[openstack-dev] [magnum] Magnum conductor async container operations

SURO suro.patz at gmail.com
Thu Dec 17 20:35:29 UTC 2015

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 
[ 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 

[1] - 
(Line 33)

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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