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On Wed, May 20, 2015 at 5:23 PM, Zhi Yan Liu <lzy.dev at gmail.com> wrote:
> On Wed, May 20, 2015 at 5:06 PM, Daniel P. Berrange <berrange at redhat.com> wrote:
>> On Wed, May 20, 2015 at 12:01:37AM +0200, Flavio Percoco wrote:
>>> On 19/05/15 17:19 +0100, Daniel P. Berrange wrote:
>>> >In Nova we are attempting to model[1] the glance image metadata and
>>> >properties using the Nova object model (now oslo.versionedobjects).
>>> >
>>> >The one item I'm stuck on understanding is the 'locations' field
>>> >and more specifically the 'metadata' element in each location
>>> >entry
>>> >
>>> >
>>> >In the file glance/api/v2/images.py I can see this description
>>> >of the data format:
>>> >
>>> >       'locations': {
>>> >           'type': 'array',
>>> >           'items': {
>>> >               'type': 'object',
>>> >               'properties': {
>>> >                   'url': {
>>> >                       'type': 'string',
>>> >                       'maxLength': 255,
>>> >                   },
>>> >                   'metadata': {
>>> >                       'type': 'object',
>>> >                   },
>>> >               },
>>> >               'required': ['url', 'metadata'],
>>> >           },
>>> >           'description': _('A set of URLs to access the image file kept in '
>>> >                            'external store'),
>>> >
>>> >
>>> >As you can see here, 'metadata' is just said to be of type 'object'.
>>> >
>>> >Is there somewhere that actually describes what is valid contents
>>> >for this field ? Is it sufficient to assume the metadata will only
>>> >ever be a dict of strings, or can the metadata be a complex type
>>> >with arbitrarily nested data structures ?
>>>
>>> It's just arbitrary metadata for now, we don't have a specific format.
>>> I'm curious to know if there are folks using this field. We do (did)
>>> have a use case for it.
>>
>> Yep, I'd be curious to understand just what it is used for in practice ?
>> Is the data to be stored in there determined by python code, or by the
>> local administrator or both ?
>>
>>
>> Regards,
>> Daniel
>> --
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>
>
> Yes, It determined by python code in nova as a part of image download
> plugin, and administrator needs to prepare it based on particular
> deployment environment as well. Current a usage is to accelerate image
> download from nfs store to nova compute node, now there is only one
> particular plugin in nova upstream tree [0]. (from the logic in
> _file_system_lookup(), i think a predefined 'id' is needed in the
> metadata of the location entry).
>
> [0] https://github.com/openstack/nova/blob/master/nova/image/download/file.py#L150
>
> zhiyan

Btw, for your question:

>>> >Is there somewhere that actually describes what is valid contents
>>> >for this field ? Is it sufficient to assume the metadata will only
>>> >ever be a dict of strings, or can the metadata be a complex type
>>> >with arbitrarily nested data structures ?

for current nova in-tree image download plugin (above [0]), the schema
of location metadata should be this:
https://github.com/openstack/glance_store/blob/master/glance_store/_drivers/filesystem.py#L72

zhiyan