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?
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?)
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 ;)
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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