Niocchi - Java crawl library implementing synchronous I/O multiplexing

Niocchi - Java crawl library implementing synchronous I/O multiplexing

Niocchi is a java crawler library implementing synchronous I/O multiplexing.
This specific type of implementation allows crawling tens of thousands of hosts in parallel on a single low end server. Niocchi has been designed for big search engines that need to crawl massive amount of data, but can also be used to write no frills crawlers. It is currently used in production by Enormo and Vitalprix.

javadoc

Index

1. Introduction
2. Requirements
3. License
4. Package organization
5. Architecture
6. Usage
7. Caveats
8. To Do
9. Download
10. Change history
11. About the authors

Introduction

Most of the java crawling libraries use standard java IO package.
That means crawling N documents in parallel requires at least N running
threads. Even if each thread is not taking a lot of resources while
fetching the content, that approach becomes costly when crawling at a
large scale. On the contrary, doing synchronous I/O multiplexing by using the NIO
package introduced in java 1.4 allows the crawling of many documents in
parallel using one single thread.

Requirements

Niocchi requires java 1.5 or above.

License

This software is licensed under the Apache license version 2.0.

Package organization

• org.niocchi.core holds the library itself.
• org.niocchi.gc holds an implementation example of a very simple crawler that reads the URL to crawl from a file and saves the crawled documents.
• org.niocchi.monitor holds a utility thread that can be used by the crawler to provide real time information through a telnet connexion.
• org.niocchi.rc holds an implementation example of a RedirectionController.
• org.niocchi.resources holds a few implementation examples of the Resource and ResourceCreator classes.
• org.niocchi.urlpools holds a few implementation examples of the URLPool class.

Architecture

• A Query encapsulates a URL and implements methods to check its
  status after being crawled.
• A Resource holds the crawled content and implements methods to
  save it.
• Each Query is associated to a Resource. To crawl one URL, one
  Resource needs to be taken from the pool of resources. Once the URL is
  crawled and its content processed, the Resource is returned to the
  pool. The number of available Resources is fixed and controls how many
  URL can be crawled in parallel at any time. This number is set through
  the ResourcePool constructor.
• When a Query is crawled, its associated Resource will be
  processed by one of the workers.
• The URLPool acts as a source of URLs to crawl into which the
  crawled taps. It's an interface that must be implemented to provide
  URLs to the crawler.
• The crawler has been designed as "active", meaning it consumes
  URLs from the URLPool, as opposed to being "passive" and waiting to be
  given URL. When the crawler starts, it will get URLs to crawl from the
  URLPool until all resources are consumed or hasNextQuery() return false
  or getNextQuery() returns null. Each time a Query is crawled and
  processed and its Resource returned to the ResourcePool, the crawler
  requests other URLs to crawl from the URLPool until all resources are
  consumed or hasNextQuery() return false or getNextQuery() returns null.
  If all URLs have been crawled and no more are immediately available to
  crawl, the crawler will recheck every second for available URLs to
  crawl.
• When a Query has been crawled, it is put into a FIFO of queries
  to be processed. One of the Workers will take it and process the
  content of its associated Resource. The work is done in the
  processResource() method. The Query is then returned to the URLPool
  which can examine the crawl status and the result of the processing.
  Lastly, the Query associated Resource is returned to the Resource pool.
• In order to not block during host name resolution, the Crawler
  uses two additional threads. ResolverQueue resolves the URL coming from
  the URLPool and RedirectionResolverQueue resolves the URLs gotten from
  redirections.