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Applications of Data Structures and Algorithms

Danfeng YaoCS 16

3/13/2006

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Overview

Data structures in web interfaceGoogle – Indexing– PageRanking– Crawling

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Forward/backward buttons of Browser

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Forward/backward buttons and Stack

www.brown.edu

Back Forward

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Forward/backward buttons and Stack

www.dam.brown.edu

www.cs.brown.edu

www.brown.edu

www.engin.brown.edu

Back Forward

Old pages

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Then click the Back button once

www.cs.brown.edu

www.engin.brown.edu

www.brown.edu

www.dam.brown.edu

Back Forward

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Then click the Forward button once

www.dam.brown.edu

www.cs.brown.edu

www.engin.brown.edu

www.brown.edu

ForwardBack

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Planar point location: where is the mouse click?

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Simple point location: Binary decision tree

Build a binary tree– internal nodes corresponding to line segments – external nodes corresponding to regions

a

b

c

a

b

d

below

c

d

below

left

above

above

left

right

right

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The need for search engines to scale up What a search engine faces– Storage for index files (and maybe

documents themselves too)– Index system processes hundreds of

gigabytes– Queries at a rate of thousands per

secondAdvances in hardware technology– Faster CPU– Cheaper memory and disk space

But still, slow disk seek (~10ms) and operating system instabilityWhat will make today’s search engine scale?

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Google facts

Will be on the stock market soon – Estimated annual profit $150 million to

$350 million– Estimated annual revenue $500 million to

$1 billion– Estimated market value $12 billion to $20

billion

The heart of Google software is PageRankTM

Google has integrity– No one can buy a higher PageRank

Sergey Brin

Larry Page

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Data structures in Google– Compact data structures – Avoid disk seeks whenever possible

Data structures for indexing– Link structures– Inverted index

Page rankingCrawling

Google’s approach

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Web as a directed graph

(Brown, Brown CS)

(Brown CS, CS16)

(Brown CS, rt)

A hyperlink is an edge

A web page is a vertex

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Storing the link structures

Want to maintain the link relationship of two pages– Used for crawling, ranking, …

Main problem: how to store the set of pairs efficientlyURL is too long and has variable length– Storing (URLi, URLj) has too

much overhead and is slowUse a more compact docID and support fast docID/URLconversion docID URL

http://www.cs.brown.edu/people/rt/

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Forward index and inverted index

vitae securityinformationdesignalgorithm

algorithm

vitae graphdrawingdesignalgorithm

Forward

Inverted

graph

Hit 3: algorithmHit 2: algorithmHit 1: algorithm

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PageRanking: bringing order to the web

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GoogleBot: where to crawl?Through addURL forms or linksDeep crawling– BFS or DFS?– Rumored to crawl in

PageRank orderFresh crawling– Recrawling to keep index

updated

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More Google facts

Free lunch every day!Bring pets to workOn-site massage

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Bibliography

The Anatomy of a Large-Scale Hypertextual Web Search Engine. Sergey Brin and Lawrence PageThe PageRank Citation Ranking: Bringing Order to the Web. Lawrence Page