Web crawler

WEB CRAWLERs

Mr. Abhishek Gupta

content
•
• Why is web crawler required?
• How does web crawler work?
• Crawling strategies
Breadth first search traversal
depth first search traversal
• Architecture of web crawler
• Crawling policies
• Distributed crawling

WEB CRAWLERS
 The process or program used by search engines to
download pages from the web for later processing by a
search engine that will index the downloaded pages to
provide fast searches.
 A program or automated script which browses the World
Wide Web in a methodical, automated manner
 also known as web spiders and web robots.
 less used names- ants, bots and worms.

content
• What is a web crawler?

WHY CRAWLERS?
Internet has a
wide expanse of
Information.
 Finding
relevant
information
requires an
efficient
mechanism.
Web Crawlers
provide that
scope to the
search engine.

content

• It starts with a list of URLs to visit, called the seeds . As
the crawler visits these URLs, it identifies all the
hyperlinks in the page and adds them to the list of visited
URLs, called the crawl frontier
• URLs from the frontier are recursively visited according
to a set of policies.

How does web crawler

Googlebot, Google’s
Web Crawler

New url’s can be
specified here. This is
google’s web Crawler.

Crawling Algorithm
Initialize queue (Q) with initial set of known URL’s.
Until Q empty or page or time limit exhausted:
Pop URL, L, from front of Q.
If L is not an HTML page (.gif, .jpeg, .ps, .pdf, .ppt…)
exit loop.
If already visited L, continue loop(get next url).
Download page, P, for L.
If cannot download P (e.g. 404 error, robot excluded)
exit loop, else.
Index P (e.g. add to inverted index or store cached copy).
Parse P to obtain list of new links N.
Append N to the end of Q.

Keeping Track of Webpages to
Index

Crawling Strategies
Alternate way of looking at the problem.
Web is a huge directed graph, with documents as vertices
and hyperlinks as edges.
 Need to explore the graph using a suitable
graph traversal algorithm.
 W.r.t. previous ex: nodes are represented
by
rectangles and directed edges are
drawn as arrows.

Breadth-First Traversal
Given any graph and a set of seeds at which to start, the graph can be
traversed using the algorithm
1. Put all the given seeds into the queue;
2. Prepare to keep a list of “visited” nodes (initially empty);
3. As long as the queue is not empty:
a. Remove the first node from the queue;
b. Append that node to the list of “visited” nodes
c. For each edge starting at that node:
i. If the node at the end of the edge already appears on the list of “visited”
nodes or it is already in the queue, then do nothing more with that
edge;
ii. Otherwise, append the node at the end of the edge to the end of the
queue.

content
Depth first search traversal
• Parallel crawling

Depth First Crawlers
Use depth first search (DFS) algorithm
• Get the 1st link not visited from the start page
• Visit link and get 1st non-visited link
• Repeat above step till no non-visited links
• Go to next non-visited link in the previous level and repeat 2nd
step

Depth-First vs. Breadth-First
• depth-first goes off into one branch until it reaches a
leaf node
• not good if the goal node is on another branch
• neither complete nor optimal
• uses much less space than breadth-first
• much fewer visited nodes to keep track of
• smaller fringe

• breadth-first is more careful by checking all
alternatives
• complete and optimal
• very memory-intensive

content

ARCHITECTURE OF crawler
Doc

Robots

URL

Fingerprint

templates

set

DNS

Parse

www

Content
Seen?

URL
Filter

Fetch

URL Frontier

Dup
URL
Elim

Architecture
URL Frontier: containing URLs yet to be fetches in the current
crawl. At first, a seed set is stored in URL Frontier, and a crawler
begins by taking a URL from the seed set.
 DNS: domain name service resolution. Look up IP address for
domain names.
 Fetch: generally use the http protocol to fetch the URL.
 Parse: the page is parsed. Texts (images, videos, and etc.) and
Links are extracted.


Content

Seen?: test whether a web page with the
same content has already been seen at another
URL. Need to develop a way to measure the
fingerprint of a web page.

Architecture(cont)
 URL Filter:

Whether the extracted URL should be excluded from the
frontier (robots.txt).
 URL should be normalized (relative encoding).
 en.wikipedia.org/wiki/Main_Page
 <a href="/wiki/Wikipedia:General_disclaimer"
title="Wikipedia:General
disclaimer">Disclaimers</a>
 Dup URL Elim: the URL is checked for duplicate
elimination.


Crawling Policies
• Selection Policy that states which pages to download.
• Re-visit Policy that states when to check for changes to
the pages.
• Politeness Policy that states how to avoid overloading
Web sites.
• Parallelization Policy that states how to coordinate
distributed Web crawlers.

