Thanks Alex for your detailed inspection of my work. Comments inline..

On 3 February 2015 at 21:32, Alexandre Levine <>

>  I'm writing this in regard to several reviews concering tagging
> functionality for EC2 API in nova.
> The list of the reviews concerned is here:
> I don't think it's a good idea to merge these reviews. The analysis is
> below:
> *Tagging in AWS*
> Main goal for the tagging functionality in AWS is to be able to
> efficiently distinguish various resources based on user-defined criteria:
> "Tags enable you to categorize your AWS resources in different ways, for
> example, by purpose, owner, or environment.
> ...
> You can search and filter the resources based on the tags you add."
> (quoted from here:
> It means that one of the two main use-cases is to be able to use Tags as
> filter when you describe something. Another one is to be able to get
> information about particular tag with all of the resources tagged by it.
> Also there is a constraint:
> "You can tag public or shared resources, but the tags you assign are
> available only to your AWS account and not to the other accounts sharing
> the resource."
> The important part here is "shared resources" which are visible to
> different users but tags are not shared - each user sees his own.
> *Existing implementation in nova *Existing implementation of tags in
> nova's EC2 API covers only instances. But it does so in both areas:
> 1. Tags management (create, delete, describe,...)
> 2. Instances filtering (describe_instances with filtering by tags).
> The implementation is based on storing tags in each instance's metadata.
> And nova DB sqlalchemy level uses "tag:" in queries to allow instances
> describing with tag filters.
> I see the following design flaws in existing implementation:
> 1. It uses instance's own metadata for storing information about assigned
> tags.
> Problems:
> - it doesn't scale when you want to start using tags for other resources.
> Following this design decision you'll have to store tags in other resources
> metadata, which mean different services APIs and other databases. So
> performance for searching for tags or tagged resources in main use cases
> should suffer. You'll have to search through several remote APIs, querying
> different metadatas to collect all info and then to compile the result.
> - instances are not shared resources, but images are. It means that, when
> developed, metadata for images will have to store different tags for
> different users somehow.
> 2. EC2-specific code ("tag:" searching in novaDB sqlalchemy) leaked into
> lower layers of nova.
> - layering is violated. There should be no EC2-specifics below EC2 API
> library in nova, ideally.
All of the Nova-EC2 mapping happens in Nova's DB currently. See
InstanceIdMapping model in nova/db/sqlalchemy/ EC2 API which
resides in Nova will keep using the Nova database as long as it is

> - each other service will have to implement the same solution in its own
> DB level to support tagging for EC2 API.
> *Proposed review changes*
> The review in question introduces tagging for volumes and snapshots. It
> follows design decisions of existing instance tagging implementation, but
> realizes only one of the two use cases. It provides "create", "delete",
> "describe" for tags. But it doesn't provide describe_volumes or
> describe_snapshots for filtering.
I honestly forgot about those two methods. I can implement them.

> It suffers from the design flaws I listed above. It has to query remote
> API (cinder) for metadata. It didn't implement filtering by "tag:" in
> cinder DB level so we don't see implementation of describe_volumes with
> tags filtering.
Cinder do support filtering based on tags, and I marked the work as TODO in . This was
not the reason why I didn't implement describe_volumes and
describe_snapshots. Those two methods just missed my attention :)

Nova's EC2 API's tag filtering is also done in-memory presently if I'm
correct, as Nova's API doesn't support filtering only on the basis of tag
names or tag values alone..

> *Current stackforge/ec2-api tagging implementation*
> In comparison, the implementation of tagging in stackforge/ec2-api, stores
> all of the tags and their links to resources and users in a separate place.
> So we can efficiently list tags and its resources or filter by tags during
> describing of some of the resources. Also user-specific tagging is
> supported.
> *Conclusion *Keeping in mind all of the above, and seeing your discussion
> about deprecation of EC2 API in nova, I don't feel it's a good time to add
> such a half-baked code with some potential problems into nova.
> I think it's better to concentrate on cleaning up, fixing, reviving and
> making bullet-proof whatever functionality is currently present in nova for
> EC2 and used by clients.
EC2 API already shares database with Nova's, so the tight coupling between
EC2 API and Nova's database is not going to go away till the time EC2 API
server/controller is present in Nova. Nova instance metadata is being used
as EC2 instance tags, and what the above-referenced spec is doing is is
very similar: Cinder volume metadata is being used as EC2 volume tags, and
similarly for volume snapshots. I don't see a difference between instances
and volumes and volume snapshots in the sense that they all are
non-share-able (yet).

I completely understand that these patches look like feature additions. I
started working on them first in January 2014 ( ), and at that time it was just a
sincere effort to improve EC2 API using the first possible way I could find
out. Since we have not deprecated the in-Nova EC2 support yet, and we are
yet to reach a concrete plan to move forward, I am tempted to ask for
allowing this patch to be considered for review..

I am fine if people think these patches shouldn't be allowed to go in. I
can only wish that the patches got more attention when it was possible to
get them merged :)

Rushi Agrawal

> Best regards,
>   Alex Levine
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