W3C home > Mailing lists > Public > public-hydra@w3.org > July 2014

Re: Express "go to specific page" for a collection

From: Philipp Zins <pipo@senaeh.de>
Date: Thu, 10 Jul 2014 13:35:37 +0200
Message-ID: <CACkJGAsY2093QqC_sz9h-c-4LciDg8W_cntK=_MP9qKh_8-dDA@mail.gmail.com>
To: public-hydra@w3.org
Hi Tomasz,

thank you for your answer. Very helpful.

A small addition to
> This is interesting. I haven't seen the JSON Hyper-Schema before and
> it doesn't have the shortcomings I explained above in that the "core
> representation" stays intact when adding hypermedia definition.

JSON Hyper-Schema doesn't really tell you how it should be used. I
think most people use it only for automatic API tests or return it on
an OPTIONS request. I don't know if anyone uses JSON Hyper-Schema the
way we like to use it (embedded as "_schema" for every resource in
every response), but I think its not prohibited by the spec. Just
wanted to point that out, if you should want to look for further
examples.

> >
> > Yes, generating (or evaluating) forms is one of the benefits of this
> > approach which we use. E.g. in a "create new user form" we could
> > automatically set the "required" attribute on the input element for the user
> > name.
> >
> Hydra gives you all that functionality, doesn't it?

That would be great! :) As I said, I still couldn't make an in-depth
look into Hydra. I just skimmed through the spec :)

