James definitely is talking about what I have in mind.
I've posted this response in better markup on my blog:
HATEOAS: URITemplateAspects, LinkAwareModels and
LinkTemplateProcessors<
http://brettdargan.com/blog/2009/04/29/hateoas-uritemplateaspects-linkawaremodels-and-linktemplateprocessors/>
I don't think I have confused HATEOAS with "connectedness", I have a
reasonable understanding of resource state, application state and HATEOAS.
See
http://www.stucharlton.com/blog/archives/000141.html" and
http://tech.dir.groups.yahoo.com/group/rest-discuss/message/12497,
http://tech.dir.groups.yahoo.com/group/rest-discuss/message/12497, which I
see you have contributed to.
I think this final point made by Roy at
http://roy.gbiv.com/untangled/2008/rest-apis-must-be-hypertext-driven is
pretty applicable here:
A REST API should be entered with no prior knowledge beyond the initial URI
(bookmark) and set of standardized media types that are appropriate for the
intended audience (i.e., expected to be understood by any client that might
use the API). From that point on, all application state transitions must be
driven by client selection of server-provided choices that are present in
the received representations or implied by the user’s manipulation of those
representations. The transitions may be determined (or limited by) the
client’s knowledge of media types and resource communication mechanisms,
both of which may be improved on-the-fly (e.g., code-on-demand). [Failure
here implies that out-of-band information is driving interaction instead of
hypertext.]
My point is you can't adhere to the HATEOAS constraint, without providing
links between resources, so the server can guide the client to other
application states.
I agree that forms are a better example for explaining state transitions,
especially since resource state is likely to be changed by providing forms
(as will the User Agent, as Roy mentions); or equivalent form
templates/prototypes for other media types.
But to me providing forms or equivalents for other media types is a
different and easier problem to solve.
I'm very interested in how to evolve existing systems; I want to leverage
the vast islands of information that already exist.
I just want to do it in an elegant way from a code and a web perspective,
including the fact that uri's shouldn't change.
In a standard, layered design, you would have these components (see
http://brettdargan.com/blog/wp-content/uploads/2009/04/hateoas_4_legacy-300x225.jpg
):
1. Code-On-Demand
2. View
3. Controller
4-6. Contains the Proposed Abstractions
7. Model
8. Persistent Store - Db; file; jcr; webdav
9. Proxies
Here are some options I have for adding links around some existing system
(there are likely more)
Due to the time constraints, the Pros, will be sparse.
Option 1: Add Links via Code-On-Demand.
Add links to my html page via javascript.
I could take my existing html representation and use dhtml to add links.
Cons:
* code-on-demand not available for all user agents or may be disabled
* Proxies can't cache the entire representation
* leak of possible application states to clients that don't need to know
* Detailed knowledge of resource and subresources required to provide "link
rich" representations
* Alternative media types may require alternative languages or structure to
perform add the links. ie. use of xsl for xml; javascript with svg.
Option 2: Add Links via Views
Cons:
* We all know this is bad
* Detailed knowledge of resource and subresources required to provide "link
rich" representations
* different views/media types require the same logic and detailed knowledge
of the model
* different views/media types may require some application state knowledge.
an admin user may have other links than anon.
Option 3: Add Links via Controllers
Pros:
* Can handle the inclusion/exclusion of links depending on the current
application state, regardless of the media type to respond with
Cons:
* Detailed knowledge of resource and subresources required to provide "link
rich" representations
Option 4: Add Links via Models
Pros:
* Detailed knowledge of the model and related models belong in the Model
Cons:
* Detailed knowledge of resource links aren't the domain of the models
* Not all possible related models are likely to be represented; to balance
performance with types of usage, often some relationships are not modelled,
so as to not eager load, very large graphs etc.
* Links are very static, The "next available" application state, still needs
to be determined in another layer, like Controller
Option 5: Add Links via Persistent Store
Store partial links in the db
Use knowledge of the partial links within another layer.
Worst option
Pros:
* Db driven, easy to update
* Tables have relationships
Cons:
* Update to change a link, will not play nice with intermediaries nor "cool
uris, that don't change"
* Having "link rich" resources between tables
* Relationships are not always defined in Foreign Keys
* Links are very static, The "next available" application state, still needs
to be determined in another layer, like Controller
Thoughts
Models shouldn't have relationships to hardcoded resources, like
"/resource/50", but they could have a semantic relationship to
"/resource/{id}" or "/resource/{id};role={role}" under certain conditions.
There is a place for such LinkAwareModels (Component no. 5), as long as the
links are isolated and only contain semantic relationships, they are not
urls.
The Links from Models to Resources are isolated and encapsulated within its
own abstraction; which gives us component no. 6 URITemplateAspects (maybe
just simplify this to LinkTemplateAspect).
Possible "Application States" are dependent on the request, even though a
request must be STATELESS, possible "Application States" may still be
determined by the security access; restrictions of media-types or client,
which should all be part of the request. If you do stuff with user roles,
then make sure you encode within your url.
Component no. 4 is our LinkTemplateProcessor(s) and its responsibility is to
evaluate URITemplates within the LinkAwareModels/entities and to determine
appropriate state transitions based on the request.
This leads us to the final Option ( well, for this post ;) )
Option 6: Semantic relationships of a "Model to resources" definied via
URITemplateAspect and a LinkTemplateProcessor to evaluate the relationships
(URITemplates) and sub-entities to determine other "application states"
Pros:
* Detailed knowledge of the model and related models belong in the Model
* Detailed knowledge of the model to resource of interest is a concern
isolated in the URITemplateAspects
* Any possible relationships to other resources can be defined
* Links are very dynamic. The possible state transitions can be determined
by the LinkTemplateProcessor, but it can do so decoupled from the detailed
relationships of the model to other resources.
The semantic relationships that are defined in the LinkAwareModels, can
include uri and other attributes, like rel. The LinkTemplateProcessor can
evaluate a lot of the current application state (but not all as the User
Agent may have representations from other servers or representations that
have been modified by Code-On-Demand) in combination with the semantic
relationship to determine the ultimate value of the link to use (relative
preferred, of course) and it can determine if that link is a valid state
transition. Only if you are an admin do you see this link; or the link
retains the current media-type used for this request; so a request for
/country/AU.html includes links to cities.html, not just cities. The default
negotiation might be something if that is not specified.
* The Processor can operate on objects, prior to rendering to a particular
media-type, so if you have a single object model that can render to multiple
media-types, you could have less code
Conclusion
The use of these abstractions, in no way make it RESTful, that is up to the
developers, but hopefully by following some abstractions, like these we will
get some clean code and some better understanding of RESTful APIs.
I have some old code at
http://brettdargan.com/blog/2009/01/14/hateoas-for-legacy-object-models/ ,
to show this working.
One day I'll get a chance to clarify terms and simplify the code ;)
