Presentation on theme: "Mining the WebChakrabarti and Ramakrishnan1 Overview of Web-Crawlers Neal Richter & Anthony Arnone Nov 30, 2005 – CS Conference Room These slides are."— Presentation transcript:
Mining the WebChakrabarti and Ramakrishnan1 Overview of Web-Crawlers Neal Richter & Anthony Arnone Nov 30, 2005 – CS Conference Room These slides are edited versions of the chapter 2 lecture notes from “Mining the Web” by Soumen Chakrabarti. “Programming Spiders, Bots, and Aggregators” in Java by Jeff Heaton also is a good reference.
Crawling the Web Web pages Few thousand characters long Served through the internet using the hypertext transport protocol (HTTP) Viewed at client end using `browsers’ Crawler To fetch the pages to the computer At the computer Automatic programs can analyze hypertext documents
Mining the WebChakrabarti and Ramakrishnan3 HTML HyperText Markup Language Lets the author specify layout and typeface embed diagrams create hyperlinks. expressed as an anchor tag with a HREF attribute HREF names another page using a Uniform Resource Locator (URL), URL = protocol field (“HTTP”) + a server hostname (“www.cse.iitb.ac.in”) + file path (/, the `root' of the published file system).
Mining the WebChakrabarti and Ramakrishnan4 HTTP(hypertext transport protocol) Built on top of the Transport Control Protocol (TCP) Steps(from client end) resolve the server host name to an Internet address (IP) Use Domain Name Server (DNS) DNS is a distributed database of name-to-IP mappings maintained at a set of known servers contact the server using TCP connect to default HTTP port (80) on the server. Enter the HTTP requests header (E.g.: GET) Fetch the response header –MIME (Multipurpose Internet Mail Extensions) –A meta-data standard for and Web content transfer Fetch the HTML page
Mining the WebChakrabarti and Ramakrishnan5 Crawl “all” Web pages? Problem: no catalog of all accessible URLs on the Web. Solution: start from a given set of URLs Progressively fetch and scan them for new outlinking URLs fetch these pages in turn….. Submit the text in page to a text indexing system and so on……….
Mining the WebChakrabarti and Ramakrishnan6 Crawling procedure Simple Great deal of engineering goes into industry- strength crawlers Industry crawlers crawl a substantial fraction of the Web E.g.: Alta Vista, Northern Lights, Inktomi No guarantee that all accessible Web pages will be located in this fashion Crawler may never halt ……. pages will be added continually even as it is running.
Mining the WebChakrabarti and Ramakrishnan7 Crawling overheads Delays involved in Resolving the host name in the URL to an IP address using DNS Connecting a socket to the server and sending the request Receiving the requested page in response Solution: Overlap the above delays by fetching many pages at the same time
Mining the WebChakrabarti and Ramakrishnan8 Anatomy of a crawler. Page fetching threads Starts with DNS resolution Finishes when the entire page has been fetched Each page stored in compressed form to disk/tape scanned for outlinks Work pool of outlinks maintain network utilization without overloading it Dealt with by load manager Continue till the crawler has collected a sufficient number of pages.
Mining the WebChakrabarti and Ramakrishnan9 Typical anatomy of a large-scale crawler.
Mining the WebChakrabarti and Ramakrishnan10 Large-scale crawlers: performance and reliability considerations Need to fetch many pages at same time utilize the network bandwidth single page fetch may involve several seconds of network latency Highly concurrent and parallelized DNS lookups Use of asynchronous sockets Explicit encoding of the state of a fetch context in a data structure Polling socket to check for completion of network transfers Multi-processing or multi-threading: Impractical Care in URL extraction Eliminating duplicates to reduce redundant fetches Avoiding “spider traps”
Mining the WebChakrabarti and Ramakrishnan11 DNS caching, pre-fetching and resolution A customized DNS component with….. 1. Custom client for address resolution Tailored for concurrent handling of multiple outstanding requests Allows issuing of many resolution requests together –polling at a later time for completion of individual requests Facilitates load distribution among many DNS servers. 2. Caching server With a large cache, persistent across DNS restarts Residing largely in memory if possible. 3. Prefetching client Parse a page that has just been fetched & extract host names from HREF targets Query DNS cache via UDP Don’t wait for it, Check later (when you need it)
Mining the WebChakrabarti and Ramakrishnan12 Multiple concurrent fetches Managing multiple concurrent connections A single download may take several seconds Open many socket connections to different HTTP servers simultaneously Multi-CPU machines not useful crawling performance limited by network and disk Two approaches 1. using multi-threading 2. using non-blocking sockets with event handlers
Mining the WebChakrabarti and Ramakrishnan13 Multi-threading logical threads physical thread of control provided by the operating system (E.g.: pthreads) OR concurrent processes fixed number of threads allocated in advance programming paradigm create a client socket connect the socket to the HTTP service on a server Send the HTTP request header read the socket (recv) until no more characters are available close the socket. use blocking system calls
Mining the WebChakrabarti and Ramakrishnan14 Multi-threading: Problems performance penalty mutual exclusion concurrent access to data structures slow disk seeks. great deal of interleaved, random input-output on disk Due to concurrent modification of document repository by multiple threads
Mining the WebChakrabarti and Ramakrishnan15 Non-blocking sockets and event handlers non-blocking sockets connect, send or recv call returns immediately without waiting for the network operation to complete. poll the status of the network operation separately “select” system call lets application suspend until more data can be read from or written to the socket timing out after a pre-specified deadline Monitor polls several sockets at the same time More efficient memory management code that completes processing not interrupted by other completions No need for locks and semaphores on the pool only append complete pages to the log
Mining the WebChakrabarti and Ramakrishnan16 Link extraction and normalization Goal: Obtaining a canonical form of URL URL processing and filtering Avoid multiple fetches of pages known by different URLs Relative URLs need to be interpreted w.r.t to a base URL. many IP addresses / Mirror???
