EC2: add support for volume and snapshot tags

Implements blueprint ec2-volume-and-snapshot-tags
Previously-approved: juno

Change-Id: I1891d0f2a7b9189441943445507ab87c035f3108

specs/kilo/approved/ec2-volume-and-snapshot-tags.rst

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+..
+ This work is licensed under a Creative Commons Attribution 3.0 Unported
+ License.
+
+ http://creativecommons.org/licenses/by/3.0/legalcode
+
+========================================================
+Tags support in EC2 API for volumes and volume snapshots
+========================================================
+
+https://blueprints.launchpad.net/nova/+spec/ec2-volume-and-snapshot-tags
+
+Expose volume and volume snapshot metadata as EC2 tags in the EC2 API.
+
+Problem description
+===================
+
+OpenStack's EC2 API has little support for 'tags' (resource metadata).
+Only instance metadata are exposed in the EC2 API, so a user
+can create, delete and list only instance metadata. OpenStack Cinder API
+has support for metadata as well, for both volumes and volume snapshots,
+and we just need to expose it into the EC2 API. This blueprint aims to
+do just that.
+
+Use Cases
+---------
+
+As metadata feature is already provided in OpenStack APIs, the end users will
+now be able to use tags for volumes and volume snapshots. This will increase
+the parity between OpenStack and EC2 APIs.
+
+Project Priority
+----------------
+
+None
+
+Proposed change
+===============
+
+* EC2 API's 'CreateTags' method only used to work when one is creating
+  tags for an instance resource. After this patch, one will be also
+  able to create tags for volume and volume snapshot resources.
+
+* A user will be able to call the 'DeleteTags' API to delete any tag
+  associated with a volume or a volume snapshot.
+
+* While calling the 'DescribeTags' API, tags of volumes and volume
+  snapshots will be listed along with instance tags (provided tags
+  for these resources are present, obviously).
+
+* Support for specifying volume and volume snapshot IDs, and 'volume'
+  and 'snapshot' as resources as parameters while calling 'DescribeTags'
+  is added.
+
+* As this is the first time the supported resources for tags are becoming
+  plural in number, the code is made more generic so as to allow addition
+  of further resources easier.
+
+* Implementation detail: In the DescribeInstances API, user can specify
+  both resource ID and resource type as filters. If the query says filter
+  by resource IDs (vol-00000001 and ami-00000001) and also filter by
+  resource type (instances and volumes), the current implementation takes
+  the intersection of the resources (volumes in this case) and then checks
+  if those resources are implemented.
+
+Alternatives
+------------
+
+Alternative is: EC2 tags be different from volume tags by using scoped keys.
+So a user creating a tag stack=beta in EC2 API will, in the Cinder API, see
+it as EC2:stack=beta. This way, a user using the Cinder APIs will be able
+to clearly see which metadata entries are created using EC2 API and which
+are created by the OpenStack API.
+
+I think it makes sense to keep the EC2 API layer as transparent as possible.
+This means not going with the alternative proposed above. This also falls in
+line with what we have presently for instance metadata.
+
+Regarding the implementation detail specified above, an alternative is:
+Do not allow resource IDs and resource type to be specified in the same
+query.
+
+There is no doc of AWS which says such an API call is not allowed (atleast I
+can't find it). This implementation is easier, but IMO the way in which
+it is implemented right now gives a better user experience. Probably logging
+would help if we are dropping out a resource ID or resource type from the
+query.
+
+
+Data model impact
+-----------------
+
+None
+
+REST API impact
+---------------
+
+Only EC2 API will be affected. Affected API calls are: CreateTags, DeleteTags
+DescribeTags.
+
+Security impact
+---------------
+
+None
+
+Notifications impact
+--------------------
+
+None
+
+Other end user impact
+---------------------
+
+None
+
+Performance Impact
+------------------
+
+Insignificant. Note that in the case of DescribeTags, as we keep on adding
+resources, an API call will be made to all of them (e.g. Glance, Cinder, etc)
+when a DescribeTags call is made without specifying a resource type.
+
+Other deployer impact
+---------------------
+
+None
+
+Developer impact
+----------------
+
+None
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  rushiagr (Rushi Agrawal)
+
+Work Items
+----------
+
+* Reorganize EC2 'tags' code to allow addition of new resources
+* Add support for EC2 tags for volumes
+* Add support for EC2 tags for volume snapshots
+
+
+Dependencies
+============
+
+None
+
+
+Testing
+=======
+
+Comprehensive unit tests to test the functionality will be written.
+
+Documentation Impact
+====================
+
+EC2 API document should be updated to reflect the changes done to the EC2 API
+under this blueprint.
+
+
+References
+==========
+
+EC2 API reference:
+* CreateTags http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-query-CreateTags.html
+* DeleteTags http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-query-DeleteTags.html
+* DescribeTags http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-query-DescribeTags.html