[openstack-dev] [TripleO] Should we have a TripleO API, or simply use Mistral?
dprince at redhat.com
Fri Jan 15 13:30:17 UTC 2016
On Thu, 2016-01-14 at 16:04 -0500, Tzu-Mainn Chen wrote:
> ----- 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
> > >  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 ; 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
> learned from Tuskar's limitations. There were actually two issues
> 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
> (environment selection), because there was no Heat API in place for
> My takeaway from the first issue was that any potential TripleO API
> the future needed to be very low-level, a light skimming on top of
> 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
> unclear) whether that feature truly belonged in Heat. What does a
> 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
> in the future, arriving at a point where no new features would ever
> 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
> 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
> provide that, allowing us to switch code behind the scenes (say, if
> the capabilities feature lands in Heat).
I think the above example would work equally well in a generic workflow
sort of tool. You could image that the inputs to the workflow remain
the same... but rather than running our own code in some interim step
we simply call Heat directly for the capabilities map feature.
So regardless of whether we build our own API or use a generic workflow
too I think we still have what I would call a "release valve" to let us
inject some custom code (actions) into the workflow. Like we discussed
last week on IRC I would like to minimize the number of custom actions
we have (with an eye towards things living in the upstream OpenStack
projects) but it is fine to do this either way and would work equally
well w/ Mistral and TripleO API.
> 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
> library makes sense to me. It gives consumers a stable platform upon
> which to use TripleO, while allowing us to be flexible behind the
> 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
> invisible to the API consumer.
I mentioned this in my "Driving workflows with Mistral" thread but with
regards to stability I view say Heat's v1 API or Mistral's v2 API as
both being way more stable that what we could ever achieve with TripleO
API. The real trick to API stability with something like Heat or
Mistral is how we manage the inputs and outputs to Stacks and Workflows
themselves. So long as we are mindful of this I can't image an end user
(say a GUI writer or whoever) would really care whether they POST to
Mistral or something we've created. The nice thing about using other
OpenStack projects like Heat or Mistral is that they very likely have
better community and documentation around these things as well that we
would ever have.
The more I look at using Mistral for some of the cases that have been
brought up the more it seems to make sense for a lot of the workflows
we need. I don't believe we can achieve better stability by creating
what sounds more and more like a shim/proxy API rather than using the
versioned API's that OpenStack already provides.
There may be some corner cases where a "GUI helper" API comes into play
for some sort of caching or something. I'm not blocking anyone from
creating these sorts of features if they need them. And again if it is
something that could be added to an upstream OpenStack project like
Heat or Mistral I would look there first. So perhaps Zaqar for
websockets instead of rolling our own, this sort of thing.
What does concern me is that we are overstating what TripleO API should
actually contain should we choose to pursue it. Initially it was
positioned as the "TripleO workflow API". I think we now agree that we
probably shouldn't put all of our workflows behind it. So if our stance
has changed would it make sense to compile a new list of what we
believe belongs behind our own TripleO API vs. what we consider
> > > 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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