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

SURO suro.patz at gmail.com
Thu Dec 17 00:25:19 UTC 2015

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]
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. 
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'.

[1] - 

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