Crawling and web indexes. Today’s lecture Crawling Connectivity servers

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  • Slide 1
  • Crawling and web indexes
  • Slide 2
  • Todays lecture Crawling Connectivity servers
  • Slide 3
  • Basic crawler operation Begin with known seed pages Fetch and parse them Extract URLs they point to Place the extracted URLs on a queue Fetch each URL on the queue and repeat 20.2
  • Slide 4
  • Crawling picture Web URLs crawled and parsed URLs frontier Unseen Web Seed pages 20.2
  • Slide 5
  • Simple picture complications Web crawling isnt feasible with one machine All of the above steps distributed Even non-malicious pages pose challenges Latency/bandwidth to remote servers vary Webmasters stipulations How deep should you crawl a sites URL hierarchy? Site mirrors and duplicate pages Malicious pages Spam pages Spider traps incl dynamically generated Politeness dont hit a server too often 20.1.1
  • Slide 6
  • 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 20.1.1
  • Slide 7
  • 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 20.1.1
  • Slide 8
  • 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 20.1.1
  • Slide 9
  • Updated crawling picture URLs crawled and parsed Unseen Web Seed Pages URL frontier Crawling thread 20.1.1
  • Slide 10
  • 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 20.2
  • Slide 11
  • 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 20.2
  • Slide 12
  • 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 URL, create a file URL/robots.txt This file specifies access restrictions 20.2.1
  • Slide 13
  • 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: 20.2.1
  • Slide 14
  • Processing steps in crawling Pick a URL from the frontier 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 For each extracted URL Ensure it passes certain URL filter tests Check if it is already in the frontier (duplicate URL elimination) E.g., only crawl.edu, obey robots.txt, etc. Which one? 20.2.1
  • Slide 15
  • Basic crawl architecture WWW DNS Parse Content seen? Doc FPs Dup URL elim URL set URL Frontier URL filter robots filters Fetch 20.2.1
  • Slide 16
  • 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 20.2.2
  • Slide 17
  • Parsing: URL normalization When a fetched document is parsed, some of the extracted links are relative URLs E.g., at http://en.wikipedia.org/wiki/Main_Page http://en.wikipedia.org/wiki/Main_Page we have a relative link to /wiki/Wikipedia:General_disclaimer which is the same as the absolute URL http://en.wikipedia.org/wiki/Wikipedia:General_disclaimer http://en.wikipedia.org/wiki/Wikipedia:General_disclaimer During parsing, must normalize (expand) such relative URLs 20.2.1
  • Slide 18
  • 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 20.2.1
  • Slide 19
  • Filters and robots.txt Filters regular expressions for URLs 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 20.2.1
  • Slide 20
  • Duplicate URL elimination For a non-continuous (one-shot) crawl, test to see if an extracted+filtered URL has already been passed to the frontier For a continuous crawl see details of frontier implementation 20.2.1
  • Slide 21
  • Distributing the crawler Run multiple crawl threads, under different processes potentially at different nodes Geographically distributed nodes Partition hosts being crawled into nodes Hash used for partition How do these nodes communicate? 20.2.1
  • Slide 22
  • Communication between nodes The output of the URL filter at each node is sent to the Duplicate URL Eliminator at all nodes WWW Fetch DNS Parse Content seen? URL filter Dup URL elim Doc FPs URL set URL Frontier robots filters Host splitter To other hosts From other hosts
  • Slide 23
  • URL frontier: two main considerations Politeness: do not hit a web server too frequently Freshness: crawl some pages more often than others E.g., pages (such as News sites) whose content changes often These goals may conflict each other. (E.g., simple priority queue fails many links out of a page go to its own site, creating a burst of accesses to that site.) 20.2.3
  • Slide 24