Selection policy
 Search engines covers only a fraction of Internet.
 This requires download of relevant pages, hence a
good selection policy is very important.
 Common Selection policies:
Restricting followed links
Path-ascending crawling
Focused crawling
Crawling the Deep Web

Re-Visit Policy





Web is dynamic; crawling takes a long time.
Cost factors play important role in crawling.
Freshness and Age- commonly used cost functions.
Objective of crawler- high average freshness;
low average age of web pages.
 Two re-visit policies:
Uniform policy
Proportional policy

Politeness Policy
Crawlers can have a crippling impact on the overall
performance of a site.
The costs of using Web crawlers include:
Network resources
Server overload
Server/ router crashes
Network and server disruption
A partial solution to these problems is the robots
exclusion protocol.

Robot Exclusion
• How to control those robots!
Web sites and pages can specify that robots should not
crawl/index certain areas.
Two components:
• Robots Exclusion Protocol (robots.txt): Site wide specification of
excluded directories.
• Robots META Tag: Individual document tag to exclude indexing or
following links.

Robots Exclusion Protocol
• Site administrator puts a “robots.txt” file at the root
of the host’s web directory.
• http://www.ebay.com/robots.txt
• http://www.cnn.com/robots.txt
• http://clgiles.ist.psu.edu/robots.txt

• File is a list of excluded directories for a given robot
(user-agent).
• Exclude all robots from the entire site:
User-agent: *
Disallow: /
New Allow:
• Find some interesting robots.txt

Robot Exclusion Protocol Examples
• Exclude specific directories:
User-agent:
Disallow:
Disallow:
Disallow:

*
/tmp/
/cgi-bin/
/users/paranoid/

• Exclude a specific robot:

User-agent: GoogleBot
Disallow: /

• Allow a specific robot:

User-agent: GoogleBot
Disallow:
User-agent: *
Disallow: /

Robot Exclusion Protocol Has Not Well
Defined Details
• Only use blank lines to separate different User-agent
disallowed directories.
• One directory per “Disallow” line.
• No regex (regular expression) patterns in directories.

Parallelization Policy
The crawler runs multiple processes in parallel.
The goal is:
To maximize the download rate.
To minimize the overhead from parallelization.
To avoid repeated downloads of the same page.
The crawling system requires a policy for assigning the
new URLs discovered during the crawling process.

content
• Mechanism used

distributed WEB CRAWLING
• A distributed computing technique whereby search
engines employ many computers to index the Internet
via web crawling.
• The idea is to spread out the required resources of
computation and bandwidth to many computers and
networks.
• Types of distributed web crawling:
1. Dynamic Assignment
2. Static Assignment

DYNAMIC ASSIGNMENT
• With this, a central server assigns new URLs to different
crawlers dynamically. This allows the central server
dynamically balance the load of each crawler.
• Configurations of crawling architectures with dynamic
assignments:
• A small crawler configuration, in which there is
a central DNS resolver and central queues per Web
site, and distributed down loaders.
• A large crawler configuration, in which the DNS resolver
and the queues are also distributed.

STATIC ASSIGNMENT
• Here a fixed rule is stated from the beginning of the
crawl that defines how to assign new URLs to the
crawlers.
• A hashing function can be used to transform URLs into a
number that corresponds to the index of the
corresponding crawling process.
• To reduce the overhead due to the exchange of URLs
between crawling processes, when links switch from one
website to another, the exchange should be done in
batch.

FOCUSED CRAWLING
• Focused crawling was first introduced by Chakrabarti.
• A focused crawler ideally would like to download only
web pages that are relevant to a particular topic and
avoid downloading all others.
• It assumes that some labeled examples of relevant and
not relevant pages are available.

STRATEGIES OF FOCUSED
CRAWLING
• A focused crawler predict the probability that a link to a
particular page is relevant before actually downloading
the page. A possible predictor is the anchor text of links.
• In another approach, the relevance of a page is
determined after downloading its content. Relevant
pages are sent to content indexing and their contained
URLs are added to the crawl frontier; pages that fall
below a relevance threshold are discarded.

EXAMPLES
•
•
•
•

Yahoo! Slurp: Yahoo Search crawler.
Msnbot: Microsoft's Bing web crawler.
Googlebot : Google’s web crawler.
WebCrawler : Used to build the first publicly-available
full-text index of a subset of the Web.
• World Wide Web Worm : Used to build a simple index of
document titles and URLs.
• Web Fountain: Distributed, modular crawler written in
C++.
• Slug: Semantic web crawler

Important questions
1)Draw a neat labeled diagram to explain how does a web
crawler work?
2)What is the function of crawler?
3)How does the crawler knows if it can crawl and index data
from website? Explain.
4)Write a note on robot.txt.
5)Discuss the architecture of a search engine.
7)Explain difference between crawler and focused crawler.

Web crawler

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