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By Tyrell Perera ([EMAIL PROTECTED]), Virtusa Corp. (http://www.virtusa.com)
= 1 Introduction =
== 1.1 What is Nutch? ==
Nutch is an effort to build a Free and Open Source search engine. It uses
Lucene for the search and index component. The fetcher (robot) has been written
from scratch solely for this project.
Nutch has a highly modular architecture allowing developers to create plug-ins
for activities such as media-type parsing, data retrieval, querying and
clustering.
''Doug Cutting'' is the lead developer of Nutch.
== 1.2 What is Lucene? ==
Lucene is a Free and Open Source search and index API released by the Apache
Software Foundation. It is written in Java and is released under the Apache
Software License.
Lucene is just the core of a search engine. As such, it does not include things
like a web spider or parsers for different document formats. Instead these
things need to be added by a developer who uses Lucene.
Lucene does not care about the source of the data, its format, or even its
language, as long as you can convert it to text. This means you can use Lucene
to index and search data stored in files: web pages on remote web servers,
documents stored in local file systems, simple text files, Microsoft Word
documents, HTML or PDF files, or any other format from which you can extract
textual information.
Lucene has been ported or is in the process of being ported to various
programming languages other than Java:
* {{{ Lucene4c - C }}}
* {{{ CLucene - C++ }}}
* {{{ MUTIS - Delphi }}}
* {{{ NLucene - .NET }}}
* {{{ DotLucene - .NET }}}
* {{{ Plucene - Perl }}}
* {{{ Pylucene - Python }}}
* {{{ Ferret and RubyLucene â Ruby }}}
== 1.3 What License? ==
Both Nutch and Lucene are Apache projects and carry the Apache license
(http://www.opensource.org/licenses/apache2.0.php).
= 2 The Design of Nutch =
== 2.1 Core Components of Nutch ==
The Nutch search engine consists, very roughly, of three components:
1. The Crawler, which discovers and retrieves web pages
2. The âWebDBâ, a custom database that stores known URLs and fetched page
contents
3. The âIndexerâ, which dissects pages and builds keyword-based indexes
from them
After the initial creation of an Index, it is usual to perform periodic updates
of the index, in order to keep it up-to-date. We will look into the details of
index maintenance in the parts following this.
== 2.2 The Nutch Web Application ==
Apart from the above three components, it has a Search Web Application. This
application is a JSP application that can be configured and deployed in a
servlet container.
== 2.3 The Nutch API ==
All components listed above use the nutch API. The users can utilize the API
via two approaches, which depends on the task at hand.
1. Through the nutch Shell Script for administrative tasks, such as creating
and maintaining indexes
2. Through the Search Web Application, in order to perform a search using
keywords
= 3 Implementing a Nutch Search =
Implementing our own version of Nutch is fairly easy, provided that you;
1. have a basic understanding of how a web search engine works and
2. are comfortable working in a command line and finally
3. have a fair knowledge of Java and Servlet containers
If you said âyesâ to all three questions above, you have a very high
probability of having your Nutch implementation up and running by the end of
the steps which follows.
== 3.1 Before We Begin ==
=== 3.1.1 Download Nutch ===
Go to http://www.apache.org/dyn/closer.cgi/lucene/nutch/ and select a mirror to
download Nutch. The version described in this document is version 0.7. After
downloading the archive, extract it to your disk.
NOTE: This document assumes that the archive was extracted to
/home/tyrell/nutch-0.7 change this path to reflect your location.
=== 3.1.2 Download and Install a Servlet Container ===
Apache Tomcat is a popular Open Source servlet container. We will use this to
deploy the Nutch search web application in this document. The version referred
to in this document is version 5.5. You can download Tomcat from
http://tomcat.apache.org/download-55.cgi
=== 3.1.3 To Cygwin or not to Cygwin ===
To run the Nutch shell scripts and create the indexes, we require a UNIX like
environment. If you do not have access to a UNIX environment, you can use
Cygwin as an alternative. More details and download information for Cygwin can
be found at http://www.cygwin.com/
=== 3.2 Creating the Index ===
In order for the nutch web application to function, it will require at least
one search index. A search index in nutch is represented in the file system as
a directory. However, it is much more than that
