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Suro,

FYI. In before, we tried a distributed lock implementation for bay operations (here are the patches [1,2,3,4,5]). However, after several discussions online and offline, we decided to drop the blocking implementation for bay operations, in favor of non-blocking implementation (which is not implemented yet). You can find more discussion in here [6,7].

For the async container operations, I would suggest to consider a non-blocking approach first. If it is impossible and we need a blocking implementation, suggest to use the bay operations patches below as a reference.

[1] https://review.openstack.org/#/c/171921/
[2] https://review.openstack.org/#/c/172603/
[3] https://review.openstack.org/#/c/172772/
[4] https://review.openstack.org/#/c/172773/
[5] https://review.openstack.org/#/c/172774/
[6] https://blueprints.launchpad.net/magnum/+spec/horizontal-scale
[7] https://etherpad.openstack.org/p/liberty-work-magnum-horizontal-scale

Best regards,
Hongbin

-----Original Message-----
From: Adrian Otto [mailto:adrian.otto at rackspace.com] 
Sent: December-16-15 10:20 PM
To: OpenStack Development Mailing List (not for usage questions)
Cc: suro at yahoo-inc.com
Subject: Re: [openstack-dev] [magnum] Magnum conductor async container operations


> On Dec 16, 2015, at 6:24 PM, Joshua Harlow <harlowja at fastmail.com> wrote:
> 
> SURO wrote:
>> Hi all,
>> Please review and provide feedback on the following design proposal 
>> for implementing the blueprint[1] on async-container-operations -
>> 
>> 1. Magnum-conductor would have a pool of threads for executing the 
>> container operations, viz. executor_threadpool. The size of the 
>> executor_threadpool will be configurable. [Phase0] 2. Every time, 
>> Magnum-conductor(Mcon) receives a container-operation-request from 
>> Magnum-API(Mapi), it will do the initial validation, housekeeping and 
>> then pick a thread from the executor_threadpool to execute the rest 
>> of the operations. Thus Mcon will return from the RPC request context 
>> much faster without blocking the Mapi. If the executor_threadpool is 
>> empty, Mcon will execute in a manner it does today, i.e. 
>> synchronously - this will be the rate-limiting mechanism - thus 
>> relaying the feedback of exhaustion.
>> [Phase0]
>> How often we are hitting this scenario, may be indicative to the 
>> operator to create more workers for Mcon.
>> 3. Blocking class of operations - There will be a class of 
>> operations, which can not be made async, as they are supposed to 
>> return result/content inline, e.g. 'container-logs'. [Phase0] 4. 
>> Out-of-order considerations for NonBlocking class of operations - 
>> there is a possible race around condition for create followed by 
>> start/delete of a container, as things would happen in parallel. To 
>> solve this, we will maintain a map of a container and executing 
>> thread, for current execution. If we find a request for an operation 
>> for a container-in-execution, we will block till the thread completes 
>> the execution. [Phase0]
> 
> Does whatever do these operations (mcon?) run in more than one process?

Yes, there may be multiple copies of magnum-conductor running on separate hosts.

> Can it be requested to create in one process then delete in another? 
> If so is that map some distributed/cross-machine/cross-process map 
> that will be inspected to see what else is manipulating a given 
> container (so that the thread can block until that is not the case... 
> basically the map is acting like a operation-lock?)

That’s how I interpreted it as well. This is a race prevention technique so that we don’t attempt to act on a resource until it is ready. Another way to deal with this is check the state of the resource, and return a “not ready” error if it’s not ready yet. If this happens in a part of the system that is unattended by a user, we can re-queue the call to retry after a minimum delay so that it proceeds only when the ready state is reached in the resource, or terminated after a maximum number of attempts, or if the resource enters an error state. This would allow other work to proceed while the retry waits in the queue.

> If it's just local in one process, then I have a library for u that 
> can solve the problem of correctly ordering parallel operations ;)

What we are aiming for is a bit more distributed. 

Adrian

>> This mechanism can be further refined to achieve more asynchronous 
>> behavior. [Phase2] The approach above puts a prerequisite that 
>> operations for a given container on a given Bay would go to the same 
>> Magnum-conductor instance.
>> [Phase0]
>> 5. The hand-off between Mcon and a thread from executor_threadpool 
>> can be reflected through new states on the 'container' object. These 
>> states can be helpful to recover/audit, in case of Mcon restart. 
>> [Phase1]
>> 
>> Other considerations -
>> 1. Using eventlet.greenthread instead of real threads => This 
>> approach would require further refactoring the execution code and 
>> embed yield logic, otherwise a single greenthread would block others to progress.
>> Given, we will extend the mechanism for multiple COEs, and to keep 
>> the approach straight forward to begin with, we will use 'threading.Thread'
>> instead of 'eventlet.greenthread'.
>> 
>> 
>> Refs:-
>> [1] -
>> https://blueprints.launchpad.net/magnum/+spec/async-container-operati
>> ons
>> 
> 
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