How a Search Engine Works_report

8/8/2019 How a Search Engine Works_report

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DEPARTMENT OF COMPUTER SCIENCE AND

ENGINEERING

SYNERGY INSTITUTE OF ENGINEERING &

TECHNOLOGY, DHENKANAL

SEMINAR ON

How a search engineworks?


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Seminar Report 10 How a search engine works

Dept. of CSE S.I.E.T, Dhenkanal

Guided by : XxXSubmitted by:

SOVAN MISRACS-O7-42

07012301472


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DEPARTMENT OF COMPUTER SCIENCE &

ENGINEERING

SYNERGY INSTITUTE OF ENGINEERING

AND TECHNOLOGY

DHENKANAL

CERTIFICATE

Certified that this is a bonafide record of the seminar entitledHOW A

SEARCH ENGINE WORKSdone by the following student SOVAN

MISRAof the 7th semester, Computer Science and Engineering in the

year 2010 in partial fulfilment of the requirements of the award of

Degree of Bachelor of Technology in Computer Science and

Engineering of Synergy Institute Of Engineering And Technology,

Dhenkanal

XxX XxX

Seminar Guide Head of the Department

Dept. of CSE S.I.E.T, Dhenkanal3


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ACKNOWLEDGEMENT

I thank my seminar guide XxX, Lecturer, SIET, for her proper

guidance, and valuable suggestions. I am indebted to XxX, the HOD,

Computer Science department & other faculty members for giving me

an opportunity to learn and do this seminar. If not for the above

mentioned people my seminar would never have been completed

successfully. I once again extend my sincere thanks to all of them.

SOVAN MISRA



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HOW SEARCH ENGINE WORKS?

INTRODUCTION

What is a search engine?

Search engine is a software program that searches a database and gathers

reports, information that contains or is related to specified terms.Or

It is a website whose primary function is providing a search for gathering

and reporting informations available on the Internet or a portion of

internet.

Why Search Engine?

In todays world we have million and billions of information available inthe vast World Wide Web (WWW). If one has to search some information

it will kill lots of time of the user. For this purpose we should have certain

tools for making this searching automatic, Quick, and Effortless.

So, to reduce the problem to a , more or less manageable solution, Web

Search Engine were introduced a few years ago.

Different Search engines:



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History of search engines:-

In 1990, the first search engine ARCHIE was released, at that time there is

no World Wide Web. Data resided on defence contractor, university, and

government computers and techies were the only people accessing the data.

The computers are interconnected by Telnet*.

File Transfer Protocol (FTP) used for transferring file from computer to

computer.There is no such thing called a Browser. So, information or data are

transferred in their native format and viewed using the associated file type

software.

Archie searched FTP servers and indexed their files into a searchable

directory.

In 1991, Ghopherspace come into existence with the advantage of Gopher.

It catalogued FTP sites and the resulting catalogue become known as

Gopher space.

In 1994, WebCrawler, a new type of search engine that indexed the entirecontents pf a webpage, was introduced.

In between 1995-1998, many changes and development occurred in the

world of search engines. Meta tags* is the webpage were first utilized by

some search engines to determine relevancy.

Search engine rank-checking software was introduced. It provides an

automated tool to determine web sites position and ranking within the

major search engines.

In around 1998, search engine Algorithms was introduced to optimize the

searching.In 2000, Marketer determines that pay-per click campaigns were an easy

yet expensive approach for gaining top search rankings. To elevate sites in

the searching engine ranking websites started adding useful and relevant

content while optimizing their

WebPages for each specific search engines. And still the search engines

optimization (SEO) is going on by improving the algorithms.

TYPE OF SEARCH ENGINES:



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On the basis of working, search engine is categories in the following

groups:

* Crawler-based search engine.

* Directories.* Hybrid search engines.

* Meta search engine.

CRAWLER BASED SEARCH ENGINE:

It uses automated software programs to survey and categorises WebPages,

which is known as spiders ,crawlers ,robots and bots.

A spider will find a web page, download it and analyses the information

presented on the WebPages. The webpage will then be added to the search

engines database.

When a user performs a search , the search engine will check its database

of WebPages for the key word the user searched.

The results are ordered as per the bots algorithm in the search engine result

pages (SERPs).

Ex:-

GOOGLE (www.google.com)

ASK (www.ask.com)

http://www.google.com/http://www.ask.com/http://www.google.com/http://www.ask.com/


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SPIDERS ALGORITHMS :

All spiders use the following algorithms for retrieving documents from the

web:

The algorithm uses a list of known URLs. This lists contains at least one

URL to start with.

The document is parsed to retrieve information for the index database andto extract the embedded link to other documents.



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The URL of the links found in the document are added to the list of known

URLs.

If the list is empty or some limit exceed (number of documents retrieved,

size of the indexed database, etc) the algorithm stops, otherwise the

algorithm continues at steps 2.

Crawlers program treats World Wide Web as big graph having pages as

nodes and the hyperlinks as arcs.

Crawlers works with a simple goal, indexing all keywords in the

webpage titles.

Three Data structures is needed for crawlers or spider algorithms

A large linear array, URL_Table.

Heap

Hash table



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URL_table:

It is a large linear array that contains millions of entries.

Each entry contains two pointers:

Pointer to URL

Pointer to Title.

These are variables length strings and kept as heap.

Heap:It is a large unstructured chunk of virtual memory to which strings can be

appended.

