Santiago,
your proposal expects the client software to know the literals "onliner" and
"offliner" and their actual meaning. No machine can ever understand that
this will take machines offline. So at least these two words and their
actual intention must be known to the programmer of the client. Also, he
must know definitively how to apply the "item" template. As he must know
that, I cannot see what is more "loosely" and more "hypermedia" than just
sending the list of links directly? Just do not see the difference here wrt
to "hypermedia" and "coupling". So I do not agree to your point here. What
your proposal does better than mine is supporting "bad" design, as I
described later. With my proposal, there is less chance to do "bad" design.
And I think an API should prevent "bad" design if any possible.
Please also not that my prosposal was not (a) and (b) as you cited, but
using the "anchor" attribute, which, BTW, I actually understand as the
intended solution to exactly this problem (separating target and method
without the need to invent another means of templating).
And, really, I do not see that the inability to support polymorphic
resources and binary resources as being just "not perfect". This is a
showstopper in my eyes as the idea of the "Link:" header actually came out
of the need that binary content cannot have easily parseable links. So we
should provide a solution that supports both "problems", not one that
ignores them. My proposal (anchor attribute) supports both as I wrote. So it
IS better, just by counting features. ;-)
Regards
Markus
From: Santiago Pericas-Geertsen [mailto:Santiago.PericasGeertsen_at_oracle.com]
Sent: Montag, 14. November 2011 16:06
To: jsr339-experts_at_jax-rs-spec.java.net
Subject: [jsr339-experts] Re: [jax-rs-spec users] Re: Re: Re: Re: Improving
Hypermedia Support
On Nov 12, 2011, at 9:53 AM, Markus KARG wrote:
But, as a machine can neither guess nor understand tooltips, it just could
pick one of two strategies: (a) It is explicitly written to orchestrate this
particular service, so it just knows the syntax of the URI (I mean, if I
write an eBay client I obviously know the API, which contains the particular
URI syntax).
This strategy seems to be against hypermedia and loose coupling. Ideally,
we want clients to only know a single URI and then be dynamically informed
of the rest (as these may change even from run to run). I'm not saying that
URI templates is the best solution, but it's certainly better than clients
needing to know the exact URI structure to follow a link. Templates only
assume that clients know how to identify a machine and can call a method to
instantiate the template.
One doesn't need a template then (you only need a template, if you *do not*
know the syntax and want some background magic to handle this).
That's precisely the point, not knowing the syntax. If I know the exact
syntax, then there's no point of including these links in the
representations as they are already known to the client.
(a) Polymorhy: Maybe there are "machine" instances that need to have
differnent URIs! You would need to provide a different template, but how to?
(b) Binary content: Maybe the machine "name" is not (or not easily)
parseable from the content! How would your client software guess the machine
name to pass into the template?
Agree, it isn't perfect. But I can't agree that the strategy you outline
above is better.
But, actually, if I were you, I would provide a different model in fact to
solve your task: Not sending all the transition links for all the machines
inside of the cluster document, but let the cluster document contain only
the machines "self" URIs
This is certainly an option. In fact, given that what has been proposed are
all API calls, you could design your application this way --this is the
reason why I don't want to "bake" anything in annotations and framework
logic.
-- Santiago
(so you only learn the list of machines in the first step -- maybe you
neither want to know the state nor want to shut down a machine, so you don't
really lose anything in the first step). If a link is needed for a machine
(since you then want to know the status or want to shutdown one machine), I
can request the machine's representation in a second roundtrip and find it's
transition links inside that document. This imposes one more http request,
but it provides self-controlled machines, which in fact solves not only the
templates problem, the polymorphy problem and the binary content problem,
but it is also more scalable as for lots of machines you would only ask for
as much state as you can see on the screen (not requesting machines states
you won't read ever), and it is much more object oriented: If I want to shut
down a machine, I press it's power switch. I do not press a swith on a
different device. :-)
Regards
Markus
From: Santiago Pericas-Geertsen [mailto:Santiago.PericasGeertsen_at_oracle.com]
Sent: Dienstag, 8. November 2011 20:09
To: jsr339-experts_at_jax-rs-spec.java.net
Subject: [jsr339-experts] Re: [jax-rs-spec users] Re: Re: Re: Re: Improving
Hypermedia Support
On Nov 8, 2011, at 12:39 PM, Markus KARG wrote:
Can you please post the WIKI link again? Thanks.
Here it is:
http://java.net/projects/jax-rs-spec/pages/HypermediaExample
-- Santiago
From: Santiago Pericas-Geertsen [mailto:Santiago.PericasGeertsen_at_oracle.com]
Sent: Montag, 7. November 2011 19:11
To: jsr339-experts_at_jax-rs-spec.java.net
Subject: [jsr339-experts] Re: [jax-rs-spec users] Re: Re: Re: Improving
Hypermedia Support
Hi Markus,
Could you apply this to the example we have on the wiki? In particular,
given a collection item Y, how can I get the link to Y? I may want to start
machine A and stop machine B; so getting the links to all the machines in
the cluster is not enough.
-- Santiago
On Nov 6, 2011, at 8:45 AM, Markus KARG wrote:
Santiago,
this way:
Link: <absolute-uri-A1>; rel="name-of-collection-A"
Link: <absolute-uri-A2>; rel="name-of-collection-A"
Link: <absolute-uri-B1>; rel="name-of-collection-B"
Link: <absolute-uri-B2>; rel="name-of-collection-B"
Here you have four absolute URIs. A1 and A2 are part of collection A, B1 and
B2 are part of collection B.
If the client wants to visit all resources references by collection B, it
would just have to invoke A1 and A2, identified via collection A.
An alternative way would be using the anchor attribute to identify the
resource as a fragment of the source resource (e. g.
anchor="#name-of-collection-A").
A third option would be a link-extension like "Link: <absolute-uri>;
collection="name-of-collection".
Where do you see a problem? No need for neither templates nor client side
application logic.
Regards
Markus
From: Santiago Pericas-Geertsen [mailto:Santiago.PericasGeertsen_at_oracle.com]
Sent: Dienstag, 1. November 2011 20:17
To: jsr339-experts_at_jax-rs-spec.java.net
Subject: [jsr339-experts] Re: [jax-rs-spec users] Re: Re: Improving
Hypermedia Support
* I do not understand the problem with the collection. Why not sending just
several URIs with the same relation in that case (one for each item of the
collection)?
As link headers? How would you know which one corresponds to which item in
the collection? There's no ordering for link headers.
-- Santiago