A web crawler is a program that browses the World Wide Web methodically by following links from page to page and downloading each page to be indexed later by a search engine. It initializes seed URLs, adds them to a frontier, selects URLs from the frontier to fetch and parse for new links, adding those links to the frontier until none remain. Web crawlers are used by search engines to regularly update their databases and keep their indexes current.

2. Definition
•A web crawler (also known as a web spider or web
robot) is a program or automated script which
browses theWorldWideWeb in a methodical,
automated manner.This process is calledWeb
crawling or spidering.
•Without crawlers, search engines would not exist.

3. The working of a web crawler is as follows:
• Initializing the seed URL or URLs
• Adding it to the frontier
• Selecting the URL from the frontier
• Fetching the web-page corresponding to that URLs
• Parsing the retrieved page to extract the URLs[21]
• Adding all the unvisited links to the list of URL i.e.
into the frontier
• Again start with step 2 and repeat till the frontier is
empty.

4. Architecture of
a web crawler
Figure shows the generalized
architecture of web crawler.
It has three main
components: a frontier
which stores the list of URL’s
to visit, Page Downloader
which download pages from
WWW andWeb Repository
receives web pages from a
crawler and stores it in the
database.

5. Flow of a basic crawler

6. The behaviour of aWeb crawler is the
outcome of a combination of policies:
• A selection policy: states which pages to download.
• A re-visit policy: states when to check for changes to the
pages.
• A politeness policy: states how to avoid overloading
Web sites.
• A parallelization policy: states how to coordinate
distributedWeb crawlers.

7. Usage
• Web crawlers are mainly used to create a copy of all the
visited pages for later processing by a search engine, that will
index the downloaded pages to provide fast searches.
• Crawlers can also be used for automating maintenance tasks
on aWeb site, such as checking links or validating HTML
code.Also, crawlers can be used to gather specific types of
information fromWeb pages, such as harvesting e-mail
addresses (usually for spam).

8. Types
•FocusedWeb Crawler
•IncrementalCrawler
•Distributed Crawler
•ParallelCrawler

9. Conclusion
•Web Crawler is the vital source of information
retrieval which traverses theWeb and
downloads web documents that suit the
user's need.Web crawler is used by the search
engine and other users to regularly ensure
that their database is up-to-date.

10. References
• IOSR Journal of Computer Engineering (IOSR-JCE) e-
ISSN: 2278-0661, p- ISSN: 2278-8727Volume 16, Issue 1,
Ver.VI (Feb. 2014), PP 01-05 www.iosrjournals.org
• Information form the webpages through:
• www.wikipedia.org
• www.sciencedaily.com