Infinite Programming Tips: NutchTutorial

Tuesday, December 6, 2011

NutchTutorial

Source : http://wiki.apache.org/nutch/NutchTutorial

Introduction

Apache Nutch is an open source Web crawler written in Java. By using it, we can find Web page hyperlinks in an automated manner, reduce lots of maintenance work, for example checking broken links, and create a copy of all the visited pages for searching over. That’s where Apache Solr comes in. Solr is an open source full text search framework, with Solr we can search the visited pages from Nutch. Luckily, integration between Nutch and Solr is pretty straightforward as explained below.

Apache Nutch release 1.3 has Solr integration embedded, greatly simplifying Nutch-Solr integration. It also removes the legacy dependence upon both Apache Tomcat for running the old Nutch Web Application and upon Apache Lucene for indexing. Just download a 1.3 binary release from here.

Contents

Steps

1 Setup Nutch from binary distribution

Unzip your binary Nutch package to $HOME/nutch-1.3
cd $HOME/nutch-1.3/runtime/local

From now on, we are going to use ${NUTCH_RUNTIME_HOME} to refer to the current directory.

2. Verify your Nutch installation

run "bin/nutch" - You can confirm a correct installation if you seeing the following:

Usage: nutch [-core] COMMAND

Some troubleshooting tips:

Run the following command if you are seeing "Permission denied":

chmod +x bin/nutch

Setup JAVA_HOME if you are seeing JAVA_HOME not set. On Mac, you can run the following command or add it to ~/.bashrc:

export JAVA_HOME=/System/Library/Frameworks/JavaVM.framework/Versions/1.6/Home

3. Crawl your first website

Add your agent name in the value field of the http.agent.name property in conf/nutch-site.xml, for example:

<property>
 <name>http.agent.name</name>
 <value>My Nutch Spider</value>
</property>

mkdir -p urls
create a text file nutch under /urls with the following content (one URL per line for each site you want Nutch to crawl).

http://nutch.apache.org/

* Edit the file conf/regex-urlfilter.txt and replace

# accept anything else
+.

with a regular expression matching the domain you wish to crawl. For example, if you wished to limit the crawl to the nutch.apache.org domain, the line should read:

 +^http://([a-z0-9]*\.)*nutch.apache.org/

This will include any URL in the domain nutch.apache.org.

3.1 Using the Crawl Command

Now we are ready to initiate a crawl, use the following parameters:

-dir dir names the directory to put the crawl in.
-threads threads determines the number of threads that will fetch in parallel.
-depth depth indicates the link depth from the root page that should be crawled.
-topN N determines the maximum number of pages that will be retrieved at each level up to the depth.
Run the following command:

bin/nutch crawl urls -dir crawl -depth 3 -topN 5

Now you should be able to see the following directories created:

crawl/crawldb
crawl/linkdb
crawl/segments

NOTE: If you have a Solr core already set up and wish to index to it, you are required to add the -solr <solrUrl> parameter to your crawl command e.g.

bin/nutch crawl urls -solr http://localhost:8983/solr/ -depth 3 -topN 5

If not then please skip to here for how to set up your Solr instance and index your crawl data.

Typically one starts testing one's configuration by crawling at shallow depths, sharply limiting the number of pages fetched at each level (-topN), and watching the output to check that desired pages are fetched and undesirable pages are not. Once one is confident of the configuration, then an appropriate depth for a full crawl is around 10. The number of pages per level (-topN) for a full crawl can be from tens of thousands to millions, depending on your resources.

3.2 Using Individual Commands for Whole-Web Crawling

NOTE: If you previously modified the file conf/regex-urlfilter.txt as covered here you will need to change it back.

Whole-Web crawling is designed to handle very large crawls which may take weeks to complete, running on multiple machines. This also permits more control over the crawl process, and incremental crawling. It is important to note that whole Web crawling does not necessarily mean crawling the entire World Wide Web. We can limit a whole Web crawl to just a list of the URLs we want to crawl. This is done by using a filter just like we the one we used when we did the crawl command (above).

