[openstack-dev] [TripleO] Is Swift a good choice of database for the TripleO API?

Dan Prince dprince at redhat.com
Tue Dec 22 19:29:18 UTC 2015

On Tue, 2015-12-22 at 15:36 +0000, Dougal Matthews wrote:
> Hi all,
> This topic came up in the 2015-12-15 meeting[1], and again briefly
> today.
> After working with the code that came out of the deployment library
> spec[2] I
> had some concerns with how we are storing the templates.
> Simply put, when we are dealing with 100+ files from tripleo-heat-
> templates
> how can we ensure consistency in Swift without any atomicity or
> transactions.
> I think this is best explained with a couple of examples.
>  - When we create a new deployment plan (upload all the templates to
> swift)
>    how do we handle the case where there is an error? For example, if
> we are
>    uploading 10 files - what do we do if the 5th one fails for some
> reason?
>    There is a patch to do a manual rollback[3], but I have concerns
> about
>    doing this in Python. If Swift is completely inaccessible for a
> short
>    period the rollback wont work either.
>  - When deploying to Heat, we need to download all the YAML files
> from Swift.
>    This can take a couple of seconds. What happens if somebody starts
> to
>    upload a new version of the plan in the middle? We could end up
> trying to
>    deploy half old and half new files. We wouldn't have a consistent
> view of
>    the database.
> We had a few suggestions in the meeting:
>  - Add a locking mechanism. I would be concerned about deadlocks or
> having to
>    lock for the full duration of a deploy.
>  - Store the files in a tarball (so we only deal with one file). I
> think we
>    could still hit issues with multiple operations unless we
> guarantee that
>    only one instance of the API is ever running.

I may be partial to trying out the above solutions because I was in the
meeting. Before we write these off shouldn't we prototype them out

Anyways, I feel like the title of this thread is a bit loaded. Swift
may not be the best "database"... but if our goal is to use it for
object storage while we design set of workflows to help us deploy
things then I think it might fit in nicely.


> I think these could potentially be made to work, but at this point
> are we
> using the best tool for the job?
> For alternatives, I see a can think of a couple of options:
> - Use a database, like we did for Tuskar and most OpenStack API's do.
> - Invest time in building something on Swift.
> - Glance was transitioning to be a Artifact store. I don't know the
> status of
>   this or if it would meet out needs.
> Any input, ideas or suggestions would be great!
> Thanks,
> Dougal
> [1]: http://eavesdrop.openstack.org/meetings/tripleo/2015/tripleo.201
> 5-12-15-14.03.log.html#l-89
> [2]: https://specs.openstack.org/openstack/tripleo-specs/specs/mitaka
> /tripleo-overcloud-deployment-library.html
> [3]: https://review.openstack.org/#/c/257481/
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