Add community-level image sharing

Add community-level image sharing

This spec is mostly based on an earlier blueprint created by Iccha Sethi, and later picked up by Louis Taylor. We at Symantec have already implemented a similar function within our local environment. We want to re-target this feature to Newton release.

The blueprint may be found at: https://blueprints.launchpad.net/glance/+spec/community-level-v2-image-sharing

This feature will allow image owners to share images across multiple tenants/projects without explicitly creating members individually through the glance API V2. “Community images” will not appear in users’ default image listings.

This new feature adds a new value for the visibility of an image to the underlying data model, named community.

Problem description

Currently, there is no provision for an image to be available for use by users other than the owner of the image, unless the image is either made public or explicitly shared by the owner of the image with other users. If the number of users who require access to the image is large, the overhead of explicit sharing can become a burden on the owner of the image. Also, if such images are not needed by the majority of users, then the image listing results will be spammed if they are all made public.

Public images appear in image-list for all users. This can be undesirable because:

  1. An open source project wants to make a prepared VM image available so users simply boot an instance from that image to use the project’s software. The developers of the project are too busy writing software to worry about the hassle of maintaining a list of “customers” as they’d have to do with current v2 image sharing. At the same time, the cloud provider hosting the prepared image doesn’t want to make this image public, as that would imply a support relationship that doesn’t exist between image consumers and the cloud provider. Moreover, if too many such images get published, then image listing will be spammed as this type of image should not be seen by most of the users.
  2. A cloud provider wants to discourage users from using a public image that is getting old. For example, an image which is missing some security patches or is no longer supported by the vendor. The cloud provider doesn’t want to delete the image because some users still require it. Users could need the old image to rebuild a server, or because they have custom patches for that particular image.

In both cases, the vendor can make the image a “community” image and alleviate the challenges presented above. This means that:

a) The community image won’t appear in users’ default image lists. This means they won’t know about it unless they are motivated to seek it out, for example by asking other users for the UUID.

b) Since the image is no longer “public,” it doesn’t imply the same level of support from the vendor as a provider-supplied public image.

Proposed change

An additional value for the visibility enum will be added in the JSON schema, named 'community'. This makes the possible values of visibility:

['public', 'private', 'shared', 'community']

Images with these visibility values have the following properties:

  • public:
    • Who: all users:
      • have this image in the default image-list
      • can see image-detail for this image
      • can boot from this image
  • private:
    • Who: users with tenant_id == owner_tenant_id only:
      • have this image in the default image-list
      • see image-detail for this image
      • can boot from this image
  • shared:
    • Who: users with tenant_id == owner_tenant_id:
      • have this image in the default image-list
      • can see image-detail for this image
      • can boot from this image
      • can see full member list for this image
    • Who: users with tenant_id in the member-list with member_status == 'accepted' of the image:
      • have this image in the default image-list
      • can see image-detail for this image
      • can boot from this image
      • can see a member list containing only themselves for this image
    • Who: users with tenantId in the member-list with member_status == 'pending' or member_status == 'rejected':
      • do not have this image in their default image-list
      • can see image-detail for this image
      • can boot from this image
      • can see a member list containing only themselves for this image
  • community:
    • Who: users with tenant_id == owner_tenant_id:
      • have this image in the default image-list
      • can see image-detail for this image
      • can boot from this image
    • Who: all users:
      • do not have this image in their default image-list
      • can see image-detail for this image
      • can boot from this image

Membership Behaviour and Transitions

After much discussion, it has been decided that this topic is complex enough to break out into its own separate proposal. Between the significant number of state transitions, and the problem use cases brought up by several people, fully designing what the “correct” behaviour shall be is outside the scope of this spec.

The behaviour for the initial implementation for community images shall respect the current paradigm in glance - where changing visiblities does not affect an image’s internal member-list. To be precise, consistent with current behavior:

  • An image’s member-list persists even when the visibility of an image has changed to a value that renders the member-list inert.
  • Image member operations (that is, creating image members or changing the member_status of image members) are allowed only when an image has a visibility under which the member-list is not inert. The justification for this is that it is confusing to users to be able to perform member operations on an image when these operations have no effect on the accessibility of the image.

Alternatives

Adding image aliases

A completely different way of solving the usecase for cloud providers (discouraging users from using an older version of a public image) could be to create a mechanism to make an image alias, which could point at the newest version of the public image. There is an abandoned blueprint for this feature [1]. This, however, is much harder to implement and does not fit with the other use cases.

[1]https://blueprints.launchpad.net/glance/+spec/glance-image-aliases

Adding a special case of image sharing

Another method of implementing this functionality is to add a membership record for an image that has a target of "community" (i.e. it is shared with all tenants) with membership_status = "community". This marks it as a community image very simply and requires few modifications to existing code.