Step-by-Step: Concepts

Nutch data is composed of:

The crawl database, or crawldb. This contains information about every URL known to Nutch, including whether it was fetched, and, if so, when.
The link database, or linkdb. This contains the list of known links to each URL, including both the source URL and anchor text of the link.
A set of segments. Each segment is a set of URLs that are fetched as a unit. Segments are directories with the following subdirectories:
- a crawl_generate names a set of URLs to be fetched
- a crawl_fetch contains the status of fetching each URL
- a content contains the raw content retrieved from each URL
- a parse_text contains the parsed text of each URL
- a parse_data contains outlinks and metadata parsed from each URL
- a crawl_parse contains the outlink URLs, used to update the crawldb

Step-by-Step: Seeding the crawldb with a list of URLs

Option 1: Bootstrapping from the DMOZ database.

The injector adds URLs to the crawldb. Let's inject URLs from the DMOZ Open Directory. First we must download and uncompress the file listing all of the DMOZ pages. (This is a 200+ MB file, so this will take a few minutes.)

wget http://rdf.dmoz.org/rdf/content.rdf.u8.gz
gunzip content.rdf.u8.gz

Next we select a random subset of these pages. (We use a random subset so that everyone who runs this tutorial doesn't hammer the same sites.) DMOZ contains around three million URLs. We select one out of every 5,000, so that we end up with around 1,000 URLs:

mkdir dmoz
bin/nutch org.apache.nutch.tools.DmozParser content.rdf.u8 -subset 5000 > dmoz/urls

The parser also takes a few minutes, as it must parse the full file. Finally, we initialize the crawldb with the selected URLs.

bin/nutch inject crawldb dmoz

Now we have a Web database with around 1,000 as-yet unfetched URLs in it.

Option 2. Bootstrapping from an initial seed list.

This option shadows the creation of the seed list as covered here.

bin/nutch inject crawldb urls

Step-by-Step: Fetching

To fetch, we first generate a fetch list from the database:

bin/nutch generate crawldb crawldb/segments

This generates a fetch list for all of the pages due to be fetched. The fetch list is placed in a newly created segment directory. The segment directory is named by the time it's created. We save the name of this segment in the shell variable s1:

s1=`ls -d crawldb/segments/2* | tail -1`
echo $s1

Now we run the fetcher on this segment with:

bin/nutch fetch $s1

Then we parse the entries:

bin/nutch parse $s1

When this is complete, we update the database with the results of the fetch:

bin/nutch updatedb crawldb $s1

Now the database contains both updated entries for all initial pages as well as new entries that correspond to newly discovered pages linked from the initial set.

Now we generate and fetch a new segment containing the top-scoring 1,000 pages:

bin/nutch generate crawldb crawldb/segments -topN 1000
s2=`ls -d segments/2* | tail -1`
echo $s2

bin/nutch fetch $s2
bin/nutch parse $s2
bin/nutch updatedb crawldb $s2

Let's fetch one more round:

bin/nutch generate crawldb crawldb/segments -topN 1000
s3=`ls -d segments/2* | tail -1`
echo $s3

bin/nutch fetch $s3
bin/nutch parse $s3
bin/nutch updatedb crawldb $s3

By this point we've fetched a few thousand pages. Let's index them!

Step-by-Step: Invertlinks

Before indexing we first invert all of the links, so that we may index incoming anchor text with the pages.

bin/nutch invertlinks crawldb/linkdb -dir crawldb/segments

We are now ready to search with Apache Solr.

4. Setup Solr for search

download binary file from here
unzip to $HOME/apache-solr-3.X, we will now refer to this as ${APACHE_SOLR_HOME}
cd ${APACHE_SOLR_HOME}/example
java -jar start.jar

5. Verify Solr installation

After you started Solr admin console, you should be able to access the following links:

http://localhost:8983/solr/admin/
http://localhost:8983/solr/admin/stats.jsp

6. Integrate Solr with Nutch

We have both Nutch and Solr installed and setup correctly. And Nutch already created crawl data from the seed URL(s). Below are the steps to delegate searching to Solr for links to be searchable:

cp ${NUTCH_RUNTIME_HOME}/conf/schema.xml ${APACHE_SOLR_HOME}/example/solr/conf/
restart Solr with the command “java -jar start.jar” under ${APACHE_SOLR_HOME}/example
run the Solr Index command:

bin/nutch solrindex http://127.0.0.1:8983/solr/ crawldb crawldb/linkdb crawldb/segments/*

This will send all crawl data to Solr for indexing. For more information please see bin/nutch solrindex

If all has gone to plan, we are now ready to search with http://localhost:8983/solr/admin/. If you want to see the raw HTML indexed by Solr, change the content field definition in schema.xml to:

<field name="content" type="text" stored="true" indexed="true"/>

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