[saga-rg] SAGA thread model

[Thilo Kielmann]

Giving it another thought, I think it isn't as bad as my last mail assumed
to be.

The difference is that SAGA tasks aren't threads. They are kind-of single
operations (e.g., a single file.read, but no sequence of multiple of such 
operations). It is just that the obvious implementation in Java would be
using threads for everything asynchronous...

Still, if state sharing between multiple tasks (or between a task and
the main thread) is desired, we need to start out by defining data consistency
of all local and remote objects...


Thilo

On Mon, Jul 17, 2006 at 04:16:26AM +0200, Thilo Kielmann wrote:
> Date: Mon, 17 Jul 2006 04:16:26 +0200
> From: Thilo Kielmann <kielmann at cs.vu.nl>
> To: saga-rg at ggf.org
> Subject: [saga-rg] SAGA thread model
> 
> I am sorry to say, but SAGA's task model seems to me severely flawed.
> This is for two reasons:
> 
> 1. the "main" thread (executing sync operations) 
>    needs to be considered as yet another task
> 2. there must be a concise definition of the shared state. current solutions
>    are ad-hoc and mostly undefined.
>    shared state is:
>      - local objects, shared between multiple tasks of the same process
>        here: definition of synchronization between tasks
>      - remote objects, in the service(s)
>        here: definition of legal execution orders
> 
> so far, I can see only a few incidental definitions, but they are far from
> being concise.
> 
> "tasks in a bulk operation have to be independent"
> "a task cancel is doing 'best effort' but can not guarantee cancelation"
> 
> The latter, BTW, is a special case, because this is about connection 
> termination for which you can formally prove that there is no protocol that 
> can guarantee this AND notify both parties of successful termination.
> 
> To be constructive:
> what the task model must do first thing is
> - define tasks
> - define which data is shared between tasks and which concurrency control
>   happens on this shared data
> 
> That is the only way to define clearly what tasks will do in the event of
> sharing, really.
> 
> You may want to look at:
> 
> http://www.amazon.com/gp/product/0201695812/qid=1153102128/sr=2-3/ref=pd_bbs_b_2_3/002-2045221-3597631?s=books&v=glance&n=283155
> 
> This is: 
> Doug Lea, "Concurrent Programming in Java: Design Principles and Patterns"
> 
> This book uses 280 pages on objects, shared state and concurrency control
> before using 95 pages for the thread operations...
> 
> 
> -- 
> Thilo Kielmann                                 http://www.cs.vu.nl/~kielmann/
> 



-- 
Thilo Kielmann                                 http://www.cs.vu.nl/~kielmann/