Wednesday, January 16, 2008

Comments, REST, Interactions, and Extensible Hypermedia

This post is to mostly keep track of the numerous blog threads going on about IDLs and schemas for REST. I find myself with more to say that wit to organize it... :(

First my summaries of opinions, then links to my comments here and there.

Some of my opinions on a RESTful Client Engine, and what types of server-side changes would NOT break a client:
  • URIs are discovered (except the first)
  • Server-provided data extensions (hidden fields with defaults in a form) are treated like "does not understand" but still submitted properly
  • Changes to which HTTP verb are used. The server can swap PUT for POST without issue (verb is discovered in form not spec'ed in a schema)
  • Changes in state path. "checkout" could directly be a single form with POST, a GET link to a single for with POST, a POST to a form with reliable PUT semantics, .......
Links,opinions on the changing landscape of content (and predictions of the future of hypermedia content):
  • Sam Ruby's suggestions for HTML5 Distributed Extensibility is a fabulous starting point.
  • URI Templates, HTML Forms, XForms, Web Forms, WADL
  • XSD is being extended to better support versioning
  • Data schemas designed for extensibility should allow everything possible to be optional
  • Microformats enable opportunistic clients to machine process
  • GRDDL leaps from microformats, xml, some json up to RDF
  • HTML+Microformats can do a ton of this already, but with no machine processable anything
  • Prediction: in 10 years all of this this will have exploded/merged into one or a few really cool and evolvable data/interaction schema systems
References to programming and type systems that address these issues more appropriately than any IDL (as far as I can get into them so far... :)

The following is a guide to my recent comments on these threads from around the web:

Subbu says:

Should this idea be extended to the rest of non-user facing resource-oriented applications? I don't think so. Here is why.

The idea of hypermedia embedding all the action controls necessary to interact with the server works well for an arbitrary number of universal clients interacting with a given server. In this case, the server offering a set of resources specifies all the ordering/interaction rules within the representation. Most application clients, on the other hand, interact with more than one server, and the ordering constraints can not be set by any given server. The clients know how to compose applications out of resources offered by various servers, and each client needs to be able to exercise control over composition. To be able to exercise such a control, client applications can not be universal, and the benefits that John lists above cannot be completely realized.
My comments is that clients should be opportunistic: if they understand some shared semantic (like a RESTful shopping API or task manager) then they can automate some interactions.

On JJ Dubray's blog I make several comments.

In response to "REST creates strong coupling", I say:
  • JJ is ignoring the shared definitions of MIME types
  • The globally shared "Provider external" (Pe) semantics in REST are (URI, GET representation, hyperlinks)
  • in WS-* the Pe globally is 0 (zero), only particular shared uses have a shared semantic.
In response to "REST and inter-actions", I say (and say and say):
  • For me REST in the enterprise isn't about scalability, but rather independent evolution and support for partial failure.
  • In maybe 10 years there will be an XML schema language that properly supports versioning and extensibility
In Steve Vinoski's blog on "IDLs vs Human Documentation" I comment that:
  • I side with Patrick Mueller: there is something of value in more than just prose to document RESTful systems
  • Hypermedia MIME types are _more_ than just a data schema (embedded forms to signify actions)
  • A list of RESTful interactions that _shouldn't_ break a programmed client (listed below)


JJ said...


I think we are in violent agreement now, at least when I read your latest comments on Steve's post, I don't see anything I would disagree with.

I don't think I can be in the position to comment on SDL vs IDL, this is really for the REST community to define what's best for REST. I don't want to be part of this debate/work.

I do think that it was worthwhile have all our discussions, even if they were rough at times.

thanks again for all your efforts in keeping the dialog open and meaningful.


JJ said...

One more thing, the REST community has a tremendous competitive advantage over WS-*. You own the "resource" concept and the "presentation layer". WS-* will never be used at the presentation layer level. At the resource level, WS-* can compete a bit better, but the concept of URI is probably too hard to translate in WS-*.

So I have no doubt that over time REST will win if you manage this well and swiftly. Make sure:
a) you are very selective about who you bring on board, a few of the wrong people can slow you down tremendously.
b) you use ample use cases to design the specs with enough precisions.

I can't predict how long it will take you, so as an IT architect all I can do today is build my services in a REST-ready mode.

Defining what REST-ready means precisely might be the most significant thing you can do in the short term, it can guarantee lowering tremendously the barrier for overtaking WS-* when you are done.

hurry ;-)



Mike Burrows said...

Hi John, I hound your blog by way of the Kanban stuff, good to see this REST stuff too.

On URI templates and discovery, take a look at