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    Webcrawler Webcrawler Presentation Transcript

    • 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.
    • 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.
    • MSN Search : Most recent search engine, owned by Microsoft. Increasing in popularity Windows live search --- a new search platform.
    • 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
    • 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.
    • • 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
    • What is WebCrawler basically? A single piece of software ,with two different functions Building indexes of web pages. Navigate the web automatically on demand.
    • 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.
    • 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
    • Web viewed as a Graph Web site Main page pointers Sub pages NODE
    • 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
    • Architecture Robots exclusion Protocol
    • 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
    • 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
    • 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.
    • 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.
    • Applications • Search Engine Indexing • Statistical Analysis • Maintenance of Hypertext Structure (URL , Links Validation) • Resource Discovery • Attributer – A service that mines web for Copyright violations
    • THANK YOU..!!