Web Caching

Types of Web Caches

Web content can be cached at a number of different locations along the path between a client and an origin server. First, many browsers and other user agents have built-in caches. For simplicity, I’ll call these browser caches. Next, a caching proxy (a.k.a. “proxy cache”) aggregates all of the requests from a group of clients. Lastly, a surrogate can be located in front of an origin server to cache popular responses. In this book, we’ll spend more time talking about caching proxies than the others.

Browser Caches

Browsers and other user agents benefit from having a built-in cache. When you press the Back button on your browser, it reads the previous page from its cache. Nongraphical agents, such as web crawlers, cache objects as temporary files on disk rather than keeping them in memory.

Netscape Navigator lets you control exactly how much memory and disk space to use for caching, and it also allows you to flush the cache. Microsoft Internet Explorer lets you control the size of your local disk cache, but in a less flexible way. Both have controls for how often cached responses should be validated. People generally use 10–100MB of disk space for their browser cache.

A browser cache is limited to just one user, or at least one user agent. Thus, it gets hits only when the user revisits a page. As we’ll see later, browser caches can store “private” responses, but shared caches cannot.

Caching Proxies

Caching proxies, unlike browser caches, service many different ...