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

Tzu-Mainn Chen tzumainn at redhat.com
Thu Jan 14 21:04:10 UTC 2016

----- Original Message -----
> On Wed, Jan 13, 2016 at 04:41:28AM -0500, 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.
> Actually, that whole capabilities map thing is a workaround for a missing
> feature in Heat, which I have proposed, but am having a hard time reaching
> consensus on within the Heat community:
> https://review.openstack.org/#/c/196656/
> Given that is a large part of what's anticipated to be provided by the
> proposed TripleO API, I'd welcome feedback and collaboration so we can move
> that forward, vs solving only for TripleO.
> > 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?
> Actually, this is an argument for making the validation part of the
> deployment a workflow - then the interface with the storage mechanism
> becomes more easily pluggable vs baked into an opaque-to-operators API.
> E.g, in the long term, imagine the capabilities feature exists in Heat, you
> then have a pre-deployment workflow that looks something like:
> 1. Retrieve golden templates from a template store
> 2. Pass templates to Heat, get capabilities map which defines features user
> must/may select.
> 3. Prompt user for input to select required capabilites
> 4. Pass user input to Heat, validate the configuration, get a mapping of
> required options for the selected capabilities (nested validation)
> 5. Push the validated pieces ("plan" in TripleO API terminology) to a
> template store
> This is a pre-deployment validation workflow, and it's a superset of the
> getDeploymentOptions feature you refer to.
> Historically, TripleO has had a major gap wrt workflow, meaning that we've
> always implemented it either via shell scripts (tripleo-incubator) or
> python code (tripleo-common/tripleo-client, potentially TripleO API).
> So I think what Dan is exploring is, how do we avoid reimplementing a
> workflow engine, when a project exists which already does that.
> > 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 it's not about limiting TripleO code at all costs, it's about
> learning from past mistakes, where long-term TripleO specific workarounds
> for gaps in other projects have become serious technical debt.
> For example, the old merge.py approach to template composition was a
> workaround for missing heat features, then Tuskar was another workaround
> (arguably) for missing heat features, and now we're again proposing a
> long-term workaround for some missing heat features, some of which are
> already proposed (referring to the API for capabilities resolution).

This is an important point, thanks for bringing it up!

I think that I might have a different understanding of the lessons to be
learned from Tuskar's limitations.  There were actually two issues that
arose.  The first was that Tuskar was far too specific in how it tried to
manipulated Heat pieces.  The second - and more serious, from my point of
view - was that there literally was no way for an API-based GUI to
perform the tasks it needed to in order to do the correct manipulation
(environment selection), because there was no Heat API in place for doing

My takeaway from the first issue was that any potential TripleO API in
the future needed to be very low-level, a light skimming on top of the
OpenStack services it uses.  The plan creation process that the
tripleo-common library spec describes is that: it's just a couple of
methods designed to allow a user to create an environment file, which
can then be used for deploying the overcloud.

My takeaway from the second issue was a bit more complicated.  A
required feature was missing, and although the proper functionality
needed to enable it in Heat was identified, it was unclear (and remains
unclear) whether that feature truly belonged in Heat.  What does a GUI
do then?  The GUI could take a cycle off, which is essentially what
happened here; I don't think that's a reasonable solution.  We could
hope that we arrive at a 100% foolproof and immutable deployment solution
in the future, arriving at a point where no new features would ever be
needed; I don't think that's a practical hope.

The third solution that came to mind was the idea of creating the
TripleO API.  It gives us a place to add in missing features if needed.
And I think it also gives us a useful layer of indirection.  The
consumers of TripleO want a stable API, so that a new release doesn't
force them to do a massive update of their code; the TripleO API would
provide that, allowing us to switch code behind the scenes (say, if
the capabilities feature lands in Heat).

I think I kinda view TripleO as a 'best practices' project.  Using
OpenStack is a confusing experience, with a million different options
and choices to make.  TripleO provides users with an excellent guide.
But the problem is that best practices change, and I think that
perceived instability is dangerous for adoption of TripleO.

So having a TripleO library and its associated API be a 'best practices'
library makes sense to me.  It gives consumers a stable platform upon
which to use TripleO, while allowing us to be flexible behind the scenes.
The 'best practice' for Heat capabilities right now is a workaround,
because it hasn't been judged to be suitable to go into Heat itself.
If that changes, we get to shift as well - and all of these changes are
invisible to the API consumer.


> > 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.
> What may help is if we can work through the proposed API spec, and
> identify which calls can reasonably be considered workflows vs those where
> it's really just proxying an API call with some logic?
> When we have a defined list of "not workflow" API requirements, it'll
> probably be much easier to rationalize over the value of a bespoke API vs
> mistral?
> Steve
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