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 serverWeb 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

client

client

cache

router

Web

Transparent cache Router level

cache

Web

client

client

client

router

cache

cache

Proxy cache (II)

Transparent cache Switch level

Proxy cache (III)

Web

client

client

client

L4 switch

cache

cache

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...