Open Stack

The following link is the google doc of the proposed engine/executor
message flow architecture.
https://drive.google.com/file/d/0B4TqA9lkW12PZ2dJVFRsS0pGdEU/edit?usp=sharing

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
pattern<https://github.com/openstack/oslo.messaging/tree/master/oslo/messaging/rpc>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
fake<https://github.com/openstack/oslo.messaging/blob/master/oslo/messaging/_drivers/impl_fake.py>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.
https://blueprints.launchpad.net/mistral/+spec/mistral-inproc-executor
3) Implement versioning for the engine.
https://blueprints.launchpad.net/mistral/+spec/mistral-engine-versioning
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.
https://blueprints.launchpad.net/mistral/+spec/mistral-engine-standalone-process
.

Winson


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

>
> On 25 Feb 2014, at 02:21, W Chan <m4d.coder at gmail.com> wrote:
>
> Renat,
>
> 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 compatibility.
>
>
> 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
> https://github.com/uhobawuhot/interceptor/blob/master/bin/interceptor-engine.
>  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 works.
>
>
> 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.
>
> Thanks.
>
> Renat Akhmerov
> @Mirantis Inc.
>
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>
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