In learning about REST architecture, I've noticed that Hypermedia seems to be an important part of the uniform interface constraint to become RESTful; however, I'm having a hard time understanding how this concept of hypermedia is to be consumed by a client based off of the definition of hypermedia found across the web.
From what I understand about REST, hypermedia is/are basically the links that are provided to a client in a rest response containing the representation of a resource (so that the client only has to know about the base entry point url to a REST service). The links are essentially there to help the client know what options it has regarding the representation it received (for example, if I request the /children/ resource, I might receive an xml list of children that also contains links to /children/youngest, /children/oldest/, children/create, etc... This is exactly what i have a hard time understanding... Why does the server have to return these "hypermedia" links to the client? Shouldn't the client have already known about those links? The client isn't going to read the links on its own and follow them correctly... Somebody has to write client code (html and/or javascript for example) beforehand. What good do these links do for the client if the client should have already known about them? What am i missing?
Hypermedia is indeed an important part of creating RESTful interfaces. However, I see a full hypermedia client (sometimes referred to as HATEOAS) as just a point on the spectrum of becoming more RESTful. The Richardson Maturity Model describes 3 steps to becoming more RESTful and I've found these as good guidelines when considering how much of a RESTful patten to adopt on particular projects.
IMO, the best example of a full hypermedia client is a web browser. The web browser understands that the html <a href> tag represents a link. The key to a link is that a user can discover functionality as they move through their journey. So, when designing a RESTful API, the same principle can be applied.
Why does the server have to return these "hypermedia" links to the client?
So, one reason is that clients (if written to expose links) can allow state transitions to be discoverable.
Another benefit to using links is your API becomes responsible for the structure and data of each link. So, the server-side API is generating the links for each request. This means that the server API can make changes (version or structure) to the links without breaking the client.
You can build a RESTful API without links (Richardson Level 2), but then what if you want to change the URL for one of your API requests? You need to either implement a versioning strategy (through the url or headers) or you need to create a new url for the new feature. Through time, this can get out of hand.