2014-07-10 9:45 GMT+02:00 Tomasz Pluskiewicz <tomasz@t-code.pl>:
> Hi Philipp
>
> I think you should use plain text :)
>
> Also sorry I will be a little bit off topic for part of my reply.
>
> On Thu, Jul 10, 2014 at 8:43 AM, Philipp Zins <pipo@senaeh.de> wrote:
>> Hi Markus,
>>
>>> That's great. AFAIK, we didn't have (m)any frontend developers in the
>>> group till now. So your input will be very helpful to improve Hydra.
>>
>>
>> I'm happy to give input. I think my view on a Hypermedia API is quite
>> influenced from being a frontend developer as you will see in the following
>> answer.
>>
>>> Could you tell us why you currently favor JSON Hyper-Schema and briefly
>>> explain how you use it on the frontend? For example, do you use it to
>>> automatically generate forms?
>>
>>
>> With my "current" knowledge about HATEOAS I have the following philosophy:
>> HTML is the best hypermedia format I know and our JSON hypermedia format
>> should use the same principles.
>>
>
> I've just discussed this with a colleague yesterday and my conclusion
> is that HTML and Web API hypermedia differ in a fundamental yet subtle
> way. HTML is a markup of the view while any JSON/XML/RDF/whatever that
> client and server communicate with is tha data. And so the hypermedia
> is move to another layer of applications.
>
>>
>> That said a user who browsers the web consumes the HTML document. A normal
>> user doesn't look into HTTP headers. Likewise a client should gather all the
>> data it needs from the JSON payload. There is no need for the client to
>> consume HTTP headers - they're only really needed for the browser (e.g. for
>> caching information).
>> A user interacts with the HTML document via links (anchors or buttons). They
>> know what they do, because they are labeled (e.g. "Buy now", "Click here to
>> login", "Close this window"). A good JSON hypermedia format has a similar
>> way to share vocabulary between server and client. Most formats seem to rely
>> on "rel" and Link Types for this (e.g. "next", "prev", "index", "help",
>> "edit"). I don't know if this is sufficient and I think this could be one of
>> the major advantages of JSON-LD (- using schema.org for common vocabulary
>> and Hydra for API-specific vocabulary). At least if I understand JSON-LD
>> correctly.
>> Links in HTML documents are placed relative to their content (e.g. place a
>> "share button" under every article in a blog - opposite to gathering "all"
>> links at a central place). The JSON hypermedia format should do the same.
>> Every resource has its own "links". (See the following example.)
>>
>
> This is precisely what Hydra does. SIREN for example does what you
> mention. The response is split into a semantically different
> representation, where the class, properties, links and actions are
> completely different, hence the contract is totally different that if
> HAL was used. Or no hypermedia was used at all:
>
> {
>   "class": [ "order" ],
>   "properties": { "orderNumber": 42, "itemCount": 3, "status": "pending" },
>   "entities": [ { ... }, { ... }],
>   "actions": [ { "name": "add-item", "method": "POST" } ],
>   "links": [ { "rel": [ "self" ], "href": "http://api.x.io/orders/42" }, ...]
> }
>
> I don't really understand the entities property, but that's the point.
> Wihtout SIREN the response would probably be much simpler:
>
> {
>   "class": "order",
>   "orderNumber": 42,
>   "itemCount": 3,
>   "status": "pending"
> }
>
> This is because (every?) other hypermedia format defines not only the
> semantics but also the syntax. HAL is a bit better in this regard, but
> it introduces the "_embedded" property for example. This is very much
> like the media type nonsense. With Linked Data (and Semantic Web in
> general) you always speak about resources in an abstract way. It's the
> semantics that matter. JSON-LD, Turtle, RDF/XML are just syntax. Thus
> in Hydra you won't see mime types. Properties and operations are
> defined in terms of classes, ie. that an operation returns (an
> instance of) a specific class. Or requires an instance as input.
>
> And so because Linked Data operates on a model, Hydra extends existing
> data rather than turn in upside-down. You can extend individual
> instances or their classes, because the format makes it possible. And
> so, yes, each instance can have its own link and operations.
> Furthermore if any Web API switched from Hydra to some other
> RDF-compatible hypermedia description the actual data will be
> unchanged. And so any client that ignores hypermedia would never
> notice the difference. With a move from HAL to SIREN all clients
> break, because the messages change completely.
>
>>
>> A form isn't much different from a common link: it just gathers some
>> additional data and than you can follow "the link of the form" via a submit
>> button. You can even specify the HTTP method which should be used. A JSON
>> hypermedia format should include the same information: what data is needed
>> to follow the link and what HTTP method should be used?
>>
>
> I actually think that the term "hypermedia format" is not correct in
> terms of Hydra. There already is a format and its called RDF. In
> itself it is not enough for self-descriptive representations. However
> Hydra is not a new format. Nor does it change RDF in any way. It is
> merely an extension, which allows augmenting existing data with
> description of hypermedia controls.
>
>>
>> With these requirements in mind that is how we use JSON Hyper-Schema (basic
>> example):
>>
>> // GET https://api.example.com/users
>> {
>>   // Every "_schema" of a resource is a valid JSON Hyper-Schema.
>>   // Every resource has its own "_schema": Here we have a users collection
>> as one resource
>>   // containing two users which each are a resource, too. If you want these
>> users could also contain
>>   // nested resources like friends, hobbies, etc.
>>   "_schema": {
>>     // We mostly need the "links" field from JSON Hyper-Schema, but you
>> could really use all of JSON
>>     // Hyper-Schemas features (e.g. create a schema definition for the users
>> collection itself).
>>     "links": [
>>       {
>>         "rel": "self",
>>         "href": "https://api.example.com/users",
>>         "method": "GET"  // GET is default as in the browser. You don't to
>> specify it.
>>       },
>>       {
>>         "rel": "create",
>>         "href": "https://api.example.com/users",
>>         "method": "POST",
>>         // This schema definition tells us that we need a "name" to create a
>> new user and can
>>         // optionally pass a "description".
>>         // A schema definition like this refers to the request body for
>> POST, PUT and PATCH
>>         // (if you need PATCH) and to the query string for GET and DELETE.
>>         // (This is similar to UBER.)
>>         "schema": {
>>           "type": "object",
>>           "properties": {
>>             "name": {
>>               "type": "string"
>>             },
>>             "description": {
>>               "type": "string"
>>             }
>>           },
>>           "required": [ "name" ]
>>         }
>>       }
>>     ]
>>   },
>>   // Some arbitrary data on the collection resource.
