REST HATEOAS: How to know what to POST?

Question

I still don't understand how the client knows what data to POST when creating a resource. Most tutorials/articles omit this and in their examples, a client always seems to know a priori what to post (i.e. using out-of-band information). Like in this example, the consumer knows that he has to place the order by setting what <drink\> he wants.

I can only image a few approaches and I don't know if they are valid:

1. Returning an empty resource
The client discovers a link to /resource with a link to /resource/create and relation "create". A GET to /resource/create returns an empty resource (all attributes are empty) and a link to /resource/create with relation "post". The client then sets values to all attributes and POSTs this to /resource/create which returns a 201 (Created). This means that the CRUD operations are not located at the resource endpoint but to URI like /resource/create and that the client might set attributes the server ignores (like a creation date which is set on the server side)

2. Returning a form
Basically the same approach as above, despite the fact that not a resource is returned but some meta-information about what fields to post and what datatypes each attributes needs to have. Like in this example. Still, the creation endpoint is not located at /resource but on /resource/create

3. Creating by updating
A POST to /resource immediatly creates an empty resource and returns a link to this resource. The client then can follow this link to update the resource with the necessary data doing PUTs.

So what is the best approach that still follows the HATEOAs paradigm and why are all of these tutorials (and even books like REST in Practice) omitting this problem?

UPDATE:
I recently found out the Sun Cloud API seems to be pretty close to an "ideal" REST HATEOAS API. It not only defines some resources and does hyperlinking between them, it also defines media types and versioning. With all this theoretical discussion, it's pretty good to have a concrete exmaple. Maybe this helps some readers of this question.

Answer 1

Most tutorials and books about REST are very misleading, because there are many misconceptions about REST and no authoritative source other than Fielding's dissertation itself, which is incomplete.

REST is not CRUD. A POST is not a synonym to CREATE. POST is the method to be used for any action that isn't already standardized by HTTP. If it's not standardized by HTTP, its semantics are determined by the target resource itself, and the exact behavior has to be documented by the resource media-type.

With HATEOAS, a client should not rely on out-of-band information for driving the interaction. The documentation should focus on the media-types, not on the URIs and methods. People rarely get this right because they don't use media-types properly, and instead document URI endpoints.

For instance, in your example, everything has the application/xml media-type. That's the problem. Without proper media-types, there's no way to document resource-specific semantics when everything has the same media-type without relying on URI semantics, which would break HATEOAS. Instead, a drink should have a media-type like application/vnd.mycompany.drink.v1+xml, and your API documentation for that media-type can describe what to expect when using POST with a rel link.