Hey, yeah, so I started the Convection wiki and discussed that idea at the 
Portland summit in 2013.  Mirantis picked up the Convection proposal and 
decided to run with it in a project named Mistral from what I understand...  
Their was apparently some concerns over name infringement, which is why the 
name Convection was not kept when they began their Mistral proof-of-concept.  
Convection was an "idea proposed."  There is no code for Convection, and no 
active contributors to Convection.  In the spirit of collaboration, if folks 
are interested in contributing to a Workflow system, I would encourage you to 
get involved in Mistral and see if it fits your needs... If not, I'm sure the 
Mistral (and other openstack folks) would love to have a discussion about that.

Kind regards,

From: Joshua Harlow <harlo...@yahoo-inc.com<mailto:harlo...@yahoo-inc.com>>
Reply-To: "OpenStack Development Mailing List (not for usage questions)" 
Date: Friday, February 28, 2014 2:46 PM
To: "OpenStack Development Mailing List (not for usage questions)" 
W Chan <m4d.co...@gmail.com<mailto:m4d.co...@gmail.com>>
Subject: Re: [openstack-dev] [Mistral] Porting executor and engine to 

Convection? Afaik u guys are building convection (convection was just an idea, 
I see mistral as the POC/impl) ;)


So questions around taskflow:

  1.  Correct u put it in your task, there was previous ideas/work done by the 
team @ https://etherpad.openstack.org/p/BrainstormFlowConditions but from 
previous people that have build said systems it was determined that actually 
there wasn't much use for conditionals being useful (yet). But expression 
evaluation, not sure what that means, being a library, any type of expression 
evaluation is just whatever u can imagine in python. Conditional tasks (and 
such) being managed by taskflows engines we can reconsider & might even be 
possible but this is imho dangerous territory that is being approached, 
expression evaluation and conditional branching and loops is basically a 
language specification ;)
  2.  I don't see taskflow managing a catalog (currently), that seems out of 
scope of a library that provides the execution, resumption parts (any consumer 
of taskflow should be free to define and organize there catalog as they choose).
  3.  Negative, taskflow is a execution and state-management library (not a 
full framework imho) that helps build the upper layers that services like 
mistral can use (or nova, or glance or…). I don't feel its the right place to 
have taskflow force a DSL onto people, since the underlying primitives that can 
form a upper level DSL are more service/app level choices (heat has there DSL, 
mistral has theres, both are fine, and both likely can take advantage of the 
same taskflow execution and state-management primitives to use in there 

Hope that helps :)


From: W Chan <m4d.co...@gmail.com<mailto:m4d.co...@gmail.com>>
Reply-To: "OpenStack Development Mailing List (not for usage questions)" 
Date: Friday, February 28, 2014 at 12:02 PM
To: "OpenStack Development Mailing List (not for usage questions)" 
Subject: Re: [openstack-dev] [Mistral] Porting executor and engine to 

This is a great start.  I think the sooner we have this discussion the better.  
Any uncertainty in the direction/architecture here is going to stall progress.  
How about Convection?  What's the status of the Convection project and where 
it's heading?  Should we have similar discussion with the contributors of that 

I have a few questions about TaskFlow.
1) How does it handle conditional loop and expression evaluation for decision 
branching?  I've looked at the Taskflow wiki/code briefly and it's not obvious. 
 I assume it would be logic that user will embed within a task?
2) How about predefined catalog of standard tasks (i.e. REST call, SOAP call, 
Email task, etc.)?  Is that within the scope of Taskflow or up to TaskFlow 
consumers like Mistral?
3) Does TaskFlow have its own DSL?  The examples provided are mostly code based.


On Fri, Feb 28, 2014 at 10:54 AM, Joshua Harlow 
<harlo...@yahoo-inc.com<mailto:harlo...@yahoo-inc.com>> wrote:
Sounds good,

Lets connect, the value of central oslo connected projects is that shared 
libraries == share the pain. Duplicating features and functionality is always 
more pain. In the end we are a community, not silos, so it seems like before 
mistral goes down the path of duplicating more and more features (I understand 
the desire to POC mistral and learn what mistral wants to become, and all that) 
that we should start the path to working together. I personally am worried that 
mistral will start to apply for incubation and then the question will come up 
as to this (mistral was doing POC, kept on doing POC, never came back to using 
common libraries, and then gets asked why this happened).

