Designing Evolvable Web APIs with ASP.NET

Appendix D. Caching in Action

As we’ve seen, there are quite a few moving parts involved with HTTP caching. To illustrate how everything works together, let’s take a look at a common scenario involving two clients, an HTTP cache and the origin server. For the sake of brevity the body and all headers are not shown in the responses.

First, Client A does an initial request, as shown in Figure D-1.

Figure D-1. Client A does an initial GET

1. The cache receives the request, and seeing that it is a GET request checks whether it has a cached response. It doesn’t, so the cache forwards it on to the origin server.
2. The origin server generates a response, including ETag and max-age headers.
3. The cache receives the response and caches the result using a hash of the request URI and the Accept header value.
4. The cache then returns the response, including an additional AGE header to inform the client of the age of the representation.
5. Client A receives the representation and stores the ETag and Expires information.

Fifteen minutes later, Client B makes a request to the same resource, as shown in Figure D-2.

Figure D-2. Client B does an initial GET

1. The cache receives the request and checks whether it has a copy of the representation.
2. It sees from the matching on the URI and Accept that the representation ...