[ovirt-devel] Evaluating Errai JAX-RS for oVirt

[Juan Hernandez]

On 11/19/2014 06:11 PM, Christian Sadilek wrote:
> Hi Juan,
> 
> I have had this argument many times and to me it does not come down to
> “coupling” vs. “no coupling” but to “implicit coupling” vs. “explicit
> coupling”.
> 
> Even if you don’t use a shared interface (and, btw, you don’t have to
> share the interface in Errai) you still have coupling. In most cases,
> you can’t simply change the server-side REST endpoint without breaking
> existing clients and without changing your JavaScript code. The
> difference here is merely that in Errai it will fail at compile time
> while in other cases you will fail at runtime.
> 
> Cheers,
> Christian
> 

I don't have any objection to use JAX-RS, Errai or any other client side
technology. My concern is about using the server *implementation
classes*, that is probably what you describe as "explicit coupling". The
Java interfaces that we currently use are by no means intended for use
outside of the server, they are an implementation details. We don't have
(and won't have) any commitment to keep them backwards compatible, or to
keep them at all, or even to keep JAX-RS (in the server side). So using
those interfaces, for any purpose other than the server side
implementation, is extremely risky.

It would be nice to have the oVirt RESTAPI defined as an stable set of
Java interfaces (with or without JAX-RS) and enjoy type safety and
compile type checks, but that is not the case today (and it won't be in
the 3.6 time frame). Today the only stable definition is the XML schema
and the RSDL. That we commit to keep backwards compatible.

As I said this doesn't mean that you can't use Errai JAX-RS, just that
if you do so you will have to create and maintain your own interfaces.

> On Nov 19, 2014, at 11:15 AM, Juan Hernandez <jhernand at redhat.com
> <mailto:jhernand at redhat.com>> wrote:
> 
>> On 11/19/2014 04:58 PM, Martin Betak wrote:
>>> Hi all,
>>>
>>> I've been exploring the possibilities of utilizing Errai JAX-RS for
>>> REST client in 
>>> our current GWT-based frontend. This would be a complement to
>>> Vojtech's oVirt.js library
>>> which is very native to javascript and utilizes dynamic discovery of
>>> resources and actions.
>>>
>>> Errai JAX-RS would enable us to reuse our restapi definitions of
>>> resource interfaces and entities 
>>> and generate proxies automatically thus eliminating the need for
>>> extensive manual
>>> code generation.
>>>
>>
>> I strongly discourage this approach, as it will couple the client and
>> the RESTAPI server implementation classes, similar coupling to what we
>> have today between the client and the backend entities and parameters
>> classes.
>>
>> The contract of the RESTAPI is the XML schema that defines the entities
>> and the RSDL that defines resources and actions. Everything else is
>> subject to change and there is (and won't be) any backwards
>> compatibility commitment. The resource interfaces in particular are not
>> part of the contract, they are internal implementation details.
>>
>> If you eventually decide to go this direction then you should create
>> your own resource interfaces.
>>
>>> [errai jax-rs] https://docs.jboss.org/author/display/ERRAI/Errai+JAX-RS
>>>
>>> Unfortunately in my quest I stumbled upon some issues I couldn't
>>> exactly google or solve 
>>> and I would like to ask Mark or Christian, whether the below is
>>> somehow possible with Errai:
>>>
>>> 1) Adding custom header to every request (some form of request
>>> interceptors)
>>> You, see our api returns by default XML and we of course want to
>>> consume JSON.
>>> Thus adding 'Content-Type: application/json' as well as other custom
>>> authentication headers
>>> is necessary for our usage.
>>>
>>> 2) Using custom JSONProvider/ObjectMapper instead of the default
>>> Jackson one. 
>>> In our JSONProvider [1] we use custom ObjectMapper [2] mainly to map
>>> JAX-B annotations
>>> to json. Without this for example errai generates marshaller for the
>>> VMs resource
>>>
>>> public class VMs {
>>>    @XmlElement(name = "vm")
>>>    protected List<VM> vMs;
>>> }
>>>
>>> that expects json field called "vMs" instead of the JAX-B
>>> specification "vm" which is the actual
>>> format returned from the API. There may be other differencies such as
>>> [3] so I think the best solution
>>> would be if we could tell errai to use specific JSONProvider for
>>> generating the marshallers.
>>>
>>> This are the issues that so far seem to be the most blocking ones,
>>> others may arise along the way.
>>>
>>> Thank you for any help provided.
>>>
>>> Best regards,
>>>
>>> Martin
>>>
>>> [1]
>>> https://github.com/oVirt/ovirt-engine/blob/master/backend/manager/modules/restapi/interface/definition/src/main/java/org/ovirt/engine/api/json/JSONProvider.java
>>> [2]
>>> https://github.com/oVirt/ovirt-engine/blob/master/backend/manager/modules/restapi/interface/definition/src/main/java/org/ovirt/engine/api/json/CustomObjectMapper.java
>>> [3]
>>> https://github.com/oVirt/ovirt-engine/blob/master/backend/manager/modules/restapi/interface/definition/src/main/java/org/ovirt/engine/api/json/CustomBeanFactory.java
>>> [resource schema]
>>> https://github.com/oVirt/ovirt-engine/blob/master/backend/manager/modules/restapi/interface/definition/src/main/resources/api.xsd

-- 
Dirección Comercial: C/Jose Bardasano Baos, 9, Edif. Gorbea 3, planta
3ºD, 28016 Madrid, Spain
Inscrita en el Reg. Mercantil de Madrid – C.I.F. B82657941 - Red Hat S.L.