[occi-wg] Modeling proposal for ReST/Cloud based commands.

Sam Johnston samj at samj.net
Wed Oct 14 19:17:08 CDT 2009


Ironically Adrian wrote this post after being prompted by myself and Andy
Edmonds from the OCCI-WG following criticism of exactly this approach. I
definitely think links are a great way of satisfying the hypertext
constraint (formerly known as HATEOAS) but I'm intrigued by what specific
problems he has with this approach - e.g. "what could possibly go wrong"?

In many (most?) cases the "actions" are not parametrised in which case the
entity-body is empty, but the alternative, doing an empty POST to a
parametrised (GET-style) URL, feels [very] wrong. While URLs should be
considered opaque, I'm ok with using either semicolons or path segments
('/') to delineate actions:


but the following "feels funky":


I much prefer:

POST http://example.com/disk01;resize

So what we are currently doing for "actions" is a post/redirect/get
pattern<http://en.wikipedia.org/wiki/Post/Redirect/Get>which I feel is
quite elegant in that it allows us to handle more
challenging realities such as asynchronous responses (e.g. returning HTTP
201 Accepted with a Location: header which can be monitored ala pubsubhubbub
or polled).

Adrian was talking about servers returning 409 Conflict but I don't have a
problem with that either - if so the client knows it can try again later and
doing anything else (like saving and replaying the request on the server
side) doesn't seem right to me either as it's a form of server-side state.

Anyway it's good to see this all starting to coalesce - just need to sort
out the nitty gritty details.


On Thu, Oct 15, 2009 at 1:43 AM, Bill Burke <bburke at redhat.com> wrote:

> Well, the more restful way of doing it would be:
> GET of /objects/robot/3333 returns:
> {"robot" :
>    {"name" : "Mr. Roboto",
>     "links" : [ {"rel" : "kill", "href" : "http://.../objects/robot/3333/kill",
> "type" : "application/x-form-urlencoded}]
>    }
> }
> So, the idea is you get the representation of the robot, and the links
> define what relationships are to the robot.  One of the relationships is
> "kill".  You follow the link by posting to it:
> POST /objects/robot/333/kill
> death=slow
> GET of /objects/robot/333/kill might return:
> "This robot died a slow death"
> The thing is here that the URL patterns are irrelevant, the URLs are
> opaque, you navigate to them by linking.
> Adrian Cole wrote:
>> sorry.. the rendering is bad in firefox.
>> Here's the matrix example:
>> POST http://localhost:8080/objects/robot/action/kill;death=slow HTTP/1.1
>> Here's the entity example (objects renamed from a real cloud api):
>> POST http://localhost:8080/objects/robot/action HTTP/1.1
>> Content-Length: 25
>> {"kill":{"death":"slow"}}
>> This is probably more high-level, but if it is rest to address resources
>> as /path/to and operate them with POST, DELETE, etc, it may come naturally
>> for some who are FSMs to offer state transition commands.  In this case,
>> simple command patterns come in handy and are currently in use in compute
>> clouds, for example.
>> I hope this background helps.  If the topic doesn't fit the list, let me
>> know.
>> Cheers,
>> -Adrian
>> On Wed, Oct 14, 2009 at 2:55 PM, Bill Burke <bburke at redhat.com <mailto:
>> bburke at redhat.com>> wrote:
>>    REST is not an RPC model...Or maybe I'm misinterpreting you.  Also,
>>    the code examples in your blog are cut off.
>>    Adrian Cole wrote:
>>        Hi, all.
>>        I'd like your opinion on modeling ReST commands on the server
>>        side.  I've placed a suggestion for thought on my blog [1].
>>         Please let me know what you think.  The basic idea is to prefer
>>        not using entities on commands as it makes retry logic easier.
>>         In such case, we can use Matrix Params instead.
>>        Cheers.
>>        -Adrian
>>        founder jclouds
>>        [1]
>> http://anyweight.blogspot.com/2009/10/rest-be-nice-no-entities-please.html
>> http://www.reddit.com/r/programming/comments/9u2ic/rest_be_nice_no_entities_please/
>>        http://digg.com/programming/ReST_be_nice_no_entities_please
>>    --    Bill Burke
>>    JBoss, a division of Red Hat
>>    http://bill.burkecentral.com
> --
> Bill Burke
> JBoss, a division of Red Hat
> http://bill.burkecentral.com
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