>>   "total": 2,
>>   "users": [
>>     // Every item of "users" is also a resource, so they have "_schema"
>> definitions.
>>     {
>>       "_schema": {
>>         "links": [
>>           {
>>             "rel": "self",
>>             "href": "https://api.example.com/users/xxx-123"
>>           }
>>         ]
>>       },
>>       // Some arbitrary data on this user resource.
>>       "id": "xxx-123",
>>       "name": "John",
>>       "description": null
>>     },
>>     // Every item of "users" is also a resource, so they have "_schema"
>> definitions.
>>     {
>>       "_schema": {
>>         "links": [
>>           {
>>             "rel": "self",
>>             "href": "https://api.example.com/users/abc-124"
>>           },
>>           // I can dynamically add or remove "link" objects. We don't need
>> to use the same
>>           // for every user. Maybe I can remove this one, because it is an
>> intern and not an
>>           // admin.
>>           // We rarely need URI templates, as we include an absolute base
>> ("https://api.example.com/users/abc-124")
>>           // and define query params with "schema", if we need them.
>>           {
>>             "rel": "remove",
>>             "href": "https://api.example.com/users/abc-124",
>>             "method": "DELETE"
>>           }
>>         ]
>>       },
>>       // Some arbitrary data on this user resource.
>>       "id": "abc-124",
>>       "name": "Mica",
>>       "description": "Just an intern..."
>>     }
>>   ]
>> }
>>
>
> This is interesting. I haven't seen the JSON Hyper-Schema before and
> it doesn't have the shortcomings I explained above in that the "core
> representation" stays intact when adding hypermedia definition.
> However the first sentence reads "JSON Schema is a JSON based format
> for defining the structure of JSON data", and so it is still a new
> format, albeit JSON-based.
>
> All in all I see a lot of similarities between JSON Hyper-Schema and
> Hydra. You could almost envision a tool to covert the former into the
> latter.
>
>>
>> Yes, generating (or evaluating) forms is one of the benefits of this
>> approach which we use. E.g. in a "create new user form" we could
>> automatically set the "required" attribute on the input element for the user
>> name.
>>
>
> Hydra gives you all that functionality, doesn't it?
>
>>
>>> What are those benefits? Apart from the "go to specific page" feature,
>>> what else is currently missing in Hydra in your opinion?
>>
>>
>> I can't say what is missing in Hydra yet as I hadn't enough time to look
>> deeply into Hydra. A "go to specific page" feature would be very easy with
>> JSON Hyper-Schema: just add a new "link" object on the collection which has
>> a schema definition that requires a "page" param with an int value. (The
>> server could automatically include a "min" and "max" page, too. Very handy.)
>>
>
> Unlike next/prev page, which Hydra defines out of the box, there
> probably won't be a goToSpecificPage property, because it requires the
> use of a IriTemplate. However all the building blocks are there.
>
> Markus, please correct me here. Also I think that the vocabulary
> documentation page is a bit lacking in this regard. I would add a
> complete example of how a IriTemplate fits into a resource
> representation.
>
>>
>> I hope you have a picture of what we want to achieve now :)
>>
>> Btw, I'm coming from Leipzig. I saw that one person in the Hydra group is
>> associated with the university of Leipzig. What a coincidence! We're
>> literally neighbors :)
>>
>> Cheers,
>> Pipo
>>
>>
>> 2014-07-08 13:17 GMT+02:00 Markus Lanthaler <markus.lanthaler@gmx.net>:
>>
>>> Hi Pipo,
>>>
>>> On Tuesday, July 08, 2014 11:42 AM, Philipp Zins wrote:
>>> > > Does your use case really require it?
>>> >
>>> > I would say, yes. If you compare some GUI pagination patterns (see
>>> > https://gist.github.com/mislav/622561) you can see that roughly 50%
>>> > allow the user to jump to an arbitrary page.
>>>
>>> When it comes to GUIs it is indeed an often used pattern. The question
>>> however is, if this really needs to be supported by the API directly as
>>> well. It depends entirely on the application I would say. How often does a
>>> user jump from page 1 to page 213 and then back to page 49? If it doesn't
>>> happen often (and it isn't needed to support certain use cases), it's better
>>> to avoid supporting it directly IMO. The client can prefetch the next x
>>> pages and then render direct links just to those pages (1 2 3 4 .... last
>>> page).
>>>
>>> It all depends on the use case. I would thus like to have a concrete
>>> scenario that really requires it, i.e., that can't be solved otherwise. Do
>>> you have one?
>>>
>>>
>>> > What do you think about Jindřich comment with the anonymous anonymous
>>> > property?
>>>
>>> Jindřich said:
>>>
>>>   Instead of using hydra:freetextQuery for hydra:property mapping in
>>>   the hydra:IriTemplateMapping, I think you can use an anonymous
>>>   property with rdfs:range defined to be xsd:positiveInteger:
>>>
>>>   "property" : {
>>>     "rdfs:range" : "xsd:positiveInteger"
>>>   }
>>>
>>>   Although I'm not sure if using anonymous properties in such case is
>>>   an accepted practice in Hydra.
>>>
>>> I wouldn't use an anonymous property in this case as it is crucial for the
>>> client to understand what the property is about. Just telling the client
>>> that the value should be a positive integer isn't enough IMO. The client
>>> needs to know that this represents the page number. It also needs to know if
>>> the first page is 0 or 1 or if the page number is actually an offset (11
>>> instead of 2 for page 2 if each page has 10 items).
>>>
>>>
>>> > Some background: I'm mainly a frontend developer switching to the
>>> > backend here and there. As we try to move more development processes
>>> > to the client (for better iteration speeds, dynamic and fluid GUIs,
>>> > etc.) we need a good interface between server and client. We really
>>>
>>> That's great. AFAIK, we didn't have (m)any frontend developers in the
>>> group till now. So your input will be very helpful to improve Hydra.
>>>
>>>
>>> > like the idea of Hypermedia APIs (instead of RPC-ish "REST" APIs) and
>>> > I try to evaluate different solutions. We currently favor JSON Hyper-
>>> > Schema directly embedded in our resources,
>>>
>>> Could you tell us why you currently favor JSON Hyper-Schema and briefly
>>> explain how you use it on the frontend? For example, do you use it to
>>> automatically generate forms?
>>>
>>>
>>> > but I would like to explore
>>> > if we can move to Hydra/JSON-LD completely in the next turn as it
>>> > seems to be a more widely used standard and because JSON-LD is
>>> > supported by all major search engines. But I don't want to loose the
>>> > benefits of JSON Hyper-Schema.
>>>
>>> What are those benefits? Apart from the "go to specific page" feature,
>>> what else is currently missing in Hydra in your opinion?
>>>
>>>
>>> Cheers,
>>> Markus
>>>
>>>
>>> --
>>> Markus Lanthaler
>>> @markuslanthaler
>>>
>>>
>>
>
>
Received on Thursday, 10 July 2014 11:36:06 UTC

This archive was generated by hypermail 2.3.1 : Tuesday, 6 January 2015 20:29:42 UTC