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					Nutch Search Engine Tool
                   Nutch overview
   A full-fledged web search engine
   Functionalities of Nutch
       Internet and Intranet crawling
       Parsing different document formats (PDF, HTML,
        XML, JS, DOC,PPT etc.)‫‏‬
       Web interface for querying the index
       Management of Recrawls
                  Nutch Architecture
   4 main components
       Crawler
       Web Database (WebDB, LinkDB, segments)‫‏‬
       Indexer
       Searcher
   Crawler and Searcher are highly decoupled
    enabling independent scaling
   Highly modular, Plugin based architechture
Nutch Architecture




     Doug Cutting, "Nutch: Open Source Web Search",
     22 May 2004, WWW2004, New York
         Steps in a Crawl+Index cycle
1. Create a new WebDB (admin db -create).
2. Inject root URLs into the WebDB (inject).
3. Generate a fetchlist from the WebDB in a new segment (generate).
4. Fetch content from URLs in the fetchlist (fetch).
5. Update the WebDB with links from fetched pages (updatedb).
6. Repeat steps 3-5 until the required depth is reached.
7. Update segments with scores and links from the WebDB
   (updatesegs).
8. Index the fetched pages (index).
9. Eliminate duplicate content (and duplicate URLs) from the indexes
   (dedup).
10.Merge the indexes into a single index for searching (merge).
                  Crawling (cont.)
   Can effectively crawl upto ~100M pages
   Crawl Statistics on KReSIT site (it.iitb)
       Took 153 mins for a deep crawl (depth = 10)
       Crawled 4171 documents
       Size of crawl on disk 168MB
       Size of index ~25MB
           Web Database (WebDB)‫‏‬
   Persistent data structure for mirroring the
    structure and properties of the web graph being
    crawled
   The WebDB stores two types of entities
       Pages
       Links
   Optimised for frequent updation
Crawl Structure of it.iitb
                             Page DB
   Page Database
       used for fetch scheduling
       Contains:
            pages indexed and sorted by MD5 and URL
            outlinks, fetch information, page score
   A set of APIs are provided to perform the
    various operations
              Sample data of PageDB
Page 1: Version: 4
URL: http://keaton/tinysite/A.html
ID: fb8b9f0792e449cda72a9670b4ce833a
Next fetch: Thu Nov 24 11:13:35 GMT 2005
Retries since fetch: 0
Retry interval: 30 days
Num outlinks: 1
Score: 1.0
NextScore: 1.0

Page 2: Version: 4
URL: http://keaton/tinysite/B.html
ID: 404db2bd139307b0e1b696d3a1a772b4
Next fetch: Thu Nov 24 11:13:37 GMT 2005
Retries since fetch: 0
Retry interval: 30 days
Num outlinks: 3
Score: 1.0
NextScore: 1.0
                              Link DB
   Link Database
       Contains:
            links sorted by MD5
            links sorted by URL
       Represents full link graph.
       Stores anchor text associated with each link
       Used for:
            Link analysis;
            Anchor text indexing.
                    Segments
   Collection of pages fetched and indexed by the
    crawler in a single run
   One segment dir for each crawl-fetch-update
    cycle at a particular depth
   Contains raw text and parsed data of the files
    crawled
   Used to return the cached copy of a page and
    in snippet generation in results page
                        Segments
   segread tool gives a useful summary of all segments.
    (Parsed, Started, Finished, Dir)
   It can also be used to dump the segment data in raw text
    format. The dump switch gives the following details
       Fetcher Output: <url, hash, fetch-date ..>. Entries that
        go into the WebDB
       Content: Raw content including http-headers and other
        meta-data. stored cached copy of a page
       ParseData & ParseText: appropriate parser plugin by
        looking at the Raw content, is used to generate this
        data
Nutch API
                         Plugins
   Provide extensions to extension-points
   Each extension point defines an interface that
    must be implemented by extension
   Some core extension points
       IndexingFilter: add meta-data to indexed fields
       Parser: to parse a new type of document
       NutchAnalyzer: language specific analyzers
                         References
   Nutch Docs: http://lucene.apache.org/nutch/
   Nutch Wiki: http://wiki.apache.org/nutch/
   Prasad Pingali, CLIA consortium, Nutch
    Workshop, 2007
   Tom White, Introduction to Nutch, java.net
    website
    (http://today.java.net/pub/a/today/2006/01/10/introduction-to-nutch-
    1.html)

				
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