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Basic crawler operation

Basic crawler operation
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WilliamsMcmahon,United States,Professional
Published Date:20-07-2017
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Introduction to Information Retrieval Introduction to Information Retrieval Basic crawler operation 1Introduction to Information Retrieval Overview ❶ Recap ❷ A simple crawler ❸ A real crawler 2Introduction to Information Retrieval Outline ❶ Recap ❷ A simple crawler ❸ A real crawler 3Introduction to Information Retrieval Search engines rank content pages and ads 4 4Introduction to Information Retrieval Google’s second price auction  bid: maximum bid for a click by advertiser  CTR: click-through rate: when an ad is displayed, what percentage of time do users click on it? CTR is a measure of relevance.  ad rank: bid × CTR: this trades off (i) how much money the advertiser is willing to pay against (ii) how relevant the ad is  paid: Second price auction: The advertiser pays the minimum amount necessary to maintain their position in the auction (plus 1 cent). 5 5Introduction to Information Retrieval What’s great about search ads  Users only click if they are interested.  The advertiser only pays when a user clicks on an ad.  Searching for something indicates that you are more likely to buy it . . .  . . . in contrast to radio and newpaper ads. 6 6Introduction to Information Retrieval Near duplicate detection: Minimum of permutation document 1: s document 2: s k k Roughly: We use as a test for: are d and d near-duplicates? 1 2 7 7Introduction to Information Retrieval Example h(x) = x mod 5 g(x) = (2x + 1) mod 5 final sketches 8 8Introduction to Information Retrieval Outline ❶ Recap ❷ A simple crawler ❸ A real crawler 9Introduction to Information Retrieval How hard can crawling be?  Web search engines must crawl their documents.  Getting the content of the documents is easier for many other IR systems.  E.g., indexing all files on your hard disk: just do a recursive descent on your file system  Ok: for web IR, getting the content of the documents takes longer . . .  . . . because of latency.  But is that really a design/systems challenge? 10 10Introduction to Information Retrieval Basic crawler operation  Initialize queue with URLs of known seed pages  Repeat  Take URL from queue  Fetch and parse page  Extract URLs from page  Add URLs to queue  Fundamental assumption: The web is well linked. 11 11Introduction to Information Retrieval Exercise: What’s wrong with this crawler? urlqueue := (some carefully selected set of seed urls) while urlqueue is not empty: myurl := urlqueue.getlastanddelete() mypage := myurl.fetch() fetchedurls.add(myurl) newurls := mypage.extracturls() for myurl in newurls: if myurl not in fetchedurls and not in urlqueue: urlqueue.add(myurl) addtoinvertedindex(mypage) 12 12Introduction to Information Retrieval What’s wrong with the simple crawler  Scale: we need to distribute.  We can’t index everything: we need to subselect. How?  Duplicates: need to integrate duplicate detection  Spam and spider traps: need to integrate spam detection  Politeness: we need to be “nice” and space out all requests for a site over a longer period (hours, days)  Freshness: we need to recrawl periodically.  Because of the size of the web, we can do frequent recrawls only for a small subset.  Again, subselection problem or prioritization 13 13Introduction to Information Retrieval Magnitude of the crawling problem  To fetch 20,000,000,000 pages in one month . . .  . . . we need to fetch almost 8000 pages per second  Actually: many more since many of the pages we attempt to crawl will be duplicates, unfetchable, spam etc. 14 14Introduction to Information Retrieval What a crawler must do Be polite  Don’t hit a site too often  Only crawl pages you are allowed to crawl: robots.txt Be robust  Be immune to spider traps, duplicates, very large pages, very large websites, dynamic pages etc 15 15Introduction to Information Retrieval Robots.txt  Protocol for giving crawlers (“robots”) limited access to a website, originally from 1994  Examples:  User-agent: Disallow: /yoursite/temp/  User-agent: searchengine Disallow: /  Important: cache the robots.txt file of each site we are crawling 16 16Introduction to Information Retrieval Example of a robots.txt ( User-agent: PicoSearch/1.0 Disallow: /news/information/knight/ Disallow: /nidcd/ ... Disallow: /news/research_matters/secure/ Disallow: /od/ocpl/wag/ User-agent: Disallow: /news/information/knight/ Disallow: /nidcd/ ... Disallow: /news/research_matters/secure/ Disallow: /od/ocpl/wag/ Disallow: /ddir/ Disallow: /sdminutes/ 17 17Introduction to Information Retrieval What any crawler should do  Be capable of distributed operation  Be scalable: need to be able to increase crawl rate by adding more machines  Fetch pages of higher quality first  Continuous operation: get fresh version of already crawled pages 18 18Introduction to Information Retrieval Outline ❶ Recap ❷ A simple crawler ❸ A real crawler 19Introduction to Information Retrieval URL frontier 20 20