[openstack-dev] [Glance][Glare] External locations design

Kairat Kushaev kkushaev at mirantis.com
Mon Aug 1 11:19:16 UTC 2016

Hello all,

I would like to start to describe some design decisions we made in Glare
code (https://review.openstack.org/#/q/topic:bp/glare-api+status:open).  If
you are not familiar with Glare I suggest you to read the following spec:


I hope it will help other folks to understand Glare approach and provide
some constructive feedback for Glare. I think that we can also use Glare
solution for Glance in near future to address some drawbacks we have in

Glare locations

Glance and Glare have possibility to set some external url as
image(artifact) location. This feature is quite useful for users who would
like to refer to some external image or artifact (for example, Fedora image
on official Fedora site) and not to store this image or artifact in the

External locations in Glance have several specialities:


   It is possible to setup multiple locations for an image. Glance uses
   special location strategy to define which location to use. This strategy
   defined in glance codebase and can be configured in glance conf.

   Glance doesn’t differ image locations specified by url and image
   locations uploaded to Glance backend. Glance has some restrictions about
   which urls to use for locations (see Glance docs for more info).

Glare external locations designed in different way to address some
drawbacks we have in Glance. So the approach is the following:


   Glare doesn’t support multiple locations, you can specify dict of blobs
   in artifact type and add url for each blob in dict. User must define a
   name(f.e. region name or priority) for blob in dict and this name can be
   used to retrieve this blob from artifact. So decision about which location
   to use will be outside of Glare.

   Glare adds a special flag to database for external locations. So they
   will be treated differently in Glare when delete artifact. If blob value is
   external url then we don’t need to pass this url to backend and just delete
   the record in DB. For now, Glare allows only http(s) locations set but it
   may be extended in future but the idea still the same: external location
   are just records in DB.

   Glare saves blob size and checksum when specifying external url. When
   user specified url Glare downloads the blob by url, calculates its size and
   checksum. Of course, it leads to some performance degradation but we can
   ensure that the external blob is immutable. We made this because security
   seems more important for Glare than performance. Also there are plans to
   extend this approach to support subscriptions for external locations so we
   can increase secureness of that operation.

I think that some of the features above can be implemented in Glance. For
example, we can treat our locations as only read-only links if external
flag will be implemented.  It will allow us to ensure that only blobs
uploaded through the Glance will be managed.

Additionally, if we will calculate checksum and size for external urls, we
can ensure that  all multiple locations refers to the same blob. So
management of multiple locations(deletion/creation) can be more secure.
Also we can ensure that the external url blob was not changed.

I understand that we need a spec for that but I would like to discuss this
at high level first. Here is etherpad for discussion:

Best regards,
Kairat Kushaev
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