Open Stack

Hi folks,

In the latest meeting we've mentioned several important architectural
points including:
- agents vs direct driver call
- asynchronous execution
- dispatching a generic REST call to a proper driver.

I would like to present how Mirantis team sees this based on our previous
experience with LBaaS and other openstack components.

We need asynchronous execution in the sense that client gets immediate
response while actual device configuration happens later.
Workflow of such operation could look like following:
1) client makes REST call; receives an object it has created/modified with
PENDING status
2) call is dispatched to a plugin, plugin creates/modifies/etc an object in
the database
3) plugin calls driver to apply new configuration to specific device
4) driver finishes applying configuration, plugin updates DB object
5) client polls objectID and gets final status of operation.

Now depending on approach we take, (3) could expand into different sequence
of operations.
One of the good options to choose could be using agent between plugin and
drivers. In this case (3) expands to:
3.1 plugin posts message to mq
3.2. message is consumed by one of the running service agents
3.3. agent calls corresponding driver directly in synchronous way.
3.4. agent posts message upon completion.
3.5. plugin consumes the message and updates DB object with final status

Such approach solves at least two potential problems:
1. plugin may be simplified since it is not required to implement call/work
item queuing
2. Applying device configuration is time consuming task which could take
seconds.
Both plugin and agent has thread limit for any concurrent operations.
Handling heavy workload in large deployments will be simple with several
agents consuming messages from mq.

Also this allows to create synchronous drivers since asyncness will be
handled by mq + agent.

Another option could be calling drivers directly without any asyncness at
all while preserving above workflow (1-5).
That could work as temporary fast solution while allowing to split it to
"plugin + agent approach" relatively easily.

Regarding the dispatching REST calls to proper driver:
In fact, VIP object should contain reference to particular device it is
created at.
http://wiki.openstack.org/LBaaS/CoreResourceModel/proposal misses that
device management part, I think it was just implied there.
Every balancer-related object references the VIP and hence references the
specific device where it was created.
E,g, when a call for any object is made, plugin needs to extract device
type from DB following those references and later plugin or agent will use
it to call particular driver.

Thanks,
 Eugene.
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