1.1. Kinds of Things on the Programmable Web

The programmable web is based on HTTP and XML. Some parts of it serve HTML, JavaScript Object Notation (JSON), plain text, or binary documents, but most parts use XML. And it’s all based on HTTP: if you don’t use HTTP, you’re not on the web.^[5]Beyond that small island of agreement there is little but controversy. The terminology isn’t set, and different people use common terms (like “REST,” the topic of this book) in ways that combine into a vague and confusing mess. What’s missing is a coherent way of classifying the programmable web. With that in place, the meanings of individual terms will become clear.

^[5] Thanks to Big Web Services’ WS-Addressing standard, it’s now possible to create a web service that’s not on the Web: one that uses email or TCP as its transport protocol instead of HTTP. I don’t think absolutely everything has to be on the Web, but it does seem like you should have to call this bizarre spectacle something other than a web service. This point isn’t really important, since in practice nearly everyone uses HTTP. Thus the footnote. The only exceptions I know of are eBay’s web services, which can send you SOAP documents over email as well as HTTP.

Imagine the programmable web as an ecosystem, like the ocean, containing many kinds of strange creatures. Ancient scientists and sailors classified sea creatures by their superficial appearance: whales were lumped in with the fish. Modern scientists classify animals according to their position in the evolutionary tree of all life: whales are now grouped with the other mammals. There are two analogous ways of classifying the services that inhabit the programmable web: by the technologies they use (URIs, SOAP, XML-RPC, and so on), or by the underlying architectures and design philosophies.