[openstack-dev] [TripleO] Should we have a TripleO API, or simply use Mistral?

Dan Prince dprince at redhat.com
Sat Jan 23 00:19:07 UTC 2016

On Fri, 2016-01-22 at 11:24 -0600, Ben Nemec wrote:
> So I haven't weighed in on this yet, in part because I was on
> vacation
> when it was first proposed and missed a lot of the initial
> discussion,
> and also because I wanted to take some time to order my thoughts on
> it.
>  Also because my initial reaction...was not conducive to calm and
> rational discussion. ;-)
> The tldr is that I don't like it.  To explain why, I'm going to make
> a
> list (everyone loves lists, right? Top $NUMBER reasons we should stop
> expecting other people to write our API for us):
> 1) We've been down this road before.  Except last time it was with
> Heat.
>  I'm being somewhat tongue-in-cheek here, but expecting a general
> service to provide us a user-friendly API for our specific use case
> just
> doesn't make sense to me.

We've been down this road with Heat yes. But we are currently using
Heat for some things that we arguable should be (a workflows tool might
help offload some stuff out of Heat). Also we haven't implemented
custom Heat resources for TripleO either. There are mixed opinions on
this but plugging in your code to a generic API is quite nice

That is the beauty of Mistral I think. Unlike Heat it actually
encourages you to customize it with custom Python actions. Anything we
want in tripleo-common can become our own Mistral action (these get
registered with stevedore entry points so we'd own the code) and the
YAML workflows just tie them together via tasks.

We don't have to go off and build our own proxy deployment workflow
API. The structure to do just about anything we need already exists so
why not go and use it?

> 2) The TripleO API is not a workflow API.  I also largely missed this
> discussion, but the TripleO API is a _Deployment_ API.  In some cases
> there also happens to be a workflow going on behind the scenes, but
> honestly that's not something I want our users to have to care about.

Agree that users don't have to care about this.

Users can get as involved as they want here. Most users I think will
use python-tripleoclient to drive the deployment or the new UI. They
don't have to interact with Mistral directly unless they really want
to. So whether we choose to build our own API or use a generic one I
think this point is mute.

> 3) It ties us 100% to a given implementation.  If Mistral proves to
> be a
> poor choice for some reason, or insufficient for a particular use
> case,
> we have no alternative.  If we have an API and decide to change our
> implementation, nobody has to know or care.  This is kind of the
> whole
> point of having an API - it shields users from all the nasty
> implementation details under the surface.

Mistal's API is a generic workflow API. It is very much the same layer
that I think we would get if we were to integrate with something like
Ansible Tower... except that Mistral is part of OpenStack. It
integrates very nicely with OpenStack services and is very customizable
with custom actions. The fact that Mistral sits much closer to
OpenStack and is essentially a light shim on top of it is to our
advantage (being TripleO). To think that we can build up a proxy API in
such a manner that we might be able to swap in an entirely new backend
(without even having a fully implement backend yet to begin with) is
for me a bit of a stretch. We've got a lot of "TripleO API" maturing
before we'll get to this point. Which is why I lean towards using a
generic workflow API to accomplis the same task.

I actually think rather than shielding users we should be more
transparent about the actual workflows that are driving deployment.
Smaller more focused workflows that we string together to drive the

> 4) It raises the bar even further for both new deployers and
> developers.
>  You already need to have a pretty firm grasp of Puppet and Heat
> templates to understand how our stuff works, not to mention a decent
> understanding of quite a number of OpenStack services.
> This presents a big chicken and egg problem for people new to
> OpenStack.
>  It's great that we're based on OpenStack and that allows people to
> peek
> under the hood and do some tinkering, but it can't be required for
> everyone.  A lot of our deployers are going to have little to no
> OpenStack experience, and TripleO is already a daunting task for
> those
> people (hell, it's daunting for people who _are_ experienced).

And on the flipside you will get more of a community around using an
OpenStack project than you ever would going off and building your own
"Deployment/Workflow API". 

