Hypermedia is the topic that generates the most intensive discussions in any REST training, workshop, or project. While the other aspects of RESTful HTTP usage are pretty straightforward to grasp and hard to argue with, the use of hypermedia in machine-to-machine communication is a bit tougher to explain.
If you are already convinced that REST is the right architectural style for you, you can probably stop reading and switch over to Mike Amundsen’s excellent “H Factor” discussions. That may be a bit tough to start with, though, so I thought it might make sense to explain some of the ways I use to “pitch” hypermedia. I’ve arrived at a number of explanations and examples of meaningful hypermedia usage, explicitly targeted at people who are not deep into REST yet:
- “Service Document” and “Registries”: Especially for people with an SOA/SOAP/WSDL/WS-* background, the idea of putting at least one level of indirection between a client (consumer) and server (provider) is well established. A link in a server response is a way for a client to not couple itself to a particular server address, and a server response including multiple links to “entry point” resources is somewhat similar to a small registry. If providing links to actual entry point resources is the only purpose of a resource, I’ve become used to calling it a “service document”. Of course these documents themselves can be linked to each other, allowing for hierarchies or other relationships; they can support queries that return a subset of services; they can be provided by different servers themselves; and they are way more dynamic than a typical registry setup. In other words, the very simple approach included in HTTP is far more powerful than what most crazily expensive registries provide.
- Extensible contracts: The merits of being able to link to resources can be exploited to add functionality to existing systems without breaking their contracts. This is most visible in server responses that have one or more places where you can put additional links. As your server or service evolves, you can add links that will be ignored by existing clients, but can be used by those that rely on them. The concept of “a link to a resource” is both sufficiently generic and specific to be meaningful enough, especially if you include a way to specify what they actually mean via a link rel=… approach (but more on that in a separate post).
- Co-Location Independence: What I mean by this slightly weird term is the fact that while resources that are exposed as part of a service interface are sometimes (or maybe even often) designed in a way that requires them to be part of the same implementation, they very often are not, i. e. they could at least in theory be part of a different system. (In fact you can reasonably argue that there should be no assumption about this at all, neither on the server nor the client side for something to be rightfully called “RESTful”, but I simply haven’t found that to be doable in practice.) In those cases where resource don’t need to be hosted by the same implementation, you can and should couple them via links and have clients navigate them instead of relying on common URI paths.
There are quite a few more examples to talk about, but I won’t do that now as I promised to publish something today and don’t want to get into the habit of keeping drafts lying around for too long again. (I know, lenghty this is probably not. Sue me.) So please let me know in the comments what you think of these three if you’re just starting to pick up REST, and what additional ways of explanations you use if you already have done so.