I'd like to make us all successful, and as a old saying goes,

“A single twig breaks, but the bundle of twigs is strong”, openstack needs to 
be a cohesive bundle and not a single twig ;)

From: Renat Akhmerov <rakhme...@mirantis.com<mailto:rakhme...@mirantis.com>>

Reply-To: "OpenStack Development Mailing List (not for usage questions)" 
Date: Friday, February 28, 2014 at 6:31 AM

To: "OpenStack Development Mailing List (not for usage questions)" 
Subject: Re: [openstack-dev] [Mistral] Porting executor and engine to 

Hi Joshua,

Sorry, I’ve been very busy for the last couple of days and didn’t respond 
quickly enough.

Well, first of all, it’s my bad that I’ve not been following TaskFlow progress 
for a while and, honestly, I just need to get more info on the current TaskFlow 
status. So I’ll do that and get back to you soon. As you know, there were 
reasons why we decided to go this path (without using TaskFlow) but I’ve always 
thought we will be able to align our efforts as we move forward once 
requirements and design of Mistral become more clear. I really want to use 
TaskFlow for Mistral implementation. We just need to make sure that it will 
bring more value than pain (sorry if it sounds harsh).

Thanks for your feedback and this info. We’ll get in touch with you soon.

Renat Akhmerov
@ Mirantis Inc.

On 27 Feb 2014, at 03:22, Joshua Harlow 
<harlo...@yahoo-inc.com<mailto:harlo...@yahoo-inc.com>> wrote:

So this design is starting to look pretty familiar to a what we have in 

Any reason why it can't just be used instead?


This code is in a functional state right now, using kombu (for the moment, 
until oslo.messaging becomes py3 compliant).

The concept of a engine which puts messages on a queue for a remote executor is 
in-fact exactly the case taskflow is doing (the remote exeuctor/worker will 
then respond when it is done and the engine will then initiate the next piece 
of work to do) in the above listed etherpad (and which is implemented).

Is it the case that in mistral the engine will be maintaining the 
'orchestration' of the workflow during the lifetime of that workflow? In the 
case of mistral what is an engine server? Is this a server that has engines in 
it (where each engine is 'orchestrating' the remote/local workflows and 
monitoring and recording the state transitions and data flow that is 
occurring)? The details @ 
 seems to be already what taskflow provides via its engine object, creating a 
application which runs engines and those engines initiate workflows is made to 
be dead simple.

>From previous discussions with the mistral folks it seems like the overlap 
>here is getting more and more, which seems to be bad (and means something is 
>broken/wrong). In fact most of the concepts that u have blueprints for have 
>already been completed in taskflow (data-flow, engine being disconnected from 
>the rest api…) and ones u don't have listed (resumption, reversion…).

What can we do to fix this situation?


From: Nikolay Makhotkin 
Reply-To: "OpenStack Development Mailing List (not for usage questions)" 
Date: Tuesday, February 25, 2014 at 11:30 PM
To: "OpenStack Development Mailing List (not for usage questions)" 
Subject: Re: [openstack-dev] [Mistral] Porting executor and engine to 

Looks good. Thanks, Winson!

Renat, What do you think?

On Wed, Feb 26, 2014 at 10:00 AM, W Chan 
<m4d.co...@gmail.com<mailto:m4d.co...@gmail.com>> wrote:
The following link is the google doc of the proposed engine/executor message 
flow architecture.  

The diagram on the right is the scalable engine where one or more engine sends 
requests over a transport to one or more executors.  The executor client, 
transport, and executor server follows the RPC client/server design 
 in oslo.messaging.

The diagram represents the local engine.  In reality, it's following the same 
RPC client/server design pattern.  The only difference is that it'll be 
configured to use a 
 RPC backend driver.  The fake driver uses in process 
queues<http://docs.python.org/2/library/queue.html#module-Queue> shared between 
a pair of engine and executor.

