[Openstack] XML and JSON for API's

Jay Pipes jaypipes at gmail.com
Fri Jun 3 13:02:52 UTC 2011


Glance will Accept XML content types as soon as we have users
demanding such. One such user, James Weir from uShareSoft, created a
blueprint on May 20th for it:

https://blueprints.launchpad.net/glance/+spec/glance-xml

Recently, the folks on the Titan team have been working on refactoring
the XML serialization stuff in Nova. The XML serialization was a
significant source of bugs AFAICT, and I've been waiting until the
great devs on the Titan team had that refactored before I ported any
of it over to Glance.

I'm not a fan of XML, but I understand people use it. Now that we have
people asking for it, including XML will be more of a priority for us.
It just hasn't been up until now because few people were asking for
it. Now that there are a few asking for it, I'll try to bump the
blueprint up to get it into the D3 or D4 milestone.

Thanks for all your input, folks! As always, we're very happy to have
contributors help us with all the XML stuff (especially in regards to
testing it) over here in the Glance project.

Cheers!
jay


On Fri, Jun 3, 2011 at 7:51 AM, Andy Bierman <biermana at brocade.com> wrote:
>
> IMO, defining an API by providing 1 example of the API is not really interoperable.
> The point of defining a schema is to specify the 'contract' the API provider
> will honor, so a developer coding against the API knows what to write
> (for that particular message exchange).
>
> As for extensibility, YANG [RFC 6020] is better than both XSD and RelaxNG at that.
> XML namespaces may seem too heavyweight, but they solve extensibility
> at the instance level.  The JSON prefix solution doesn't seem nearly as robust.
>
>
> Andy
>
> -----Original Message-----
> From: openstack-bounces+biermana=brocade.com at lists.launchpad.net [mailto:openstack-bounces+biermana=brocade.com at lists.launchpad.net] On Behalf Of George Reese
> Sent: Friday, June 03, 2011 2:37 AM
> To: Mark Nottingham
> Cc: openstack at lists.launchpad.net
> Subject: Re: [Openstack] XML and JSON for API's
>
> A lot of the arguments against both XML and JSON seem to boil down to "here's why XML sucks..."
>
> The reality is, for whatever reason, good or bad, a lot of people prefer to parse complex data in XML. There's a big market for it. Supporting both formats doesn't add horrible complexity.
>
> -George
>
> On Jun 2, 2011, at 11:41 PM, Mark Nottingham wrote:
>
>> The problem I mentioned before, though, is that XML Schema brings more issues to the table than it solves.
>>
>> 1) People inevitably use schema to generate bindings to [insert language], and because of the complexity of the underlying data model of XML (Infoset), the mapping of information items to objects can happen in a variety of different ways. This is an endless source of bugs.
>>
>> 2) It's very, very hard to define an XML Schema that's reasonably extensible; unless you use exactly the right design patterns in your schema (which are absurdly convoluted, btw), you'll end up locking out future backwards-compatible changes. The authority in this space is Dave Orchard; see his conclusions at  <http://www.pacificspirit.com/Authoring/Compatibility/ProvidingCompatibleSchemaEvolution.html>.
>>
>> 3) An XML Schema can never express all of the constraints on the format. So, you'll still need to document those that aren't captured in the schema.
>>
>> I suppose the central question is what people are using the schema for. If it's just to document the format, that's great; we can have a discussion about how to do that. If they're using it for databinding, I'd suggest that JSON is far superior, as a separate databinding step isn't needed. Finally, if they're using it for runtime validation, I'd agree with Jay below; it's much easier to use json parse + runtime value checks for validation (especially in HTTP, where clients always have to be ready for errors anyway).
>>
>> Just my .02.
>>
>> Cheers,
>>
>>
>> On 03/06/2011, at 5:20 AM, Jorge Williams wrote:
>>
>>> It's not just about the service itself  validating it, its as Joseph said, making sure that the data structures themselves are documented in detail to the client.  To my knowledge there is no accepted schema language in JSON  though JSON schema is starting to catch on.
>>>
>>> At the end of the day it should be a matter of providing our customers with a representation that they can readily use.  It could be that my perception is wrong, but it seems to me that there's support for both representations.   I'll try to get some data to back this up.
>>>
>>> -jOrGe W.
>>>
>>>
>>> On Jun 2, 2011, at 2:00 PM, Jay Pipes wrote:
>>>
>>>> On Thu, Jun 2, 2011 at 1:54 PM, Rick Clark <rick at openstack.org> wrote:
>>>>> Hi All,
>>>>> Is it required for new openstack API's to support both JSON and
>>>>> XML, or would it be acceptable to only support JSON?
>>>>
>>>> Glance currently does not support XML and I have no plans in the
>>>> immediate future to add support for it.
>>>>
>>>> IMHO, JSON can be validated just as easily as XML. Simply
>>>> json.loads(req.body) and then, if parsing succeeds, compare the
>>>> mapping against a model. No need for XSDs, WADLs, or any other
>>>> acronym.
>>>>
>>>> -jay
>>>
>>>
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>>
>> --
>> Mark Nottingham   http://www.mnot.net/
>>
>>
>>
>>
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>
> --
> George Reese - Chief Technology Officer, enStratus
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> enStratus: Governance for Public, Private, and Hybrid Clouds - @enStratus - http://www.enstratus.com To schedule a meeting with me: http://tungle.me/GeorgeReese
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