LectureWebSearchEngines.pptVIP

Lecture 16: Web Search Engines Characteristics of Web Information “Infinite” size (Surface vs. deep Web) Surface = static HTML pages Deep = dynamically generated HTML pages (DB) Semi-structured Structured = HTML tags, hyperlinks, etc Unstructured = Text Different format (pdf, word, ps, …) Multi-media (Textual, audio, images, …) High variances in quality (Many junks) “Universal” coverage (can be about any content) General Challenges in Web Search Handling the size of the Web How to ensure completeness of coverage? Efficiency issues Dealing with or tolerating errors and low quality information Addressing the dynamics of the Web Some pages may disappear permanently New pages are constantly created Tasks in Web Information Management Information Access Search (Search engines, e.g. Google) Navigation (Browsers, e.g. IE) Filtering (Recommender systems, e.g., Amazon) Information Organization Categorization (Web directories, e.g., Yahoo!) Clustering (Organize search results, e.g., Vivsimo) Information Discovery (Mining, e.g. ??) Typology of Web Search Engines Examples of Web Search Engines Basic Search Engine Technologies Component I: Crawler/Spider/Robot Building a “toy crawler” is easy Start with a set of “seed pages” in a priority queue Fetch pages from the web Parse fetched pages for hyperlinks; add them to the queue Follow the hyperlinks in the queue A real crawler is much more complicated… Robustness (server failure, trap, etc.) Crawling courtesy (server load balance, robot exclusion, etc.) Handling file types (images, PDF files, etc.) URL extensions (cgi script, internal references, etc.) Recognize redundant pages (identical and duplicates) Discover “hidden” URLs (e.g., truncated) Crawling strategy is a main research topic (i.e., which page to visit next?) Major Crawling Strategies Breadth-First (most common; balance server load) Parallel crawling Focused crawling Targeting at a subset of pages (e.g., all pages about “automobiles” ) Typically given a qu