[Engine-devel] Serial Execution of Async Tasks

[Michael Kublin]

>>>>>>>>>> Hi guys,
>>>>>>>>>>
>>>>>>>>>> As you may know the engine currently has the ability to fire
>>>>>>>>>> an
>>>>>>>>>> SPM
>>>>>>>>>> task, and be asynchronously be "woken-up" when it ends.
>>>>>>>>>> This is great, but we found the for the Live Storage
>>>>>>>>>> Migration
>>>>>>>>>> feature we need something a bit complex - the ability to
>>>>>>>>>> have a
>>>>>>>>>> series of async tasks in a single control flow.
>>>>>>>>>>
>>>>>>>>>> Here's my initial design for this, your comments and
>>>>>>>>>> criticism
>>>>>>>>>> would
>>>>>>>>>> be welcome:
>>>>>>>>>> http://wiki.ovirt.org/wiki/Features/Serial_Execution_of_Asynchronous_Tasks_Detailed_Design
>>>>>>>> -successful execution -
>>>>>>>> * "CommandBase iterates over its SPMAsyncTaskHandlers" - when?
>>>>>>> This is the new suggested format of executeCommand(). I'll
>>>>>>> clarify
>>>>>>> this too.
>>>>>>>
>>>>>>>> * If the second task is an HSM command (vs. SPM command), I
>>>>>>>> think you
>>>>>>>> should explain in the design how to handle such flows as well.
>>>>>>> HSM commands do not create AsyncTasks, as they do today - I
>>>>>>> will
>>>>>>> clarify this.
>>>>>>>
>>>>>>>> * Why do we need before task? can you give a concrete example
>>>>>>>> of what
>>>>>>>> would you do in such a method.
>>>>>>> Basically, /today/, command look like this:
>>>>>>> executeCommand() {
>>>>>>>     doStuffInTheDB();
>>>>>>>     runVdsCommand(someCommand);
>>>>>>> }
>>>>>>>
>>>>>>> endSuccessfully() {
>>>>>>>     doMoreStuffInTheDB();
>>>>>>> }
>>>>>>>
>>>>>>> endWithFailure() {
>>>>>>>     doMoreStuffForFailureInTheDB();
>>>>>>> }
>>>>>>>
>>>>>>> In the new design, the entire doStuffInTheDB() should be moved
>>>>>>> to a
>>>>>>> breforeTask of the (only) SPMAsyncTaskHandler.
>>>>>>>
>>>>>>>>
>>>>>>>> - I see you added SPMAsyncTaskHandler, any reason not to use
>>>>>>>> SPMAsyncTasK to manage it own life-cycle?
>>>>>>> Conserving today's design - The SPMAsyncTaskHandler is the
>>>>>>> place to
>>>>>>> add additional, non-SPM, logic around the SPM task execution,
>>>>>>> like
>>>>>>> CommandBase allows today.
>>>>>>>
>>>>>>>>
>>>>>>>> - In the life-cycle managed by the SPMAsyncTaskHandler there
>>>>>>>> is a
>>>>>>>> step
>>>>>>>> 'createTask - how to create the async task' can you please
>>>>>>>> elaborate
>>>>>>>> what are the options.
>>>>>>> new [any type of async task]

(I cleaned thread a little.)
The following design and it is implementation http://gerrit.ovirt.org/#/c/7956/
is bad.
I don't see any reason for creating a new SPMAsyncTaskHandler and especially in the 
way as it's done in patch.
The reason are following:
1. Performance , increased memory footprint, created CYCLIC REFERENCE.
2. Readability and robust of code: the code which is written as cyclic references is unreadable
   and difficult for debug.
3. Why I need a generic implementation and changes all over whole project because of 
   series of async commands, for me it is a private case?