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

Problems Network congestion Server overloading User-perceived latency

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

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

Proxy cache (I) Standalone cache client cacherouter Web

Transparent cache Router level cache Web client router cache Proxy cache (II)

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

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

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)

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

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

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

Dynamic data caching Active caching cache computation CPU cost Web server accelerator

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

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