[SAGA-RG] Multiple contexts of the same type

[Andre Merzky]

Quoting [Mathijs den Burger] (Apr 08 2009):
> 
> Hi all,
> 
> What is the 'right' behavior of a SAGA implementation when a user adds
> multiple contexts of the same type to a session?
> 
> Example: in an file browsing application on top of SAGA, a user creates
> two contexts of type "ftp": one with username "anonymous" and an empty
> password (for anonymous FTP access), and one with username "john" and
> password "1234" (to access a private FTP server).
> 
> Should a SAGA implementation try all contexts of type "ftp" in this
> case, or just one of them? 

All of them.

> And what if both contexts actually work for a
> certain URL, but result in different access rights?

This can't be resolved sensibly.  So, its up to the
implementation.


> In that case, a
> directory.cwd() call may succeed with one context but fail with another
> one due to insuffient permissions. Should an implementation then also
> retry the other available contexts in the cwd() method, or fail with an
> AuthorizationFailed exception?

In a late-binding implementation, one would expect the
adaptor to try the other adaptor on a second call, if that
fails.  More, the implementation may try another adaptor
altogether (e.g. gridftp, which can also talk ftp).

Yes, that may lead to name space inconsitencies.  That
cannot be helped.  The only way to avoid that is to use one
set of adaptors, and on set of contexts.


> Currently, the SAGA spec leaves the 'right' behavior entirely up to the
> application. 

Because this is in fact implementation specific.  An
early-binding implementation may not be able to try more
than one adaptor, and/or context.  A late-binding
implementation may be able to do so. 

The SAGA spec says though:

  p.81: "Multiple contexts can co-exist in one session – it
  is up to the implementation to choose the correct context
  for a specific method call."

Please check also p.18 "2.4.1 Early versus late binding",
which addresses some of your points.

> Should a multi-adaptor SAGA implementation leave the
> behavior to the adaptors? In that case, different adaptors may do
> different things. One adaptor may try all contexts with the right type,
> while another one may only try just one. Some adaptors may only try
> multiple contexts in the constructor, while others may also do this in
> the individual methods. Is each implementation supposed to prescribe
> 'the right way' to adaptor writers? Should each adaptor document its use
> of contexts? 

Yes: all these points are implementation specific, and
should be documened by the implementation/adaptors.


> Should each adaptor mimic the behavior of the underlying
> middleware? (e.g. an SSH adaptor tries multiple keys automatically, but
> an FTP adaptor only tries one username/password combination). Or would
> it be desirable if the spec prescribes something here?

MW specific semantics should not be exposed, if possible.
Instead, I'd suggest that adaptors for one specific
implementation should behave consistently, no matter what MW
is used.  

Note to self: that semantics (what context are chosen, when
the are chosen early/late bindings, adaptor versus method
level binding, ...) should be part of an SPI specification,
which some people on this list are known to ponder about.
Hey Steve ;-)

Cheers, Andre.

> 
> best,
> -Mathijs
-- 
Nothing is ever easy.