[openstack-dev] [Murano][Heat] MuranoPL questions?

Steven Dake sdake at redhat.com
Wed Mar 19 04:06:41 UTC 2014


Some of my thoughts on the evolution of the HOT DSL to date.

On 03/18/2014 05:32 PM, Ruslan Kamaldinov wrote:
> Here is my 2 cents:
> I personally think that evolving Heat/HOT to what Murano needs for it's use
> cases is the best way to make PaaS layer of OpenStack to look and feel as a
> complete and fully integrated solution.
> Standardising these things in a project like TOSCA is another direction we all
> should follow. I think that TOSCA is the place where developers (like us),
> application developers and enterprises can collaborate to produce a common
> standard for application lifecycle management in the clouds.
> But before Murano contributors jump into direction of extending HOT to the goal
> of application (or system) lifecycle management, we need an agreement that this
> is the right direction for Heat/HOT/DSL and the Orchestration program. There are
> a lot of use cases that current HOT doesn't seem to be the right tool to solve.
> As it was said before, it's not a problem to collaborate on extending it those
> use cases. I'm just unsure if Heat team would like these use cases to be solved
> with Heat/HOT/DSL. For instance:
> - definition of an application which is already exposed via REST API. Think of
>    something like Sahara (ex. Savanna) or Trove developed in-house for internal
>    company needs. app publishers wouldn't be happy if they'll be forced to
>    develop a new resource for Heat
> - definition of billing rules for an application
> If everyone agrees that this is the direction we all should follow, that we
> should expand HOT/DSL to that scope, that HOT should be the answer on "can you
> express it?", then awesome - we can start speaking about implementation details.
> If it's not the direction these projects should follow then at least finding
> where Heat ends and Murano starts to avoid any functionality duplication would
> be great.

The HOT DSL for the most part, either by design or subconscious 
development choices, enables the application of Miller's Law[1] in a 
positive way.  HOT as a DSL takes less then a few hours to learn and use 
effectively.  Its relative simplicity is its *key* advantage as a DSL.  
DSL's by their very nature declare a desired state.  It is the 
responsibility of the DSL processor to convert that desired state into 
reality.  On a fundamental level, this is precisely what Heat does.

A DSL by its very definition is meant to express a desired outcome 
without specifying the intermediate steps.  To express the intermediate 
steps would require recording state in variables and offering 
conditional operations on those variables.  This implies individual 
steps in the processing of the input to the language.  If HOT were to 
add these sorts of features, it would no longer be a DSL, but a general 
purpose language (perhaps less general purpose then python or C).  A DSL 
is by definition a declarative language. I don't like the idea of 
expanding the scope of HOT to add an imperative model of operation.

Learning imperative languages takes inordinately more time and 
brainpower then learning declarative languages, especially those which 
generally follow the advantages provided by languages operating inside 
the constraints of Miller's Law.  We want Heat to be dead simple to 
explain and learn.  Realistically I'd like folks to be able to write a 
template in under an hour with 15 minutes of explanation, and I think we 
have hit that mark.

The idea of expanding the scope of the Heat APIs and engine to include 
ALM and Workflow don't make sense to me from an engineering 
perspective.  It over-complicates the code base.  I know we have already 
covered those thoughts in detail on the mailing list previously and the 
Murano folks agree that is a bad idea.

I see a parallel between expanding the scope of HOT to support ALM and 
Workflow and expanding the scope of the heat-engine in the same fashion 
that is not appealing.  What would make more sense is to follow the 
general laws of Unix (do one thing, do it well) and layer these other 
possibly imperative languages on top of Heat using HOT and the Heat APIs 
to implement such imperative programming models. Then if someone really 
wanted to invest in the complexity of ALM or Workflow, they may be more 
willing to invest in learning the complexity of a new imperative 
programming language.

My personal opinion is expanding the scope of HOT to include imperative 
programming models is not desirable for Heat in isolation.  I understand 
such an outcome may be appealing as a holistic approach to handling the 
entire orchestration space, but feel the costs of learning an imperative 
model for HOT do not pay for the advantages of having only one language 
to program all the things.

I see no issue with HOT remaining simple and tidy focused entirely on 
orchestration (taking a desired state and converting that into reality) 
with some other imperative language layered on top to handle workflow 
and ALM.  I believe this separation of concerns is best for OpenStack 
and should be the preferred development path.


[1] http://en.wikipedia.org/wiki/The_Magical_Number_Seven,_Plus_or_Minus_Two
> Thanks,
> Ruslan
> On Wed, Mar 19, 2014 at 2:07 AM, Keith Bray <keith.bray at rackspace.com> wrote:
>> Georgy,
>> In consideration of the "can you express it" instead of the "who will
>> generate it," I see Heat's HOT evolving to support the expression of complex
>> multi-tier architectures and applications (I would argue you can already do
>> this today, perhaps with some additional features desired, e.g. Ability to
>> define cloud workflows and workflow execution rules which could come when we
>> have a workflow service like Mistral).  Therefore, I would encourage Murano
>> contributors to consider whether they can help make Heat sufficiently cover
>> desired use cases.  I have never viewed Heat templates as isolated
>> components of a multi-tier architecture.  Instead, a single template or a
>> combination of master/subordinate templates together (using references,
>> nesting, or inclusion) could express the complete architecture, both
>> infrastructure and applications.
>> If I've read your previous comments and threads correctly, you desire a way
>> to express System Lifecycle Management across multiple related applications
>> or components, whereby you view the System as a grouping of independently
>> developed and/or deployed (but systematically related) "components," whereby
>> you view Components as individual disconnected Heat templates that
>> independently describe different application stacks of the System.  Did I
>> get that correct?   If so, perhaps the discussion here is one of "scope" of
>> what can or should be expressed in a Heat template. Is it correct to state
>> that your argument is that a separate system (such as Murano) should be used
>> to express System Lifecycle Management as I've defined it here?  If so, why
>> could we not use the Heat DSL to also define the System?  The System
>> definition could be logically separated out into its own text file... But,
>> we'd have a common DSL syntax and semantics for both lower level and higher
>> level component interaction (a building block effect of sorts).
>> As for "who will generate it," ( with "it" being the Heat multi-tier
>> application/infrastructure definition) I think that question will go through
>> a lot more evolution and could be any number of sources: e.g. Solum, Murano,
>> Horizon, Template Author with a text editor, etc.
>> Basically, I'm a +1 for as few DSLs as possible. I support the position that
>> we should evolve HOT if needed vs. having two separate DSLs that are both
>> related to expressing application and infrastructure semantics.
>> Workflow is quite interesting ... Should we be able to express imperative
>> workflow semantics in HOT?  Or, should we only be able to declare workflow
>> configurations that get configured in a service like Mistral whereby
>> Mistral's execution of a workflow may need to invoke Heat hooks or Stack
>> Updates?  Or, some other solution?
>> I look forward to a design discussion on all this at the summit... This is
>> fun stuff to think about!
>> -Keith
>> From: Georgy Okrokvertskhov <gokrokvertskhov at mirantis.com>
>> Reply-To: "OpenStack Development Mailing List (not for usage questions)"
>> <openstack-dev at lists.openstack.org>
>> Date: Tuesday, March 18, 2014 1:49 PM
>> To: "OpenStack Development Mailing List (not for usage questions)"
>> <openstack-dev at lists.openstack.org>
>> Subject: Re: [openstack-dev] [Murano][Heat] MuranoPL questions?
>> I see this in the following way - who will generate HOT template for my
>> complex multi-tier applications when I have only templates for components?
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