://www.cs.utexas.edu/users/mooney/ir-course/proj3/spider-trace A larger crawl from the same page was used to assemble 300 pages that are cached at: /u/mooney/ir-code/corpora/cs-faculty/ Servlet Web Interface Demo: Servlet Web Interface Demo Web interface to using VSR to search directories of cached HTML files is at: http://www.cs.utexas.edu/users/mooney/ir-course/search.html The Java Servlet code supporting this demo is at: /u/ml/servlets/irs/Search.java Anchor Text Indexing: Anchor Text Indexing Extract anchor text (between <a> and </a>) of each link followed. Anchor text is usually descriptive of the document to which it points. Add anchor text to the content of the destination page to provide additional relevant keyword indices. Used by Google: <a href=“http://www.microsoft.com”>Evil Empire</a> <a href=“http://www.ibm.com”>IBM</a> Anchor Text Indexing (cont): Anchor Text Indexing (cont) Helps when descriptive text in destination page is embedded in image logos rather than in accessible text. Many times anchor text is not useful: “click here” Increases content more for popular pages with many in-coming links, increasing recall of these pages. May even give higher weights to tokens from anchor text. Robot Exclusion: Robot Exclusion Web sites and pages can specify that robots should not crawl/index certain areas. Two components: Robots Exclusion Protocol: Site wide specification of excluded directories. Robots META Tag: Individual document tag to exclude indexing or following links. Robots Exclusion Protocol: Robots Exclusion Protocol Site administrator puts a “robots.txt” file at the root of the host’s web directory. http://www.ebay.com/robots.txt http://www.cnn.com/robots.txt File is a list of excluded directories for a given robot (user-agent). Exclude all robots from the entire site: User-agent: * Disallow: / Robot Exclusion Protocol Examples: Robot Exclusion Protocol Examples Exclude specific directories: User-agent: * Disallow: /tmp/ Disallow: /cgi-bin/ Disallow: /users/paranoid/ Exclude a specific robot: User-agent: GoogleBot Disallow: / Allow a specific robot: User-agent: GoogleBot Disallow: User-agent: * Disallow: / Robot Exclusion Protocol Details: Robot Exclusion Protocol Details Only use blank lines to separate different User-agent disallowed directories. One directory per “Disallow” line. No regex patterns in directories. Robots META Tag: Robots META Tag Include META tag in HEAD section of a specific HTML document. <meta name=“robots” content=“none”> Content value is a pair of values for two aspects: index | noindex: Allow/disallow indexing of this page. follow | nofollow: Allow/disallow following links on this page. Robots META Tag (cont): Robots META Tag (cont) Special values: all = index,follow none = noindex,nofollow Examples: <meta name=“robots” content=“noindex,follow”> <meta name=“robots” content=“index,nofollow”> <meta name=“robots” content=“none”> Robot Exclusion Issues: Robot Exclusion Issues META tag is newer and less well-adopted than “robots.txt”. Standards are conventions to be followed by “good robots.” Companies have been prosecuted for “disobeying” these conventions and “trespassing” on private cyberspace. “Good robots” also try not to “hammer” individual sites with lots of rapid requests. “Denial of service” attack. Multi-Threaded Spidering: Multi-Threaded Spidering Bottleneck is network delay in downloading individual pages. Best to have multiple threads running in parallel each requesting a page from a different host. Distribute URL’s to threads to guarantee equitable distribution of requests across different hosts to maximize through-put and avoid overloading any single server. Early Google spider had multiple co-ordinated crawlers with about 300 threads each, together able to download over 100 pages per second. Directed/Focused Spidering: Directed/Focused Spidering Sort queue to explore more “interesting” pages first. Two styles of focus: Topic-Directed Link-Directed Topic-Directed Spidering: Topic-Directed Spidering Assume desired topic description or sample pages of interest are given. Sort queue of links by the similarity (e.g. cosine metric) of their source pages and/or anchor text to this topic description. Preferentially explores pages related to a specific topic. Robosurfer assignment in AI course. Link-Directed Spidering: Link-Directed Spidering Monitor links and keep track of in-degree and out-degree of each page encountered. Sort queue to prefer popular pages with many in-coming links (authorities). Sort queue to prefer summary pages with many out-going links (hubs). Keeping Spidered Pages Up to Date: Keeping Spidered Pages Up to Date Web is very dynamic: many new pages, updated pages, deleted pages, etc. Periodically check spidered pages for updates and deletions: Just look at header info (e.g. META tags on last update) to determine if page has changed, only reload entire page if needed. Track how often each page is updated and preferentially return to pages which are historically more dynamic. Preferentially update pages that are accessed more often to optimize freshness of more popular pages.