Web Caching

Chapter 1. Introduction

The term cache has French roots and means, literally, to store. As a data processing term, caching refers to the storage of recently retrieved computer information for future reference. The stored information may or may not be used again, so caches are beneficial only when the cost of storing the information is less than the cost of retrieving or computing the information again.

The concept of caching has found its way into almost every aspect of computing and networking systems. Computer processors have both data and instruction caches. Computer operating systems have buffer caches for disk drives and filesystems. Distributed (networked) filesystems such as NFS and AFS rely heavily on caching for good performance. Internet routers cache recently used routes. The Domain Name System (DNS) servers cache hostname-to-address and other lookups.

Caches work well because of a principle known as locality of reference. There are two flavors of locality: temporal and spatial. Temporal locality means that some pieces of data are more popular than others. CNN’s home page is more popular than mine. Within a given period of time, somebody is more likely to request the CNN page than my page. Spatial locality means that requests for certain pieces of data are likely to occur together. A request for the CNN home page is usually followed by requests for all of the page’s embedded graphics. Caches use locality of reference to predict future accesses based on previous ones. ...