Mining the WebChakrabarti and Ramakrishnan17 Canonical URL Formed by Using a standard string for the protocol Canonicalizing the host name Adding an explicit port number Normalizing and cleaning up the path
Mining the WebChakrabarti and Ramakrishnan18 Robot exclusion Check whether the server prohibits crawling a normalized URL In robots.txt file in the HTTP root directory of the server species a list of path prefixes which crawlers should not attempt to fetch. Meant for crawlers only
Mining the WebChakrabarti and Ramakrishnan19 Eliminating already-visited URLs Checking if a URL has already been fetched Before adding a new URL to the work pool Needs to be very quick. Achieved by computing MD5 hash function on the URL Exploiting spatio-temporal locality of access Two-level hash function. –most significant bits (say, 24) derived by hashing the host name plus port –lower order bits (say, 40) derived by hashing the path concatenated bits used as a key in a B-tree qualifying URLs added to frontier of the crawl. hash values added to B-tree.
Mining the WebChakrabarti and Ramakrishnan20 Spider traps Protecting from crashing on Ill-formed HTML E.g.: page with 68 kB of null characters Misleading sites indefinite number of pages dynamically generated by CGI scripts paths of arbitrary depth created using soft directory links and path remapping features in HTTP server
Mining the WebChakrabarti and Ramakrishnan21 Spider Traps: Solutions No automatic technique can be foolproof Check for URL length Guards Preparing regular crawl statistics Adding dominating sites to guard module Disable crawling active content such as CGI form queries Eliminate URLs with non-textual data types
Mining the WebChakrabarti and Ramakrishnan22 Avoiding repeated expansion of links on duplicate pages Reduce redundancy in crawls Duplicate detection Mirrored Web pages and sites Detecting exact duplicates Checking against MD5 digests of stored URLs Representing a relative link v (relative to aliases u1 and u2) as tuples (h(u1); v) and (h(u2); v) Detecting near-duplicates Even a single altered character will completely change the digest ! E.g.: date of update/ name and of the site administrator Solution : Shingling
Mining the WebChakrabarti and Ramakrishnan23 Text repository Crawler’s last task Dumping fetched pages into a repository Decoupling crawler from other functions for efficiency and reliability preferred Page-related information stored in two parts meta-data page contents.
Mining the WebChakrabarti and Ramakrishnan24 Large-scale crawlers often use multiple ISPs and a bank of local storage servers to store the pages crawled.
Mining the WebChakrabarti and Ramakrishnan25 Refreshing crawled pages Search engine's index should be fresh Web-scale crawler never `completes' its job High variance of rate of page changes “If-modified-since” request header with HTTP protocol Impractical for a large web crawler Solution At commencement of new crawling round estimate which pages have changed
Mining the WebChakrabarti and Ramakrishnan26 Determining page changes “Expires” HTTP response header For page that come with an expiry date Otherwise need to guess if revisiting that page will yield a modified version. Score reflecting probability of page being modified Crawler fetches URLs in decreasing order of score. Assumption : recent past predicts the future
Mining the WebChakrabarti and Ramakrishnan27 Estimating page change rates Brewington and Cybenko & Cho Algorithms for maintaining a crawl in which most pages are fresher than a specified epoch. Prerequisite average interval at which crawler checks for changes is smaller than the inter-modification times of a page Small scale intermediate crawler runs to monitor fast changing sites E.g.: current news, weather, etc. Patched intermediate indices into master index
Questions? Lots of Programming detail missing!!!!