The following are the stepwise changes I will make.
1) Keep the local and scalable engine structure intact.  Create the Executor 
Client at ./mistral/engine/scalable/executor/client.py.  Create the Executor 
Server at ./mistral/engine/scalable/executor/service.py and implement the task 
operations under ./mistral/engine/scalable/executor/executor.py.  Delete 
./mistral/engine/scalable/executor/executor.py.  Modify the launcher 
./mistral/cmd/task_executor.py.  Modify ./mistral/engine/scalable/engine.py to 
use the Executor Client instead of sending the message directly to rabbit via 
pika.  The sum of this is the atomic change that keeps existing structure and 
without breaking the code.
2) Remove the local engine. 
3) Implement versioning for the engine.  
4) Port abstract engine to use oslo.messaging and implement the engine client, 
engine server, and modify the API layer to consume the engine client. 


On Mon, Feb 24, 2014 at 8:07 PM, Renat Akhmerov 
<rakhme...@mirantis.com<mailto:rakhme...@mirantis.com>> wrote:

On 25 Feb 2014, at 02:21, W Chan 
<m4d.co...@gmail.com<mailto:m4d.co...@gmail.com>> wrote:


Regarding your comments on change https://review.openstack.org/#/c/75609/, I 
don't think the port to oslo.messaging is just a swap from pika to 
oslo.messaging.  OpenStack services as I understand is usually implemented as 
an RPC client/server over a messaging transport.  Sync vs async calls are done 
via the RPC client call and cast respectively.  The messaging transport is 
abstracted and concrete implementation is done via drivers/plugins.  So the 
architecture of the executor if ported to oslo.messaging needs to include a 
client, a server, and a transport.  The consumer (in this case the mistral 
engine) instantiates an instance of the client for the executor, makes the 
method call to handle task, the client then sends the request over the 
transport to the server.  The server picks up the request from the exchange and 
processes the request.  If cast (async), the client side returns immediately.  
If call (sync), the client side waits for a response from the server over a 
reply_q (a unique queue for the session in the transport).  Also, 
oslo.messaging allows versioning in the message. Major version change indicates 
API contract changes.  Minor version indicates backend changes but with API 

My main concern about this patch is not related with messaging infrastructure. 
I believe you know better than me how it should look like. I’m mostly concerned 
with the way of making changes you chose. From my perspective, it’s much better 
to make atomic changes where every changes doesn’t affect too much in existing 
architecture. So the first step could be to change pika to oslo.messaging with 
minimal structural changes without introducing versioning (could be just TODO 
comment saying that the framework allows it and we may want to use it in the 
future, to be decide), without getting rid of the current engine structure 
(local, scalable). Some of the things in the file structure and architecture 
came from the decisions made by many people and we need to be careful about 
changing them.

So, where I'm headed with this change...  I'm implementing the basic 
structure/scaffolding for the new executor service using oslo.messaging 
(default transport with rabbit).  Since the whole change will take a few 
rounds, I don't want to disrupt any changes that the team is making at the 
moment and so I'm building the structure separately.  I'm also adding 
versioning (v1) in the module structure to anticipate any versioning changes in 
the future.   I expect the change request will lead to some discussion as we 
are doing here.  I will migrate the core operations of the executor 
(handle_task, handle_task_error, do_task_action) to the server component when 
we agree on the architecture and switch the consumer (engine) to use the new 
RPC client for the executor instead of sending the message to the queue over 
pika.  Also, the launcher for ./mistral/cmd/task_executor.py will change as 
well in subsequent round.  An example launcher is here 
The interceptor project here is what I use to research how oslo.messaging 
works.  I hope this is clear. The blueprint only changes how the request and 
response are being transported.  It shouldn't change how the executor currently 

Please create a document describing the approach you’re pursuing here. I would 
expect to see the main goals you want to achieve upon completion.

Finally, can you clarify the difference between local vs scalable engine?  I 
personally do not prefer to explicitly name the engine scalable because this 
requirement should be in the engine by default and we do not need to explicitly 
state/separate that.  But if this is a roadblock for the change, I can put the 
scalable structure back in the change to move this forward.

Separation for local and scalable implementations appeared for historical 
reasons because from the beginning we didn’t see how it all would look like and 
hence we tried different approaches to implement the engine. At some point we 
got 2 working versions: the one that didn’t distribute anything (local) and 
another one that could distribute tasks over task executors via asynchronous HA 
transport (scalable). Later on we decided to leave them both since scalable is 
needed by the requirements and local might be useful for demonstration purposes 
and testing since it doesn’t require RabbitMQ to be installed. So we decided to 
refactor both and make them work similarly except the way they run tasks.


Renat Akhmerov
@Mirantis Inc.

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