[ovirt-devel] [ATN] Introduction of RESTAPI metamodel
Juan Hernández
jhernand at redhat.com
Mon Oct 26 16:32:21 UTC 2015
On 10/26/2015 04:56 PM, Roman Mohr wrote:
> Hi Juan,
>
> The way to specify the contract look pretty clean and nice.
> I would love to read a few words about the big picture. What is the
> final scenario?
>
The motivation for this change is that currently we don't have a central
place where the RESTAPI is specified, rather we have several different
places, using several different technologies:
* XML schema for the data model.
* JAX-RS for part of the operational model (without the parameters).
* rsdl_metadata.yaml for the parameters of the operational model.
This makes it difficult to infer information about the model. For
example, the generators of the SDKs have to download the XML schema, and
the RSDL (which is generated from the JAX-RS interfaces using reflection
and combining it with the information from the rsdl_metadata.yaml file)
and then they have to do their own computations to extract what they need.
Same happens with the CLI: it has to extract the information it needs
from the Python code generated for the Python SDK, yet another level of
indirection.
We are also lacking a comprehensive reference documentation of the
RESTAPI. What we currently have has been written by hand, and gets out
of sync very quickly, and we don't even notice.
To solve these issues I intend to have the specification of the RESTAPI
only in one place, and using only one technology. I decided to use Java
interfaces for that. Note however that they are just the support for the
information, like paper is the support for ink. I decided to use Java
because it is easy to create, modify and re-factor using tools familiar
to most of us.
These source of these interfaces is analysed (using QDox, currently) and
a "model" of the RESTAPI is generated in memory. This model is
independent of the supporting Java source, and easy to consume. For
example, imagine that you want to list all the types available in the
model and for each one display its documentation:
Model model = ...;
for (Type type : model.getTypes()) {
Name name = type.getName();
String doc = type.getDoc();
System.out.println(name + ": " + doc);
}
Something like this, but more elaborate, will be part of a web
application that provides comprehensive reference documentation,
assuming that we dedicate the time to write documentation comments in
the specification.
I intend to use this model also to do simplify the generators of the
SDKs and the CLI.
In addition these are some of the things that I would like to change in
the near future (for 4.0):
* Move the specification of the parameters of operations out of the
rsdl_metadata.yaml file and into the model. For example:
@Service
public VmService {
/**
* The operation to add a virtual machine.
*/
interface Add {
/**
* The representation of the virtual machine is received
* as parameter, and the representation of the created
* virtual machine is returned as result.
*/
@In @Out Vm vm();
/**
* In the future, we will be able to specify other
* parameters here.
*/
@In Boolean force();
/**
* Even with default values.
*/
@In default Boolean force() { return true; }
/**
* And we will be able to specify constraints, which
* will replace the rsdl_metadata.yaml file.
*/
@Constraint
default boolean vmNameMustNotBeNull() {
return vm().name() != null;
}
}
}
* Enforce the constraints automatically. If the constraints are in the
model, then we can just check them and reject requests before delivering
them to the application. Currently we do this manually (and often
forget) with calls to "validate(...)" methods.
* Generate the Java classes directly from the model. Instead of Model ->
XML Schema -> Java, we can do Model -> Java. This will allow us to solve
some of the XJC compiler limitations, like the horrible way we handle
arrays today.
* Replace JAX-RS with a simpler infrastructure that supports better
streaming and CDI injection.
* Add support for multiple versions of the API, using the "Version"
header, and generating different Java classes for entities and services.
For example, if we have versions 4 and 5 of the model as separate
artifacts, then we can generate "V4Vm" and "V5Vm" entity classes, and
"V4VmService" and "V5VmService" service classes. These can be used
simultaneously in the server, so we can have in the same engine
implementations for multiple versions.
The final picture isn't completely defined yet.
Regards,
Juan Hernandez
> On Mon, Oct 26, 2015 at 4:03 PM, Juan Hernández <jhernand at redhat.com
> <mailto:jhernand at redhat.com>> wrote:
>
> Hello,
>
> I will soon merge the following patches that introduce a new way to
> specify the contracts of the RESTAPI:
>
> restapi: Introduce metamodel
> https://gerrit.ovirt.org/45852
>
> restapi: Use metamodel
> https://gerrit.ovirt.org/46478
>
> restapi: Generate JAX-RS interfaces from model
> https://gerrit.ovirt.org/47337
>
>
> Looks pretty much like we are replacing one way of annotating things
> with another way of specifying things.
> Could you elaborate what the benefit of that way of description is?
>
> How would I customize endpoints with e.g. @Gzip annotations? Would I at
> the end still have my JAX-RS annotates resource classes?
>
>
> These patches introduce a new "metamodel" concept, and move the current
> specification of the RESTAPI based on XML schema and JAX-RS interfaces
> to a new "model" built on the new metamodel.
>
>
> What does this mean for you in practical terms? Currently when you want
> to introduce or modify one of the data types used by the RESTAPI you
> start by modifying the XML schema. Once the patches are merged the XML
> schema will never be touched, as it will be automatically generated from
> the "model". For example, imagine that you need to add a new "color"
> attribute to the "VM" entity. To do so with the new model you will have
> to modify the following file, which is the specification of the "Vm"
> entity, written as a Java interface:
>
>
> https://gerrit.ovirt.org/#/c/46478/16/backend/manager/modules/restapi/model/src/main/java/types/Vm.java
>
> In that interface you will have to add a line like this:
>
> String color();
>
> Note that this Java interface is just the specification of the entity,
> it won't be used at all during runtime. Instead of that the XML schema
> will be generated from it, and then Java will be generated from the XML
> schema, as we do today (this will change in the future, but not yet).
>
> Same for the services. If you want to add a new "paint" action to the
> "Vm" resource then you won't modify the JAX-RS interfaces, instead of
> that you will modify the following file, which is the specification of
> the "Vm" service, written as a Java interface:
>
>
> https://gerrit.ovirt.org/#/c/47337/6/backend/manager/modules/restapi/model/src/main/java/services/VmService.java
>
> In that interface you will need to add a sub-interface representing the
> action:
>
> interface Paint {
> }
>
> The JAX-RS interface will be generated from that. Currently these
> sub-interfaces are empty. In the future they will contain the
> specifications of the parameters (currently in the rsdl_metadata.yml
> file).
>
>
>
> These changes will currently affect only the specification of the
> RESTAPI, not the implementation, so in in the "Backend*Resource" classes
> things won't change yet.
>
>
> Currently I do not really understand where we are going here. Are we
> trying to get rid of rdsl?
>
> So basically two questions:
>
> 1) What is the final goal?
> 2) What speaks agains using Hibernate validator on Daos in combination
> with JAX-RS annotated resources (and just removing all interfaces, as
> far as I can see we only have one implementation per endpoint) and
> creating all schemas and clients through SWAGGER tooling?
>
>
> If you have doubts, please let me know.
>
> Regards,
> Juan Hernandez
>
> --
> 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
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>
> Thanks,
>
> Roman
--
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3ºD, 28016 Madrid, Spain
Inscrita en el Reg. Mercantil de Madrid – C.I.F. B82657941 - Red Hat S.L.
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