Rushi,
Thank you for the response. I totally understand the effort and your
problems with getting it through at the time. Your design is completely
inline with what's currently present in Nova for EC2, no doubt about
that. I did whatever I could to review your patches and consider if it's
worth to go forward with them in current circumstances. I believe it'll
add more complications than value if we go on with them.
The main design problems, introduced before you, won't go away:
1. User isolation for shared objects is close to impossible to implement
in this model.
2. Marked resources listing when describing tags, will require going to
all of the possible different APIs and their databases eventually and
then compiling the result.
The stackforge/ec2-api implementation fortunately had no constraints or
previously implemented code with some conceptual problems. So it could
and did store the tags and their associations with resources separately.
It allowed efficient searching for both describing tags and resources.
Strategically if, as I understand, eventual aim is to switch to separate
ec2-api solution, it makes little sense (to me) to add more
functionality, especially partial functionality (no describe_volumes,
describe_snapshots and even if added, no tagging for other resources),
to current nova code. If the decision was made to enhance nova with new
features like this, I'd still be for a separate table in DB for all of
the tags and their associations - it would've made universal, complete
and efficient solution with one effort.
And again, I more than agree with this:
"
...I can only wish that the patches got more attention when it was
possible to get them merged :)
"
But that's a different story.
Best regards,
Alex Levine
On 2/4/15 4:32 PM, Rushi Agrawal wrote:
Thanks Alex for your detailed inspection of my work. Comments inline..
On 3 February 2015 at 21:32, Alexandre Levine
<alev...@cloudscaling.com <mailto:alev...@cloudscaling.com>> wrote:
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:
https://review.openstack.org/#/q/status:open+project:openstack/nova+branch:master+topic:bp/ec2-volume-and-snapshot-tags,n,z
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:
http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/Using_Tags.html)
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/models.py. EC2 API which
resides in Nova will keep using the Nova database as long as it is
functional.
- 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
https://review.openstack.org/#/c/112325/23/nova/volume/cinder.py .
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 (
https://review.openstack.org/#/c/64690/ ), 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 :)
Regards,
Rushi Agrawal
Best regards,
Alex Levine
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