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Excerpts from Flavio Percoco's message of 2015-12-09 09:09:10 -0430:
> On 09/12/15 11:33 +0000, Kekane, Abhishek wrote:
> >Hi Devs,
> >
> > 
> >
> >We are adding support for returning ‘x-openstack-request-id’  to the caller as
> >per the design proposed in cross-project specs:
> >
> >http://specs.openstack.org/openstack/openstack-specs/specs/
> >return-request-id.html
> >
> > 
> >
> >Problem Description:
> >
> >Cannot add a new property of list type to the warlock.model object.
> >
> > 
> >
> >How is a model object created:
> >
> >Let’s take an example of glanceclient.api.v2.images.get() call [1]:
> >
> > 
> >
> >Here after getting the response we call model() method. This model() does the
> >job of creating a warlock.model object(essentially a dict) based on the schema
> >given as argument (image schema retrieved from glance in this case). Inside
> >model() the raw() method simply return the image schema as JSON object. The
> >advantage of this warlock.model object over a simple dict is that it validates
> >any changes to object based on the rules specified in the reference schema. 
> >The keys of this  model object are available as object properties to the
> >caller.
> >
> > 
> >
> >Underlying reason:
> >
> >The schema for different sub APIs is returned a bit differently. For images,
> >metadef APIs glance.schema.Schema.raw() is used which returns a schema
> >containing “additionalProperties”: {“type”: “string”}. Whereas for members and
> >tasks APIs glance.schema.Schema.minimal() is used to return schema object which
> >does not contain “additionalProperties”.
> >
> > 
> >
> >So we can add extra properties of any type to the model object returned from
> >members or tasks API but for images and metadef APIs we can only add properties
> >which can be of type string. Also for the latter case we depend on the glance
> >configuration to allow additional properties.
> >
> > 
> >
> >As per our analysis we have come up with two approaches for resolving this
> >issue:
> >
> > 
> >
> >Approach #1:  Inject request_ids property in the warlock model object in glance
> >client
> >
> >Here we do the following:
> >
> >1. Inject the ‘request_ids’ as additional property into the model object
> >(returned from model())
> >
> >2. Return the model object which now contains request_ids property
> >
> > 
> >
> >Limitations:
> >
> >1. Because the glance schemas for images and metadef only allows additional
> >properties of type string, so even though natural type of request_ids should be
> >list we have to make it as a comma separated ‘string’ of request ids as a
> >compromise.
> >
> >2. Lot of extra code is needed to wrap objects returned from the client API so
> >that the caller can get request ids. For example we need to write wrapper
> >classes for dict, list, str, tuple, generator.
> >
> >3. Not a good design as we are adding a property which should actually be a
> >base property but added as additional property as a compromise.
> >
> >4. There is a dependency on glance whether to allow custom/additional
> >properties or not. [2]
> >
> > 
> >
> >Approach #2:  Add ‘request_ids’ property to all schema definitions in glance
> >
> > 
> >
> >Here we add  ‘request_ids’ property as follows to the various APIs (schema):
> >
> > 
> >
> >“request_ids”: {
> >
> >  "type": "array",
> >
> >  "items": {
> >
> >    "type": "string"
> >
> >  }
> >
> >}
> >
> > 
> >
> >Doing this will make changes in glance client very simple as compared to
> >approach#1.
> >
> >This also looks a better design as it will be consistent.
> >
> >We simply need to modify the request_ids property in  various API calls for
> >example glanceclient.v2.images.get().
> >
> 
> Hey Abhishek,
> 
> thanks for working on this.
> 
> To be honest, I'm a bit confused on why the request_id needs to be an
> attribute of the image. Isn't it passed as a header? Does it have to
> be an attribute so we can "print" it?

The requirement they're trying to meet is to make the request id
available to the user of the client library [1]. The user typically
doesn't have access to the headers, so the request id needs to be
part of the payload returned from each method. In other clients
that work with simple data types, they've subclassed dict, list,
etc. to add the extra property. This adds the request id to the
return value without making a breaking change to the API of the
client library.

Abhishek, would it be possible to add the request id information
to the schema data in glance client, before giving it to warlock?
I don't know whether warlock asks for the schema or what form that
data takes (dictionary, JSON blob, etc.). If it's a dictionary
visible to the client code it would be straightforward to add data
to it.

Failing that, is it possible to change warlock to allow extra
properties with arbitrary types to be added to objects? Because
validating inputs to the constructor is all well and good, but
breaking the ability to add data to an object is a bit un-pythonic.

If we end up having to change the schema definitions in the Glance API,
that also means changing those API calls to add the request id to the
return value, right?

Doug

[1] http://specs.openstack.org/openstack/openstack-specs/specs/return-request-id.html

> 
> As it is presented in your email, I'd probably go with option #2 but
> I'm curious to know the answer to my question.
> 
> Cheers,
> Flavio
> 
> > 
> >
> >Please let us know which approach is better or any suggestions for the same.
> >
> > 
> >
> >[1] https://github.com/openstack/python-glanceclient/blob/master/glanceclient/
> >v2/images.py#L179
> >
> >[2] https://github.com/openstack/glance/blob/master/glance/api/v2/images.py#
> >L944
> >
> >
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