W3C home > Mailing lists > Public > public-hydra@w3.org > December 2013

RE: Hydra and the uniform interface

From: Markus Lanthaler <markus.lanthaler@gmx.net>
Date: Tue, 17 Dec 2013 21:53:52 +0100
To: "'Ryan J. McDonough'" <ryan@damnhandy.com>
Cc: "'Mark Baker'" <distobj@acm.org>, <public-hydra@w3.org>
Message-ID: <049d01cefb6a$191fc260$4b5f4720$@lanthaler@gmx.net>
On Monday, December 16, 2013 1:53 PM, Ryan J. McDonough wrote:
> On Dec 13, 2013, at 10:15 AM, Markus Lanthaler wrote:
>> On Thursday, December 12, 2013 5:12 AM, Ryan J. McDonough wrote:
>>> On Dec 11, 2013, at 5:37 PM, Markus Lanthaler wrote:
>>>> On Friday, December 06, 2013 9:59 PM, Mark Baker wrote:
>>> HTTP says that a successful POST operation could return a 200 or 201
>>> response code that may include a response body. In this case, Hydra is
>>> building on the HTTP response codes but also asserting that for this
>>> particular URI, it's also always going to return an #Order instance in
>>> response body.
>> Hmm.. no, not exactly. The spec says
>>    Optionally, it is also possible to describe what information the
>>    server expects or returns, including additional information about HTTP
>>    status codes that might be returned. This helps a developer to
>>    understand what to expect when invoking an operation. This information
>>    has, however, not to be considered as being complete; *it is merely a
>>    hint*. Developers should, e.g., expect that other HTTP status codes
>>    might be returned and program their clients accordingly.
>> and the definition of returns says
>>    hydra:returns
>>    The information returned by the Web API on success
>> So it is really just a hint. Probably this has to be clarified or
>> highlighted somehow in the spec. I've raised ISSUE-21 to track this.
> My bad, I was mainly referring to the example and not the fact that the
> Hydra spec makes this optional. My trouble with hints like this is that
> people don't always read specs and will assume return types are required,
> as opposed to hints, and will build clients that don't expect other
> response codes or entities with something other than an #Order..
> Furthermore, this particular example flow a lot like a method call and
> fits nicely into code generators.

Yeah, that's a general problem. I'm not sure how we can address that in the
spec though. I think that needs to be addresses by test cases for clients. I
think at some point we need a document describing how a client has to
interact with a Hydra-based service and could perhaps even specify an API
for that if there's enough interest.

> I don't know that I find returns to especially useful.

That depends on how you look at it. It may provide some important
information if you need to describe similar operations or to make the
semantics dependent on the value of "returns". For example, you could use it
to define a "Convert" operation that converts a "expects" into a "returns".

> But even so, I think the spec needs to be open to
> having more that one entity type on a success. In my example, I was
> getting that a successful response could be either an #Order or an
> #Invoice on success.

We currently don't specify any cardinality constraints. Thus we could simply
say it there are multiple values for "returns" they are alternatives.
Actually I think that would be very valuable and similar to how Schema.org
e.g. defines "rangeIncludes" instead of the standard "range".

> The return type model also feels a bit limiting as well. While Hydra can
> provide a hint that a given operation might return a specific type, it
> also seems to assumed that I will always getting back a JSON-LD response.
> There really needs to more flexibility here as a service may not always
> return a JSON-LD response.

It doesn't say anything about the media type - at least not till now. If you
use a Hydra in a Turtle-based API it would be sensible to expect that you'll
(be able to) get the information in Turtle.

> Let's say we have a LOL Cat picture service
> allows me to place text on an image. The POST operation would return an
> image. Hydra doesn't appear to be able to describe non-JSON-LD responses
> very well.

Yeah, binary blobs (from a Hydra POV) are currently not well-handled. I
thought we already have an issue for that but apparently we don't. So I
created ISSUE-22 [1] for this. My current idea is to define a class
hydra:BinaryBlob or something similar and then either create specific
subclasses thereof or qualifying it somehow with a media type. I'm not sure
yet myself.

>>> Browsers work well because they
>>> only know that the next link or form post will return "something."
>>> Browsers don't even assume that they'll get back HTML either.
>> No, but humans controlling them have expectations about what they get
>> when they follow a link or fill out a form. If you break those too often,
>> the user will give up and use a different site instead. In other words,
>> site's usability sucks in that case.
> I think you've misunderstood what I was getting at. No doubt changing
> too often will result in usability issue. That's not what I am getting at.
> However, it's not uncommon to follow a link and expected to see the
> I wanted but was greeted with an Ad. Or more importantly, asking me to
> to new "terms of service" or a new privacy policy. Browsers don't break
> these changes occur. But users  do have to react to these changes.