This respects the current anti-spam provisions in the glance v2 API. When an image owner makes an image a “community” image, any other tenant should be able to boot an instance from that image. The image will not show up in any tenant’s default image-list.

This method can cause a few corner cases which result in surprising API calls and some less than desirable mappings between data model level and API level values of visibility.

Data model impact

Schema changes

The visibility of the image will be stored in the database within the images table inside a new column named visibility. The visibility will be in the set of ['public', 'private', 'shared', 'community'].

The default value for visibility is shared. (See Other end user impact for a discussion.)

This change makes the is_public column redundant. If no v1 code actually uses is_public, the column will be removed.

Appropriate indexes will be added to facilitate quick responses.

Database migrations

(Note: this is a statement of expected outcomes, not an algorithm for performing the migration.)

  1. All rows with is_public == 1:

    • visibility = public
  2. For all unique image_id in image_members where deleted != 1 and for which is_public == 0 for that image:

    • visibility = shared
  3. For all other images:

    • visibility = private

    (See Other end user impact for further discussion.)

REST API impact

The changes described in this document will require an API version bump.

Image discovery

If you want to list all community images, and only community images, then you would use:

GET /v2/images?visibility=community

All other appropriate filters will be respected. Of note is the use of an owner parameter. This, when supplied together with the visibility=community filter, allows a user to request only those community images owned by that particular tenant:

GET /v2/images?visibility=community&owner={owner_tenant_id}

Note that visiblity will be considered a core property of the image object, and as such included within image lists generated via the v2 interface.

Making an image a “community image”

As permitted by the new policy.json rule communitize_images, an admin or owner would use the existing image-update call to change an image’s visiblity to 'community':

PATCH /v2/images/{image_id}

Request body:

[{ "op": "replace", "path": "/visibility", "value": "community" }]

The response and other behaviour remains the same as was previously defined for this call.

Removing community level access from an image

An admin or owner of an image can remove community-level access from an image by using the image-update call. For example, instead of setting it to 'community' as before, we set it to 'private':

PATCH /v2/images/{image_id}

Request body:

[{ "op": "replace", "path": "/visibility", "value": "private" }]

An admin or user with permission to publicize an image could replace community visibility with public.

As in the above case, the response and other behaviour remains the same as was previously defined for this call.

Security impact

See “other deployer impact”.

Notifications impact

Current notifications contain the is_public attribute, which is true if and only if the image is a public image. This must be maintained for backwards compatibility.

An additional attribute of visibility will be added for each image to indicate its visiblity, with possible values of ['public', 'private', 'shared', 'community'].

Other end user impact

Consistent with current functionality, when a visibility is specified at the time of image creation, that visibility will be respected. If the user creating the image does not have appropriate permissions to set the specified visibility value on an image, the image-create call will fail as it does now.

Default visibility

In order to preserve backward compatibility with the current workflow for sharing an image, the default visibility of an image will be shared:

  • An image with shared visibility and no image members is not accessible to anyone other than the image owner, so this respects the current behavior.
  • Currently, when an image is created with default visibility, the owner can immediately add members to the image to share it. By making the default visibility value shared, we preserve this behavior. If the default visibility were private, then this behavior would be broken as the image owner would have to make an API call to change the visibility of the image to shared before members could be added.
  • Although the Image API v1 is deprecated in Newton, it still exists and will likely be deployed in clouds using the Ocata release of OpenStack. Since the v1 API has no concept of visibility, there’s no way for a user to change the visibility to shared so that an image can have members added to it. By making the default visibility shared, we achieve the following desirable results:
    • A v1 user can create an image and add members using the current workflow, that is, sharing an image is simply a matter of putting a member on it.
    • In deployments using both v1 and v2 of the Image API, if an image is created using v1 and then an image-show call is made in v2, the visibility of shared makes it clear to the v2 user that the image is in a state where it can both accept members and be accessed by any existing members.

If the default visibility value were not shared, then v1 would need to be modified so that private images could accept members and be accessed by members. This, however, would create a situation where an image would behave radically differently depending upon whether you tried to access it by the v1 or the v2 API. We don’t want to do that.

A key question here is whether changing the default visibility from private to shared introduces a backward incompatibility into the v2 API. Here are three arguments that it does not:

  1. The current situation is that the visibility of an image created by a non-admin user has visibility != public. That situation is preserved.
  2. If we don’t make the default visibility shared, then we are in fact introducing a backward incompatibility in that any current user scripts that create an image and then immediately add members to it will break. (This was in fact a problem with an earlier version of this spec. While the API working group agreed that it was a “minor” backward incompatibility that would be permissible, they pointed out that Glance users were likely to be annoyed by the workflow change. The change in visibility value is a lesser of two evils.)
  3. What we want to be the case is that when a user creates an image without specifying a visibility, the image behaves the same way it does now. Images that behave in that way have the visibility that we now call shared. This is to be expected; previously, we had insufficient keywords to identify the various accessibility statuses of images; now we do, and we should use these keywords appropriately.

