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World Wide Web Caching

World Wide Web Caching. CS457 Seminar Yutao Zhong 11/13/2001. Problems. Network congestion Server overloading User-perceived latency. client. Web server. Web cache. Effective solution: Web caching. Storing popular objects at locations close to the clients. Different deployments

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World Wide Web Caching

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  1. World Wide Web Caching CS457 Seminar Yutao Zhong 11/13/2001

  2. Problems • Network congestion • Server overloading • User-perceived latency

  3. client Web server Web cache Effective solution: Web caching • Storing popular objects at locations close to the clients • Different deployments • client-oriented, server-oriented, proxy

  4. Advantages of Web caching • Reduces bandwidth usage • Lightens Web server loads • Lessens user-perceived latency

  5. client Web client cache router client Proxy cache (I) • Standalone cache

  6. client Web client router client cache cache cache Proxy cache (II) • Transparent cache • Router level

  7. Web client L4 switch client router client cache cache cache Proxy cache (III) • Transparent cache • Switch level

  8. Design issues • Latency caused by cache miss (cache replacement, prefetching,…) • Stale data (cache coherence) • Dynamic data (dynamic cache)

  9. Cache replacement • Traditional policies • LRU, LFU, FIFO • Key-based policies • LRU-MIN, LRU-Threshold, HyperG • Function-based policies • Bolot/Hoschka, LNC-R (least normalized cost-replacement)

  10. Cache prefetching • Prediction • server-based vs. user-based • Scenarios • poll vs. push • Target • content vs. means • preresolving, preconnecting, prewarming

  11. Cache coherence:deals with stale data • Data validation • HTTP assistance • Conditional GET (IF-Modified-Since) • Date: date • Last-Modified: date • Expires: date • Pragma: no-cache • Cache-control: max-age

  12. Cache coherence:implementing mechanisms • Client polling • Server invalidation • Adaptive TTL • Piggyback invalidation

  13. Dynamic data caching • Active caching • cache computation • CPU cost • Web server accelerator

  14. Typical systems • CERN httpd: 1994 • Harvest (Squid): 1995 • Akamai

  15. Summary • Effective solution to Internet scalability problem • Issues and techniques • Open problems • cache routing, dynamic data caching, security...

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