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

Joshua Harlow harlowja at fastmail.com
Thu Dec 17 07:39:38 UTC 2015


SURO wrote:
> Please find the reply inline.
>
> Regards,
> SURO
> irc//freenode: suro-patz
>
> On 12/16/15 7:19 PM, Adrian Otto wrote:
>>> 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?)
> Suro> @Josh, just after this, I had mentioned
>
> "The approach above puts a prerequisite that operations for a given
> container on a given Bay would go to the same Magnum-conductor instance."
>
> Which suggested multiple instances of magnum-conductors. Also, my idea
> for implementing this was as follows - magnum-conductors have an 'id'
> associated, which carries the notion of [0 - (N-1)]th instance of
> magnum-conductor. Given a request for a container operation, we would
> always have the bay-id and container-id. I was planning to use
> 'hash(bay-id, key-id) modulo N' to be the logic to ensure that the right
> instance picks up the intended request. Let me know if I am missing any
> nuance of AMQP here.

Unsure about nuance of AMQP (I guess that's an implementation detail of 
this); but what this sounds like is similar to the hash-rings other 
projects have built (ironic uses one[1], ceilometer is slightly 
different afaik, see 
http://www.slideshare.net/EoghanGlynn/hash-based-central-agent-workload-partitioning-37760440 
and 
https://github.com/openstack/ceilometer/blob/master/ceilometer/coordination.py#L48).

The typical issue with modulo hashing is changes in N (whether adding 
new conductors or deleting them) and what that change in N does to 
ongoing requests, how do u change N in an online manner (and so-on); 
typically with modulo hashing a large amount of keys get shuffled 
around[2]. So just a thought but a (consistent) hashing routine/ring... 
might be worthwhile to look into, and/or talk with those other projects 
to see what they have been up to.

My 2 cents,

[1] 
https://github.com/openstack/ironic/blob/master/ironic/common/hash_ring.py

[2] https://en.wikipedia.org/wiki/Consistent_hashing

>> 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.
> Suro> @Adrian, I think async model is to let user issue a sequence of
> operations, which might be causally ordered. I suggest we should honor
> the causal ordering than implementing the implicit retry model. As per
> my above proposal, if we can arbitrate operations for a given bay, given
> container - we should be able to achieve this ordering.
>
>
>
>>
>>> 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.
> Suro> +1
>>
>> 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-operations
>>>>
>>>>
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