[openstack-dev] [heat] [glance] Heater Proposal

Mark Washenberger mark.washenberger at markwash.net
Fri Dec 6 16:57:11 UTC 2013

On Thu, Dec 5, 2013 at 9:32 PM, Jay Pipes <jaypipes at gmail.com> wrote:

> On 12/05/2013 04:25 PM, Clint Byrum wrote:
>> Excerpts from Andrew Plunk's message of 2013-12-05 12:42:49 -0800:
>>> Excerpts from Randall Burt's message of 2013-12-05 09:05:44 -0800:
>>>>> On Dec 5, 2013, at 10:10 AM, Clint Byrum <clint at fewbar.com>
>>>>>   wrote:
>>>>>  Excerpts from Monty Taylor's message of 2013-12-04 17:54:45 -0800:
>>>>>>> Why not just use glance?
>>>>>> I've asked that question a few times, and I think I can collate the
>>>>>> responses I've received below. I think enhancing glance to do these
>>>>>> things is on the table:
>>>>>> 1. Glance is for big blobs of data not tiny templates.
>>>>>> 2. Versioning of a single resource is desired.
>>>>>> 3. Tagging/classifying/listing/sorting
>>>>>> 4. Glance is designed to expose the uploaded blobs to nova, not users
>>>>>> My responses:
>>>>>> 1: Irrelevant. Smaller things will fit in it just fine.
>>>>> Fitting is one thing, optimizations around particular assumptions
>>>>> about the size of data and the frequency of reads/writes might be an issue,
>>>>> but I admit to ignorance about those details in Glance.
>>>> Optimizations can be improved for various use cases. The design,
>>>> however,
>>>> has no assumptions that I know about that would invalidate storing blobs
>>>> of yaml/json vs. blobs of kernel/qcow2/raw image.
>>> I think we are getting out into the weeds a little bit here. It is
>>> important to think about these apis in terms of what they actually do,
>>> before the decision of combining them or not can be made.
>>> I think of HeatR as a template storage service, it provides extra data
>>> and operations on templates. HeatR should not care about how those
>>> templates are stored.
>>> Glance is an image storage service, it provides extra data and
>>> operations on images (not blobs), and it happens to use swift as a backend.
>>> If HeatR and Glance were combined, it would result in taking two very
>>> different types of data (template metadata vs image metadata) and mashing
>>> them into one service. How would adding the complexity of HeatR benefit
>>> Glance, when they are dealing with conceptually two very different types of
>>> data? For instance, should a template ever care about the field "minRam"
>>> that is stored with an image? Combining them adds a huge development
>>> complexity with a very small operations payoff, and so Openstack is already
>>> so operationally complex that HeatR as a separate service would be
>>> knowledgeable. Only clients of Heat will ever care about data and
>>> operations on templates, so I move that HeatR becomes it's own service, or
>>> becomes part of Heat.
>> I spoke at length via G+ with Randall and Tim about this earlier today.
>> I think I understand the impetus for all of this a little better now.
>> Basically what I'm suggesting is that Glance is only narrow in scope
>> because that was the only object that OpenStack needed a catalog for
>> before now.
>> However, the overlap between a catalog of images and a catalog of
>> templates is quite comprehensive. The individual fields that matter to
>> images are different than the ones that matter to templates, but that
>> is a really minor detail isn't it?
>> I would suggest that Glance be slightly expanded in scope to be an
>> object catalog. Each object type can have its own set of fields that
>> matter to it.
>> This doesn't have to be a minor change to glance to still have many
>> advantages over writing something from scratch and asking people to
>> deploy another service that is 99% the same as Glance.
> My suggestion for long-term architecture would be to use Murano for
> catalog/metadata information (for images/templates/whatever) and move the
> block-streaming drivers into Cinder, and get rid of the Glance project
> entirely. Murano would then become the catalog/registry of objects in the
> OpenStack world, Cinder would be the thing that manages and streams blocks
> of data or block devices, and Glance could go away. Imagine it... OpenStack
> actually *reducing* the number of projects instead of expanding! :)

I think it is good to mention the idea of shrinking the overall OpenStack
code base. The fact that the best code offers a lot of features without a
hugely expanded codebase often seems forgotten--perhaps because it is
somewhat incompatible with our low-barrier-to-entry model of development.

However, as a mild defense of Glance's place in the OpenStack ecosystem,
I'm not sure yet that a general catalog/metadata service would be a proper
replacement. There are two key distinctions between Glance and a
catalog/metadata service. One is that Glance *owns* the reference to the
underlying data--meaning Glance can control the consistency of its
references. I.e. you should not be able to delete the image data out from
underneath Glance while the Image entry exists, in order to avoid a
terrible user experience. Two is that Glance understands and coordinates
the meaning and relationships of Image metadata. Without these
distinctions, I'm not sure we need any OpenStack project at all--we should
probably just publish an LDAP schema for Images/Templates/what-have-you and
use OpenLDAP.

To clarify, I think these functions are critical to Glance's role as a
gatekeeper and helper, especially in public clouds--but having this role in
your deployment is probably something that should ultimately become
optional. Perhaps Glance should not be in the required path for all
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