The conclusion is that setting the default visibility to private would have a greater end user impact than the proposal to make the default visibility value shared.

Note that if the v2 API is used to set the visibility of an image to private, if the v1 API is used to share the image, the request will fail with a 409 (as it will if a member-related call is made to the v2 API). This is appropriate, as someone has explicitly disabled sharing for that image.

One final point implicit in the foregoing is worth stating explicitly. If the v1 API is used to update the is_public property of an image, it will have the following effect upon the image’s visibility:

  • If the call updates is_public to True, then the corresponding v2 visibility of the image will be public.
  • If the call sets is_public to False, then the corresponding v2 visibility of the image will be set to the default value, that is, shared.

This will preserve backward-compatible behavior in the v1 API. The arguments advanced previously in favor of shared being the default visibility apply to this case as well.

Migration of the visibility of existing images

The issue of how the database migration should work was sufficiently controversial that an operators’ survey was taken to gather data [SUR1]. The results of the survey were reported in [SUR2] (along with a recommendation for the migration path that, as you’ll see if you read on, was ultimately rejected). Briefly, operators were evenly split on whether images with visibility private should all be migrated to shared or whether only those images with members should be migrated to shared.

We’ve had a lot of discussion on this (see the comments on [GER1] for a sample). In the end, the compromise we reached is reflected in this spec. The primary justification for this migration path is that it’s worth trading some minor incompatibilities between pre-Ocata and Ocata image sharing in order to avoid end user confusion when they first use Glance after a deployment has been upgraded to Ocata and suddenly see that all the images they own now have a visibility of ‘shared’.

Thus, the end user impact of an upgrade to the Glance Ocata release will be the following:

  • Any existing “private” image with a non-empty member-list will be found to have visibility ‘shared’.

  • Any existing “private” image that has no image members will be found to have visibility ‘private’.

    • Incompatibility: v2: Unlike pre-Ocata, the visibility of the image must be changed to ‘shared’ before the member-create call will succeed. v1: Users must explicitly set is_public to False before the member-create call will succeed.
  • Any images created after the upgrade for which a visibility is not specified at the time of image creation will have visibility ‘shared’.

    • Inconsistency: The member-create call will immediately work for newly created images with default visibility, whereas migrated images will have to be dealt with as described above.
  • Pre-Ocata, an end user had “private” images that might actually be shared. The user had to do an API call to see the member-list to determine whether other users could access that image. In Ocata, an end user will have images with visibility shared that may not actually be accessible to other users. The user must do an API call to see whether the image has a non-empty member list.

    • Incompatibility: The workflow required to determine whether an image is accessible to other users is roughly the same, but it now depends upon two aspects: (1) the visibility must be ‘shared’, and (2) the image’s member-list must be non-empty.

      The upside to this incompatibility is that in the Glance Ocata release, a visibility value of private will actually mean “private” instead of “private or shared”.

[SUR1]http://lists.openstack.org/pipermail/openstack-operators/2016-November/012107.html
[SUR2]http://lists.openstack.org/pipermail/openstack-operators/2016-December/012235.html
[GER1]https://review.openstack.org/#/c/396919/

Client changes

OpenStackClient, as well as the library portion of python-glancelient, will be updated to expose this feature. The CLI glanceclient will not be supported.

Users will be able to see all community images by using openstack image list --community.

An option to openstack image set will be added named --visibility <VISIBILITY_STATUS>, where VISIBILTY_STATUS may be one of {public, private, shared, community}.

For example, to make an image a community image:

$ openstack image set --visibility community <IMAGE>

To make the image private again:

$ openstack image set --visibility private <IMAGE>

Performance Impact

None

Other deployer impact

The ability to create community images is moderated using policy.json. As mentioned above, a new rule will be created called communitize_image, which will have the default configuration of [role:admin or rule:owner].

Also users from Horizon will be able to see community images through a separate tab. Details regarding the Horizon feature can be found at: https://blueprints.launchpad.net/horizon/+spec/glance-community-images

Developer impact

None

Implementation

Assignee(s)

Primary assignee:
timothy-symanczyk

Reviewers

Core reviewer(s):
brian-rosmaita nikhil-komawar

Work Items

  • Refactor db API to use visibility rather than is_public
  • Add functionality for storing the community state in the interfaces to both db backends:
    • sqlalchemy
    • simple
  • Add functionality to enable this and accept the image using the API
  • Add unit tests to test various inputs to the API
  • Add functional tests for the lifecycle of community images
  • Update OpenStackClient to use the new API functionality
  • Bump the API version

Dependencies

None

Testing

A tempest test must be added to cover creating a community image and it transitioning between public and private states.

Documentation Impact

New features must be documented in both glance and OpenStackClient.

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