OK, I see. So, the question is how this could be handled with Hydra. Let's
just say it's an ad (in order to avoid to having to send data). The client
would then need to be able to find the link which brings it a step closer to
its goal. The client could thus go and look up the definitions of all links
and find out which one points is expected to return the type of resource it
is interested in. Hydra descriptions are not static but of course clients
need to be programmed accordingly to be adaptive enough.

> I know Stu chimed on on this thread a few weeks back, but I'd highly
> encourage you to listen to his WS-REST talk:

I think I've seen that talk already but will watch it again on the weekend.
Thanks for the pointer.

> The idea of sensors and state machines to reach the desired goal is a very
> powerful idea. This also gets around the option to specify return types.
> agent would then be able to react to the response by sensing what it got
> back by looking at the Content-Type and optionally one or more link
> (maybe a profile link relation) to determine if they have ability to
> this response. This a lot like how enterprise messaging systems work. You
> may only be able to handle certain types of messages and you have to
> determine if you can can process the message by looking at headers or
> message properties to determine if it's worth parsing the body. I see a
> property hypermedia client working in a slightly similar way.

Don't you think the information "returns" provides would be helpful to chose
the right link to follow? Very trivial example, an "alternate" link that
allows you to get the same entity in a different serialization format or
expressed using a different vocabulary (FOAF instead of Schema.org e.g.)?

Similarly, I think it is useful to generate human-readable information out
of a Hydra description. But yeah, I see how you could make the link
relation/operation explicit enough to not have to rely on other information
such as "returns".

>>> The browser
>>> knows what media types it can understand and determines if it can render
>>> it by looking at the content-type header. If it can't display it, it
>>> the user what to do with it. Each link does not scribe what's on the
>>> end of the link, or what it might look like, its just a link.
>> It does, otherwise the user wouldn't be able to select the right link to
>> follow. It is just that machines can't understand the labels or images
>> to convey that information.
> The point I was getting is that a non-human agent can only be coded to
> things that it knows and understands. I think there's little doubt that an
> automated agent would have to be coded with some upfront knowledge of the
> data model and some basic event types. This where I'm drawing the
> similarity; the agent would know the different types (i.e. #Order,
> #Product, etc.) before embarking on achieving some type of goal. With the
> understanding of what the different types mean, the agent can react to
> different responses. When you go there, you don't really need to specify
> return types. Put it another way: I think the ideal way to approach this
> is to frame this in an event-based design rather a synchronous method call
> with a return type.

Right. The question is how you select the right link once you've got the
representation. I think "returns" may help you here to make the client more
adaptive. You seem to disagree, right?

> Taking it up another level, I think another way to approach Hyrda would be
> to look what a an ideal Hydra Client would look like. There are too many
> "REST" or Hypermedia specs out there that define what things look like
> from the server side, but do very little to to describe how a client
> should consume this stuff. I think once we can define what the client
> programming model looks like, it would make the service description that
> much easier.

>>> This detail also invites itself to germinated client bindings that would
>>> ultimately couple clients to specify server implementations.
>> Yeah, that's one of the things I'm most concerned with. I tried to
>> that by not including any URLs in a Hydra API description which allow to
>> that in a practical manner. If you need to find out the URLs at runtime
>> anyway, it's that much more work to process the API description at the
>> time.
> Well I'm glad you parse what "germinated client bindings" meant :) Stupid
> auto correct.  


>>> WADL sets up
>>> a nice environment for describing an XML IDL that can be used to
>>> clients from. One of the things that a WADL Document would describe is
>>> both what a given URI would return in a successful response as well as
>>> types of errors that the client could get.
>> Right, in WADL the coupling is, as in most current API libraries (and
>> documentations for that matter), on the URL structure.
> It's not just URL structure, WADL also binds input and return types as
> well as response codes. To complicate matters, response messages were
> baked into the WADL descriptor rather than the  getting the message from
> the server. Even if you address URL bindings, fixed entities base on a
> descriptor also binds clients to specific workflows and properties and
> don't get clients to to consider that they may get response entities with
> more information than they expected and the fact that may not be talking
> directly to the service they are connected to and might get responses from
> the intermediary.

Agreed.. but can we really address this by, let's say, removing "returns"
from Hydra?

>> Thanks a lot again Ryan for trying to help here. At least we've found
>> something that has to be made much clearer in the spec. I remember you
>> working on a little prototype. How is that going?
> That's been on hold for the moment. Day job getting in the way with all of
> the end of year crunch going on. Hopefully I'll have some time over the
> holiday's to get back on that.

Please keep us posted.


[1] https://github.com/HydraCG/Specifications/issues/22

Markus Lanthaler
Received on Tuesday, 17 December 2013 20:54:28 UTC

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