  • Politeness challenges Even if we restrict only one thread to fetch from a host, can hit it repeatedly Common heuristic: insert time gap between successive requests to a host that is >> time for most recent fetch from that host 20.2.3
  • Slide 25
  • URL frontier: Mercator scheme Prioritizer Biased front queue selector Back queue router Back queue selector K front queues B back queues Single host on each URLs Crawl thread requesting URL 20.2.3
  • Slide 26
  • Mercator URL frontier URLs flow in from the top into the frontier Front queues manage prioritization Back queues enforce politeness Each queue is FIFO 20.2.3
  • Slide 27
  • Front queues Prioritizer 1K Biased front queue selector Back queue router 20.2.3
  • Slide 28
  • Front queues Prioritizer assigns to URL an integer priority between 1 and K Appends URL to corresponding queue Heuristics for assigning priority Refresh rate sampled from previous crawls Application-specific (e.g., crawl news sites more often) 20.2.3
  • Slide 29
  • Biased front queue selector When a back queue requests a URL (in a sequence to be described): picks a front queue from which to pull a URL This choice can be round robin biased to queues of higher priority, or some more sophisticated variant Can be randomized 20.2.3
  • Slide 30
  • Back queues Biased front queue selector Back queue router Back queue selector 1B Heap
  • Slide 31
  • Back queue invariants Each back queue is kept non-empty while the crawl is in progress Each back queue only contains URLs from a single host Maintain a table from hosts to back queues Host nameBack queue 3 1 B 20.2.3
  • Slide 32
  • Back queue heap One entry for each back queue The entry is the earliest time t e at which the host corresponding to the back queue can be hit again This earliest time is determined from Last access to that host Any time buffer heuristic we choose 20.2.3
  • Slide 33
  • Back queue processing A crawler thread seeking a URL to crawl: Extracts the root of the heap Fetches URL at head of corresponding back queue q (look up from table) Checks if queue q is now empty if so, pulls a URL v from front queues If theres already a back queue for vs host, append v to q and pull another URL from front queues, repeat Else add v to q When q is non-empty, create heap entry for it
  • Slide 34
  • Number of back queues B Keep all threads busy while respecting politeness Mercator recommendation: three times as many back queues as crawler threads 20.2.3
  • Slide 35
  • Connectivity servers 20.4
  • Slide 36
  • Connectivity Server [CS1: Bhar98b, CS2 & 3: Rand01] Support for fast queries on the web graph Which URLs point to a given URL? Which URLs does a given URL point to? Stores mappings in memory from URL to outlinks, URL to inlinks Applications Crawl control Web graph analysis Connectivity, crawl optimization Link analysis 20.4
  • Slide 37
  • Champion published work Boldi and Vigna http://www2004.org/proceedings/docs/1p59 5.pdf http://www2004.org/proceedings/docs/1p59 5.pdf Webgraph set of algorithms and a java implementation Fundamental goal maintain node adjacency lists in memory For this, compressing the adjacency lists is the critical component 20.4
  • Slide 38
  • Adjacency lists The set of neighbors of a node Assume each URL represented by an integer E.g., for a 4 billion page web, need 32 bits per node Naively, this demands 64 bits to represent each hyperlink 20.4
  • Slide 39
  • Adjaceny list compression Properties exploited in compression: Similarity (between lists) Locality (many links from a page go to nearby pages) Use gap encodings in sorted lists Distribution of gap values 20.4
  • Slide 40
  • Storage Boldi/Vigna get down to an average of ~3 bits/link (URL to URL edge) For a 118M node web graph How? Why is this remarkable? 20.4
  • Slide 41
  • Main ideas of Boldi/Vigna Consider lexicographically ordered list of all URLs, e.g., www.stanford.edu/alchemy www.stanford.edu/biology www.stanford.edu/biology/plant www.stanford.edu/biology/plant/copyright www.stanford.edu/biology/plant/people www.stanford.edu/chemistry 20.4
  • Slide 42
  • Boldi/Vigna Each of these URLs has an adjacency list Main idea: due to templates, the adjacency list of a node is similar to one of the 7 preceding URLs in the lexicographic ordering Express adjacency list in terms of one of these E.g., consider these adjacency lists 1, 2, 4, 8, 16, 32, 64 1, 4, 9, 16, 25, 36, 49, 64 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144 1, 4, 8, 16, 25, 36, 49, 64 Encode as (-2), remove 9, add 8 Why 7? 20.4