I would actually argue this is less of a deployers thing and more of a
development tool choice. IMO most deployers will use python-
tripleoclient or some UI and not mistralclient directly. The code I've
posted this week shows a prototype of just this, Mistral is swapped in
such that you would never know it was involved because python-
tripleoclient works like it always did. Deployers use our CLI and UI
tools like they always have, and developers gain a community of Mistral
developers (and documentation) which they can interact with on common
problems. Sounds like a win/win to me.

> 5) What does reimplementing all of our tested, well-understood Python
> into a new YAML format gain us?  This is maybe the biggest thing I'm
> missing from this whole discussion.  We lose a bunch of things (ease
> of
> transition from other Python projects, excellent existing testing
> framework, etc.), but what are we actually gaining other than the
> ability to say that we use N + 1 OpenStack services?  Because we're
> way
> past the point where "It's OpenStack deploying OpenStack" is
> sufficient
> reason for people to pay attention to us.  We need less "Ooh, neat"
> and
> more "Ooh, that's easy to use and works well."  It's still not clear
> to
> me that Mistral helps in any way with the latter.

Nobody suggested we reimplement everything. Much of the plan to move
code into tripleo-common would stay. Instead of building our own API
we'd just skip all that and focus on the code that is actually about
our deployments in the form of custom Mistral actions and YAML

The YAML workflows just ties together actions which are actually all
written in Python. YAML works quite well for this and is a whole lot
less verbose than writting everything we have in Python. There is a
reason Heat, Ansible, and Mistral use YAML for these things... and I
think it works well. Understood you have an opinion on this, but I
don't share the view that everything works better when written in
Python. Take Puppet for example, we interface with that via Hiera.

People will pay attention because we'll be able to add features faster.
By not having to build our own API and plumbing we can focus on actual
problems rather than boilerplate Python API code.

> 6) On the testing note, how do we test these workflows?  Do we know
> what
> happens when step X fails?  How do we test that they handle it
> properly
> in an automated and repeatable way?  In Python these are largely easy
> questions to answer: unit tests.  How do you unit test YAML? 

The actions are all unit testable Python.

The workflows themselves would all get tested as part of our CI. With
Mistral workflows and the integration I'm proposing with both the CLI
and UI we'd have the same API driven workflows tested in both cases. We
don't short circuit the API and call into a library like we are doing
today for tripleo-common.

>  This is a
> big reason I'm not even crazy about having Mistral on the back end of
> a
> TripleO API.  We'd be going from code that we can test and prove
> works
> in a variety of scenarios, to YAML that is tested and proven to work
> in
> exactly the three scenarios we run in CI.  This is basically the same
> situation we had with tripleo-incubator, and it was bad there too.
> I dunno.  Maybe I'm too late to this party to have any impact on the
> discussion, but I very much do not like the direction we're going and
> I
> would be remiss if I didn't at least point out my concerns with it.

You aren't late to the party. But I would encourage you to look closely
at the Mistral demos and examples that have been posted to openstack-
dev before commenting further. Try them out, try Ansible (tower), try
Mistral, and then come back and have a hard look at what we are trying
to do by building our own TripleO API.

To me the crux of the problem isn't that we should expect other
projects to build our APIs for us. Rather it is using the right tools
for the right jobs. TripleO has gotten off on the wrong path a few
times. We tried to roll our own config manage tooling and that didn't
work out so well. I hate to see us go down the path of trying to write
our own deployment/workflow API when in fact we've already got what
exactly what we need in OpenStack already. And a community already
exists around it as well...