Hash table:It is the third data structure of size n entries

Any URL can be run through a hash function to produce a non-negative

integer less than n.

All URL that hash to the value k are hooked together on a linked list

starting at the entry k of the hash table.

Every entry in the URL_table is also entered into the hash table.

The main use of hash table is to start with a URL and be able to quickly

determine whether it is already present in URL_Table.



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DATA STRUCTURE FOR CRAWLER

Building the index requires two phases: Searching (URL processing )

Indexing.

The heart of the search engine is a recursive procedure procees_url, which

takes a URL string as input

Searching is done by procedure, procees_url as follows:-

It hashes the URL to see if it is already present in url_table. If so, it

is done and returns immediately.

If the URL is not already known, its page is fetched.

The URL and title are then copied to the heap and pointers to these

two strings are entered in url_table.

The URL is also entered into the hash table.

Finally, process_url extracts all the hyperlinks from the page and

calls process_url once per hyperlink, passing the hyperlinks URL as the

input parameter

For each entry in url_table, indexing procedure will examine the title

and selects out all words not on the stop list.



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Each selected word is written on to a file with a line consisting of the

word followed by the current url_table entry number.

When the whole table has been scanned, the file is shorted by word.

Formulating quires:

Keyword submission cause a request to be done in the machine

where the index is located (web server).

Then the keyword is looked up in the index database to find the set

of URL indices for each keyword.Indexed into url_table to find all the titles and urls. Then it is stored

in the Document server.

These are then combined to form a web page and sent back to user as the

response.

Determining Relevance

Classic algorithm "TF / IDFis used for determining relevance.



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It is a weight often used in information retrieval and text mining.

This weight is a statistical measure used to evaluate how important a

word is to a document in a collection

A high weight in TF-IDF is reached by a high term frequency (in thegiven document) and a low document frequency of the term in the

whole collection of documentsTerm Frequency

Term Frequency -The number of times a given term appears in

that document.

It gives a measure of the importance of the term ti within theparticular document.

Term Frequency,

Where, ni is the number of occurrences of the considered term, andthe denominator is the number of occurrences of all terms.

E.g.

If a document contains 100 total words and the word computer

appears 3 times, then the term frequency of the word computerin thedocument is 0.03 (3/100)

Inverse Document Frequency

The inverse document frequency is a measure of the general importanceof the term (obtained by dividing the number of all documents by the

number of documents containing the term, and then taking the logarithm of

that quotient).

Where,

|D | : total number of documents in the corpus



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: number of documents where the term tiappears (that is ni!= 0)

Inverse Document Frequency

There are different ways of calculating the IDF

Document Frequency (DF) is to determine how many

documents contain the word anddivide it by the total number ofdocuments in the collection.

E.g.1) If the word computerappears in 1,000 documents out of

a total of 10,000,000 then the IDF is 0.0001(1000/10,000,000).

2) Alternately, take the log of the document frequency.

The natural alogarithm is commonly used. In this

example we would have

IDF = ln(1,000 / 10,000,000) =1/ 9.21

The final TF-IDF score is then calculated by dividing the Term

Frequency by the Document Frequency.

E.g.

The TF-IDF score forcomputerin the collection would be :

1)TF-IDF = 0.03/0.0001= 300 , by using first formula of IDF.2)If alternate formula used we would have

TF-IDF = 0.03 * 9.21 = 0.27.



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OTHER TYPE OF SERCHING TECHNIQUES:

Directories

The human editors comprehensively check the website and

rank it, based on the information they find, using a pre-defined set of

rules.There are two major directories :

Yahoo Directory (www.yahoo.com)

Open Directory (www.dmoz.org)

Hybrid Search EnginesHybrid search engines use a combination of both crawler-based

results and directory results.

http://www.yahoo.com/http://www.dmoz.org/http://www.yahoo.com/http://www.dmoz.org/


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Examples of hybrid search engines are:

Yahoo (www.yahoo.com)

Google (www.google.com)

Meta Search Engines

Also known as Multiple Search Engines or Metacrawlers.

Meta search engines query several other Web search engine

databases in parallel and then combine the results in one list.

Examples of Meta search engines include:

Metacrawler (www.metacrawler.com)

Dogpile (www.dogpile.com)

References:

http://computer.howstuffworks.com/internet/basics/search-engine.htm

http://searchenginewatch.com/2168031

http://www.infotoday.com/searcher/may01/liddy.htm

http://www.slideshare.net/jsuleiman/how-search-engines-work-

presentation

Hey!

This is Sovan

Please send your feedbacks @

[email protected]

http://www.yahoo.com/http://www.google.com/http://www.metacrawler.com/http://www.dogpile.com/http://computer.howstuffworks.com/internet/basics/search-engine.htmhttp://searchenginewatch.com/2168031http://www.infotoday.com/searcher/may01/liddy.htmhttp://www.slideshare.net/jsuleiman/how-search-engines-work-presentationhttp://www.slideshare.net/jsuleiman/how-search-engines-work-presentationhttp://www.yahoo.com/http://www.google.com/http://www.metacrawler.com/http://www.dogpile.com/http://computer.howstuffworks.com/internet/basics/search-engine.htmhttp://searchenginewatch.com/2168031http://www.infotoday.com/searcher/may01/liddy.htmhttp://www.slideshare.net/jsuleiman/how-search-engines-work-presentationhttp://www.slideshare.net/jsuleiman/how-search-engines-work-presentation

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