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

Ben Nemec openstack at nemebean.com
Tue Dec 22 17:59:55 UTC 2015

Can we just do git like I've been suggesting all along? ;-)

More serious discussion inline. :-)

On 12/22/2015 09:36 AM, 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.

There should be no need to lock the plan for the entire deploy.  It's
not like we're re-reading the templates at the end of the deploy today.
 It's a one-shot read and then the plan could be unlocked, at least as
far as I know.

The only option where we wouldn't need locking at all is the
read-copy-update model Clint mentions, which might be a valid option as
well.  Whatever we do, there are going to be concurrency issues though.
 For example, what happens if two users try to make updates to the plan
at the same time?  If you don't either merge the changes or disallow one
of them completely then one user's changes might be lost.

TBH, this is further convincing me that we should just make this git
backed and let git handle the merging and conflict resolution (never
mind the fact that it gets us a well-understood version control system
for "free").  For updates that don't conflict with other changes, git
can merge them automatically, but for merge conflicts you just return a
rebase error to the user and make them resolve it.  I have a feeling
this is the behavior we'll converge on eventually anyway, and rather
than reimplement git, let's just use the real thing.

/2 cents

>  - 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 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.

I kind of like this, in particular because it would allow us to handle
migrations between versions the same way as other OpenStack services.
I'm not entirely sure how it maps to our template configuration method
though.  Storing a bunch of template blobs in the database feels a
little square peg, round hole to me, and might undo a lot of the
benefits of using the database in the first place.

Now, on the other hand, having a database to store basic data like
metadata on plans and such might be a useful thing regardless of where
we store the templates themselves.  We could also use it for locking
just fine IMHO - TripleO isn't a tool targeted to have cloud-scale
number of users.  It's a deployer tool with a relatively limited number
of users per installation, so the scaling of a locking mechanism isn't a
problem I necessarily think we need to solve.  We have way bigger
scaling issues than that to tackle before I think we would hit the limit
of a database-locking scheme.

> - 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.2015-12-15-14.03.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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