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Hongbin,
Very useful pointers! Thanks for bringing up the relevant contexts!

The proposal to block here for consecutive operations on same container, 
is the approach to start with. We can have a wait queue implementation 
following - that way the approach will be amortized over time. If you 
feel strongly, I am okay implementing the wait queue on the first go 
itself.
[ I felt step-by-step approach carries in sizable code, easier to review ]

By the way, I think the scope of bay lock and scope of 
per-bay-per-container operation is different too, in terms of blocking.

I have a confusion about non-blocking bay-operations for horizontal 
scale [1]  -
" Heat will be having concurrency support, so we can rely on heat for 
the concurrency issue for now and drop the baylock implementation."
- if user issues two consecutive updates on a Bay, and if the updates go 
through different magnum-conductors,
they can land up at Heat in different order, resulting in different 
state of the bay. How Heat-concurrency will prevent that I am not very 
clear. [ Take an example of 'magnum bay-update k8sbay replace 
node_count=100' followed by 'magnum bay-update k8sbay replace 
node_count=10']


[1] - 
https://etherpad.openstack.org/p/liberty-work-magnum-horizontal-scale 
(Line 33)

Regards,
SURO
irc//freenode: suro-patz

On 12/17/15 8:10 AM, Hongbin Lu wrote:
> 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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