> -Ben
> On 01/13/2016 03:41 AM, Tzu-Mainn Chen wrote:
> > Hey all,
> > 
> > I realize now from the title of the other TripleO/Mistral thread
> > [1] that
> > the discussion there may have gotten confused.  I think using
> > Mistral for
> > TripleO processes that are obviously workflows - stack deployment,
> > node
> > registration - makes perfect sense.  That thread is exploring
> > practicalities
> > for doing that, and I think that's great work.
> > 
> > What I inappropriately started to address in that thread was a
> > somewhat
> > orthogonal point that Dan asked in his original email, namely:
> > 
> > "what it might look like if we were to use Mistral as a replacement
> > for the
> > TripleO API entirely"
> > 
> > I'd like to create this thread to talk about that; more of a
> > 'should we'
> > than 'can we'.  And to do that, I want to indulge in a thought
> > exercise
> > stemming from an IRC discussion with Dan and others.  All, please
> > correct me
> > if I've misstated anything.
> > 
> > The IRC discussion revolved around one use case: deploying a Heat
> > stack
> > directly from a Swift container.  With an updated patch, the Heat
> > CLI can
> > support this functionality natively.  Then we don't need a TripleO
> > API; we
> > can use Mistral to access that functionality, and we're done, with
> > no need
> > for additional code within TripleO.  And, as I understand it,
> > that's the
> > true motivation for using Mistral instead of a TripleO API:
> > avoiding custom
> > code within TripleO.
> > 
> > That's definitely a worthy goal... except from my perspective, the
> > story
> > doesn't quite end there.  A GUI needs additional functionality,
> > which boils
> > down to: understanding the Heat deployment templates in order to
> > provide
> > options for a user; and persisting those options within a Heat
> > environment
> > file.
> > 
> > Right away I think we hit a problem.  Where does the code for
> > 'understanding
> > options' go?  Much of that understanding comes from the
> > capabilities map
> > in tripleo-heat-templates [2]; it would make sense to me that
> > responsibility
> > for that would fall to a TripleO library.
> > 
> > Still, perhaps we can limit the amount of TripleO code.  So to give
> > API
> > access to 'getDeploymentOptions', we can create a Mistral workflow.
> > 
> >   Retrieve Heat templates from Swift -> Parse capabilities map
> > 
> > Which is fine-ish, except from an architectural perspective
> > 'getDeploymentOptions' violates the abstraction layer between
> > storage and
> > business logic, a problem that is compounded because
> > 'getDeploymentOptions'
> > is not the only functionality that accesses the Heat templates and
> > needs
> > exposure through an API.  And, as has been discussed on a separate
> > TripleO
> > thread, we're not even sure Swift is sufficient for our needs; one
> > possible
> > consideration right now is allowing deployment from templates
> > stored in
> > multiple places, such as the file system or git.
> > 
> > Are we going to have duplicate 'getDeploymentOptions' workflows for
> > each
> > storage mechanism?  If we consolidate the storage code within a
> > TripleO
> > library, do we really need a *workflow* to call a single
> > function?  Is a
> > thin TripleO API that contains no additional business logic really
> > so bad
> > at that point?
> > 
> > My gut reaction is to say that proposing Mistral in place of a
> > TripleO API
> > is to look at the engineering concerns from the wrong
> > direction.  The
> > Mistral alternative comes from a desire to limit custom TripleO
> > code at all
> > costs.  I think that is an extremely dangerous attitude that leads
> > to
> > compromises and workarounds that will quickly lead to a shaky code
> > base
> > full of design flaws that make it difficult to implement or extend
> > any
> > functionality cleanly.
> > 
> > I think the correct attitude is to simply look at the problem we're
> > trying to solve and find the correct architecture.  For these
> > get/set
> > methods that the API needs, it's pretty simple: storage -> some
> > logic ->
> > a REST API.  Adding a workflow engine on top of that is unneeded,
> > and I
> > believe that means it's an incorrect solution.
> > 
> > 
> > Thanks,
> > Tzu-Mainn Chen
> > 
> > 
> > 
> > [1] http://lists.openstack.org/pipermail/openstack-dev/2016-January
> > /083757.html
> > [2] https://github.com/openstack/tripleo-heat-templates/blob/master
> > /capabilities_map.yaml
> > 
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