Webcrawler

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Webcrawler

  1. 1. Introduction • In the early days of Internet Rise of Anonymous FTP sites It download the files needed The first search engine :: ARCHIE Created in 1990,downloaded directory listings of all files on anonymous FTP sites, and created searchable database.
  2. 2. Google  Became popular around 2001  Important concepts of “ link popularity” and “page rank” were introduced. Yahoo!  Prior to 2004, Yahoo! Used Google to provide users with search results.  Launched its own search engine in 2004.  Used technologies used in Inktomi and AltaVista, which Yahoo! Acquired.
  3. 3. MSN Search : Most recent search engine, owned by Microsoft. Increasing in popularity Windows live search --- a new search platform.
  4. 4. Search Engine Defined “It is a software program that helps in locating information stored on a computer system, typically on world wide web.” They are of two types : I. Crawler Based II. Human Powered
  5. 5. Crawler Based Search Engines • Create their listings Automatically e.g. GOOGLE, YAHOO • crawl or spider the web to create a directory of information. • When “changes” are made to a page Such search engines will find these changes automatically.
  6. 6. • Human-powered Directories Depend on humans for the creation of directory • Hybrid Search Engines Can accept both types of results Based on web crawlers Based on human-powered listings
  7. 7. What is WebCrawler basically? A single piece of software ,with two different functions Building indexes of web pages. Navigate the web automatically on demand.
  8. 8. KEY DESIGN GOALS Content-based indexing. Breath first search to create a broad index. Crawler behavior to include as many as web servers as possible.
  9. 9. Components in WebCrawler retrieving documents from the web under the control of search engine => front end for Crawler Start with the known set of documents access contents using different protocol handling the query processing service document metadata hyperlinks
  10. 10. Web viewed as a Graph Web site Main page pointers Sub pages NODE
  11. 11. Algorithm • • • • Select a URL from the set of candidates Download the associated web pages Extract the URL’s contained therein Add those URL’s that have not been encountered before the candidate set
  12. 12. Architecture Robots exclusion Protocol
  13. 13. MINING DNS RESOLUTION Hyperlink Extracted From Webpage FETCH MODULE High Quality High Demand Fast Changing Page URL Frontier to avoid multiple instances Typical anatomy of a large-scale crawler
  14. 14. Performance and Reliability considerations • Need to fetch many pages at same time – utilize the network bandwidth • Highly concurrent and parallelized DNS lookups • Use of asynchronous sockets – Polling socket to check for completion of network transfers – Multi-processing or multi-threading • Care in URL extraction – Eliminating duplicates to reduce redundant fetches
  15. 15. WebCrawler : Indexing Mode • Try and build an index of as much of the web as possible. • Some heuristics used : – Which documents to select if the space for storing indices is limited? (eg. SAVE 100 pages) • A reasonable approach is to ensure that documents come from as many different servers as possible. • WebCrawler uses a modified breath first search approach in order to ensure that every server has at least one document that has been indexed.
  16. 16. WebCrawler : Real-time Search • Basic motivation : Given a user’s query, try to find documents that most closely matches. A different search algorithm is used here by WebCrawler. Intuitive reasoning : – If we follow the links from a document that is similar to what the user is looking for , they will most likely lead to relevant documents.
  17. 17. Applications • Search Engine Indexing • Statistical Analysis • Maintenance of Hypertext Structure (URL , Links Validation) • Resource Discovery • Attributer – A service that mines web for Copyright violations
  18. 18. THANK YOU..!!

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