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

James Slagle james.slagle at gmail.com
Mon Jan 25 14:56:26 UTC 2016

On Mon, Jan 25, 2016 at 6:56 AM, Dougal Matthews <dougal at redhat.com> wrote:

> Great post Ben, I largely agree with what you are saying, a lot of your
> points
> are valid concerns that I share. Just a couple of comments inline as I try
> to
> avoid what others have said.
> On 22 January 2016 at 17:24, Ben Nemec <openstack at nemebean.com> 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.
> I don't really expect the API to be used directly by many users - how many
> users directly used the Tuskar API? I suspect they all used it via the CLI
> tools or the GUI. I could be completely wrong with the assumption but I
> think
> the primary entry points will be the CLI and GUI we expose. Only advanced
> users would need to consider Mistral.

If we don't expect the API to be used directly by many users, then I
question why we're discussing building one. Is it even a useful
abstraction? Or, are we just solving for the problem that the gui needs to
talk http to something?

Tuskar was the anti-pattern, not something to try and do the same again. It
wasn't just the API that wasn't heavily used, it was the CLI as well. Most
TripleO developers didn't use it in their own development, most users
didn't use it, and it wasn't used in tripleo-ci. Eventually, TripleO moved
on, and Tuskar was left behind and stopped being useful.

We ought to focus on getting the API correct and user friendly, then the
CLI will follow as almost a boilerplate addition.

If Mistral doesn't offer the general usability we'd like to see out of a
TripleO deployment API, then it isn't really a choice imo. I'm not saying
it doesn't ftr.

I'm just trying to understand the assumptions of the debate about the
choice. Because to me, it seems like there are some fundamental differences
of opinion as to why we even need an API, or how it would be used. And
those differences are driving a lot of the discussion about TripleO API or
Mistral. It feels a little bit like it's less of choosing the right tool,
and more about how do we expect either of those tools to actually be used.
If we could agree on that, then maybe it makes the technical choice much
easier to make?

I just don't want to see TripleO offer an API we don't really anticipate
anyone using other than through a CLI/GUI. I'd like to see us offer a
stable interface, and have that be the API (TripleO or Mistral), and it be
something that people want/understand how to use and it be well supported
that they use it directly (without editing yaml, see my other reply about

> Anyone directly interfacing with the API's will need to learn a ton of
> services,
> I don't think Mistral adds much more complexity here.
>> 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.
> Yeah, I agree that one of the downsides of the Mistral approach is that we
> blur the lines between these two.
>> 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.
>> 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).
>> 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.
>> 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?  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.
>> -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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-- James Slagle
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