[codes-ross-users] CODES LP Design Question

[Rob Ross]

I think the way to think about this is that the event payload carries the state associated with the operation and gets passed around between LPs as needed to simulate the steps. If you do this right, there shouldn't be local LP state associated with the operation -- everything is in the event payload that is needed.

There might be state at LPs associated with the LP itself, such as what objects exist or what names are in the namespace. Or not, if you are just assuming things exist for instance.   

John, Phil, and/or Misbah may need to correct me...of course. 

-- Rob

> On Apr 7, 2015, at 4:09 PM, Jenkins, Jonathan P. <jenkins at mcs.anl.gov> wrote:
> 
> Hi Joe,
> 
> So if I understand right, the context is you want to simulate some POSIX
> operations of interest, such as mkdir, and that the client is capable of
> performing the stat (the lookup) using purely local information (hence the
> self-event)?
> 
> In general, it's useful to think of events in these systems as RPC calls,
> where the event structure you are setting up contains the parameters, and
> there are corresponding return parameters that the calling LP expects to
> receive. In this frame of thought, both RPC calls and returns issue a
> discrete event. In your example, you could have your "mkdir" RPC event
> handler perform an additional "lookup/stat" RPC to check the path's
> existence, and upon receiving the "return" event, either make the
> directory or fail.
> 
> Unfortunately, event-driven programming more-or-less necessitates
> callback-heavy code, which can be quite awkward in some contexts. We've
> talked in the past about ways to at least standardize the way we do this
> in the context of CODES, but nothing has materialized as of yet.
> 
> Hope that helps. Something tells me that the assumption about clients
> being capable of performing metadata operations without outside
> interaction with e.g. a storage server is not quite right, though I could
> be misunderstanding.
> 
> Thanks,
> John
> 
>> On 4/7/15, 3:43 PM, "Joe Scott" <tscott2 at g.clemson.edu> wrote:
>> 
>> Hello All,
>> 
>> I am having a hard time wrapping myself around the programming paradigm
>> here, and I wonder if you might offer some guidance on a better way to
>> use CODES.
>> 
>> So I am trying to process these higher level tasks (POSIX tasks like
>> mkdir) by launching the subevents as separate processes.
>> 
>> The specific case that is tying me in knots is a user issuing a mkdir.
>> It launches the mkdir event handler, which needs to perform a lookup on
>> the path of the mkdir.
>> 
>> So I need to send an event from this client LP to itself to perform the
>> lookup.  But I also need the lookup, upon completion, to relaunch the
>> mkdir task.
>> 
>> Speaking it over with some of my lab mates, they seem to think I am
>> either overthinking it or trying to use the wrong tool for the job.
>> 
>> Is this a usecase you guys are familiar with?  Can you shed some light on
>> this situation?
>> 
>> I feel like there should be a way to do this without getting into
>> callback/completion function hell.
>> 
>> Thanks,
>> Joe Scott
>> Clemson University
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