  • Slide 43
  • Duplicate/Near-Duplicate Detection Duplication: Exact match with fingerprints Near-Duplication: Approximate match Overview Compute syntactic similarity with an edit- distance measure Use similarity threshold to detect near- duplicates E.g., Similarity > 80% => Documents are near duplicates Not transitive though sometimes used transitively
  • Slide 44
  • Computing Near Similarity Features: Segments of a document (natural or artificial breakpoints) [Brin95] Shingles (Word N-Grams) [Brin95, Brod98] a rose is a rose is a rose => a_rose_is_a rose_is_a_rose is_a_rose_is Similarity Measure TFIDF [Shiv95] Set intersection [Brod98] (Specifically, Size_of_Intersection / Size_of_Union )
  • Slide 45
  • Shingles + Set Intersection Computing exact set intersection of shingles between all pairs of documents is expensive and infeasible Approximate using a cleverly chosen subset of shingles from each (a sketch)
  • Slide 46
  • Shingles + Set Intersection Estimate size_of_intersection / size_of_union based on a short sketch ( [Brod97, Brod98] ) Create a sketch vector (e.g., of size 200) for each document Documents which share more than t (say 80%) corresponding vector elements are similar For doc D, sketch[ i ] is computed as follows: Let f map all shingles in the universe to 0..2 m (e.g., f = fingerprinting) Let i be a specific random permutation on 0..2 m Pick sketch[i] := MIN i ( f(s) ) over all shingles s in D
  • Slide 47
  • Computing Sketch[i] for Doc1 Document 1 2 64 Start with 64 bit shingles Permute on the number line with i Pick the min value
  • Slide 48
  • Test if Doc1.Sketch[i] = Doc2.Sketch[i] Document 1 Document 2 2 64 Are these equal? Test for 200 random permutations: , , 200 AB
  • Slide 49
  • However Document 1 Document 2 2 64 A = B iff the shingle with the MIN value in the union of Doc1 and Doc2 is common to both (I.e., lies in the intersection) This happens with probability: Size_of_intersection / Size_of_union B A
  • Slide 50
  • Question Document D1=D2 iff size_of_intersection=size_of_union ?
  • Slide 51
  • Mirror Detection Mirroring is systematic replication of web pages across hosts. Single largest cause of duplication on the web Host1/ and Host2/ are mirrors iff For all (or most) paths p such that when http://Host1/ / p exists http://Host2/ / p exists as well with identical (or near identical) content, and vice versa.
  • Slide 52
  • Mirror Detection example http://www.elsevier.com/ and http://www.elsevier.nl/ Structural Classification of Proteins http://scop.mrc-lmb.cam.ac.uk/scop http://scop.berkeley.edu/ http://scop.wehi.edu.au/scop http://pdb.weizmann.ac.il/scop http://scop.protres.ru/
  • Slide 53
  • Repackaged Mirrors Auctions.msn.com Auctions.lycos.com Aug 2001
  • Slide 54
  • Motivation Why detect mirrors? Smart crawling Fetch from the fastest or freshest server Avoid duplication Better connectivity analysis Combine inlinks Avoid double counting outlinks Redundancy in result listings If that fails you can try: /samepath Proxy caching
  • Slide 55
  • Maintain clusters of subgraphs Initialize clusters of trivial subgraphs Group near-duplicate single documents into a cluster Subsequent passes Merge clusters of the same cardinality and corresponding linkage Avoid decreasing cluster cardinality To detect mirrors we need: Adequate path overlap Contents of corresponding pages within a small time range Bottom Up Mirror Detection [Cho00]
  • Slide 56
  • Can we use URLs to find mirrors? www.synthesis.org a b c d synthesis.stanford.edu a b c d www.synthesis.org/Docs/ProjAbs/synsys/synalysis.html www.synthesis.org/Docs/ProjAbs/synsys/visual-semi-quant.html www.synthesis.org/Docs/annual.report96.final.html www.synthesis.org/Docs/cicee-berlin-paper.html www.synthesis.org/Docs/myr5 www.synthesis.org/Docs/myr5/cicee/bridge-gap.html www.synthesis.org/Docs/myr5/cs/cs-meta.html www.synthesis.org/Docs/myr5/mech/mech-intro-mechatron.html www.synthesis.org/Docs/m...

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