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Basic crawler operation

Basic crawler operation
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RyanCanon,United Arab Emirates,Teacher
Published Date:20-07-2017
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Introduction to Information Retrieval Introduction to Information Retrieval Crawling and web indexes www.ThesisScientist.comIntroduction to Information Retrieval Previous lecture recap  Web search  Spam  Size of the web  Duplicate detection  Use Jaccard coefficient for document similarity  Compute approximation of similarity using sketches www.ThesisScientist.comIntroduction to Information Retrieval Today’s lecture  Crawling www.ThesisScientist.comIntroduction to Information Retrieval Sec. 20.2 Basic crawler operation  Begin with known “seed” URLs  Fetch and parse them  Extract URLs they point to  Place the extracted URLs on a queue  Fetch each URL on the queue and repeat www.ThesisScientist.comIntroduction to Information Retrieval Sec. 20.2 Crawling picture URLs crawled and parsed Unseen Web URLs frontier Seed pages Web www.ThesisScientist.comIntroduction to Information Retrieval Sec. 20.1.1 Simple picture – complications  Web crawling isn’t feasible with one machine  All of the above steps distributed  Malicious pages  Spam pages  Spider traps – incl dynamically generated  Even non-malicious pages pose challenges  Latency/bandwidth to remote servers vary  Webmasters’ stipulations  How “deep” should you crawl a site’s URL hierarchy?  Site mirrors and duplicate pages  Politeness – don’t hit a server too often www.ThesisScientist.comIntroduction to Information Retrieval Sec. 20.1.1 What any crawler must do  Be Polite: Respect implicit and explicit politeness considerations  Only crawl allowed pages  Respect robots.txt (more on this shortly)  Be Robust: Be immune to spider traps and other malicious behavior from web servers www.ThesisScientist.comIntroduction to Information Retrieval Sec. 20.1.1 What any crawler should do  Be capable of distributed operation: designed to run on multiple distributed machines  Be scalable: designed to increase the crawl rate by adding more machines  Performance/efficiency: permit full use of available processing and network resources www.ThesisScientist.comIntroduction to Information Retrieval Sec. 20.1.1 What any crawler should do  Fetch pages of “higher quality” first  Continuous operation: Continue fetching fresh copies of a previously fetched page  Extensible: Adapt to new data formats, protocols www.ThesisScientist.comIntroduction to Information Retrieval Sec. 20.1.1 Updated crawling picture URLs crawled and parsed Unseen Web Seed Pages URL frontier Crawling thread www.ThesisScientist.comIntroduction to Information Retrieval Sec. 20.2 URL frontier  Can include multiple pages from the same host  Must avoid trying to fetch them all at the same time  Must try to keep all crawling threads busy www.ThesisScientist.comIntroduction to Information Retrieval Sec. 20.2 Explicit and implicit politeness  Explicit politeness: specifications from webmasters on what portions of site can be crawled  robots.txt  Implicit politeness: even with no specification, avoid hitting any site too often www.ThesisScientist.comIntroduction to Information Retrieval Sec. 20.2.1 Robots.txt  Protocol for giving spiders (“robots”) limited access to a website, originally from 1994  www.robotstxt.org/wc/norobots.html  Website announces its request on what can(not) be crawled  For a server, create a file /robots.txt  This file specifies access restrictions www.ThesisScientist.comIntroduction to Information Retrieval Sec. 20.2.1 Robots.txt example  No robot should visit any URL starting with "/yoursite/temp/", except the robot called “searchengine": User-agent: Disallow: /yoursite/temp/ User-agent: searchengine Disallow: www.ThesisScientist.comIntroduction to Information Retrieval Sec. 20.2.1 Processing steps in crawling  Pick a URL from the frontier Which one?  Fetch the document at the URL  Parse the URL  Extract links from it to other docs (URLs)  Check if URL has content already seen  If not, add to indexes E.g., only crawl .edu, obey robots.txt, etc.  For each extracted URL  Ensure it passes certain URL filter tests  Check if it is already in the frontier (duplicate URL elimination) www.ThesisScientist.comIntroduction to Information Retrieval Sec. 20.2.1 Basic crawl architecture DNS URL Doc robots set FP’s filters WWW Parse Dup Fetch URL Content URL seen? filter elim URL Frontier www.ThesisScientist.comIntroduction to Information Retrieval Sec. 20.2.2 DNS (Domain Name Server)  A lookup service on the internet  Given a URL, retrieve its IP address  Service provided by a distributed set of servers – thus, lookup latencies can be high (even seconds)  Common OS implementations of DNS lookup are blocking: only one outstanding request at a time  Solutions  DNS caching  Batch DNS resolver – collects requests and sends them out together www.ThesisScientist.comIntroduction to Information Retrieval Sec. 20.2.1 Parsing: URL normalization  When a fetched document is parsed, some of the extracted links are relative URLs  E.g., http://en.wikipedia.org/wiki/Main_Page has a relative link to /wiki/Wikipedia:General_disclaimer which is the same as the absolute URL http://en.wikipedia.org/wiki/Wikipedia:General_disclaimer  During parsing, must normalize (expand) such relative URLs www.ThesisScientist.comIntroduction to Information Retrieval Sec. 20.2.1 Content seen?  Duplication is widespread on the web  If the page just fetched is already in the index, do not further process it  This is verified using document fingerprints or shingles www.ThesisScientist.comIntroduction to Information Retrieval Sec. 20.2.1 Filters and robots.txt  Filters – regular expressions for URL’s to be crawled/not  Once a robots.txt file is fetched from a site, need not fetch it repeatedly  Doing so burns bandwidth, hits web server  Cache robots.txt files www.ThesisScientist.com