[Pulp-list] Sanity Check

[Jeff Ortel]

On 09/23/2010 11:08 AM, Bryan Kearney wrote:
> On 09/23/2010 11:58 AM, Jeff Ortel wrote:
>>
>>
>> On 09/23/2010 10:49 AM, Bryan Kearney wrote:
>>> On 09/23/2010 09:58 AM, Jeff Ortel wrote:
>>>> All,
>>>>
>>>> I'd like to get a quick sanity check on how I designed the dispatching
>>>> of API methods (that rely on RMI on the agent) through the task
>>>> subsystem. The approach is simple enough. Hopefully, the long write up
>>>> will not make it seem more complicated than it really is :)
>>>>
>>>> First, add classes to pulp.server.async.py that mirror the
>>>> pulp.server.agent.Agent class but performs RMI asynchronously by
>>>> default
>>>> and specifies the same correlation tag as the reply listener. The
>>>> pulp.server.agent.Agent does synchronous RMI by default and this way,
>>>> the replies are guaranteed to match what the reply listener is
>>>> listening
>>>> on.
>>>>
>>>> Second, the ReplyListener is added to pulp.server.async.py. It's job is
>>>> to listen for asynchronous RMI replies based on the same correlation
>>>> tag
>>>> specified when the RMI was invoked as described above. When replies are
>>>> received, the associated task is updated by calling either
>>>> Task.succeeded() or Task.failed(). The task is also updated with the
>>>> RMI
>>>> returned value or exception as appropriate.
>>>>
>>>> Third, I extended the pulp.server.tasking.task.Task as AsyncTask.
>>>> Unlike
>>>> its superclass, it expects its callable to be asynchronous. Further,
>>>> when the task runs the callable, the task's state is not advanced in
>>>> anticipation that its succeeded() or failed() methods will be called by
>>>> an external event.
>>>>
>>>> Last, and most significant, is how the API classes leverage (1-3). The
>>>> API methods such as ConsumerApi.installpackages() need to do something
>>>> on the agent and then update consumer history if the agent (RMI) was
>>>> successful. This is what makes this kind of tricky. So, what I decided
>>>> to implement this by subclassing the AsyncTask as
>>>> pulp.server.api.consumer.InstallPackages. This specialized task is
>>>> created/enqueued and returned by ConsumerApi.installpackages() giving
>>>> InstallPackages.install() as the task's target callable. When invoked,
>>>> it sends the RMI. Then, when the reply comes in and Task.succeeded() is
>>>> called by the queue. This method is overridden in InstallPackages which
>>>> calls super.succeeded() and then updates the consumer history.
>>>>
>>>> It looks like this:
>>>>
>>>> WS(controller)-->ConsumerApi.installpackages()-->FIFOQueue.enqueue(task)
>>>>
>>>> [ task runs Task.callable() ]
>>>> InstallPackages.install()-->AsyncAgent.packages.install()
>>>> [ RMI to agent ]
>>>> [ agent reply received on QPID queue ]
>>>> ReplyListener.succeeded(reply)
>>>> -->FIFOQueue.find(taskid)
>>>> -->InstallPackages.succeeded(reply)
>>>> -->Super.succeeded(reply)
>>>> -->ConsumerHistoryApi.packages_installed()
>>>>
>
>
> One follow up question.. what happens when ConsumerApi.installpackages()
> is version 2 and InstallPackages.install() is version 1?
>

They both live on the server and would be versioned together.

>
>>>
>>>
>>> Is the assumption that the task is stored in either the DB or a user
>>> session?
>>
>> Currently, tasks are persisted using an in-memory store. Eventually, we
>> need to add a non-volatile store so tasks are preserved across
>> reboot/restart.
>
> Thanks!
>
>>
>> Also... how does this patern change if I want to install
>>> packages at noon next tuesday?
>>
>> The messaging and agent frameworks already support the concept of
>> execution (or maintenance) "windows" for RMI calls. That is,
>> asynchronous RMI with a date/time window specified.
>>
>> So, we have two choices:
>>
>> 1) Dispatch though the task subsystem as defined here but include a
>> parameter to
>> specify the window as part of the RMI call to the agent.
>>
>> 2) Introduce task scheduling into the task subsystem.
>>
>
> So this would allow storing the task locally? One feedback we got was to
> not have the task execution dependent upon "the next time I poll"

In #1, the RMI request is stored on the client.  When the current time is within the 
specified window, the RMI is executed.  No communication (or polling) with the server is 
required.  Is that what you're asking?

>
> -- bk
>
>

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