[Overflow] DL and PL integration

[Brad C. <cha...@ar...>]

Hey guys;
	As I mentioned to Jarl yesterday, Jean-Marc and I had been
talking a bit about starting to more closely integrate the BL and PL,
especially with respect to returning errors and calculated values in a
node to be displayed. We kind of came up with a two step plan for
starting on this:

1. Write a stream class for the PL which derives from the C++ iostream
class. This class will be responsible for communicating directly with the
BL through a CORBA interface. The idea is that when nodes are running
they can call this stream class like:

returnValueStream << 'This is a node name' << 'This is some value info';

This kind of abstracts the actual communication to this stream class, and
allows you to turn streams on and off.

2. Write the CORBA interface and supporting code so that the stream class
can communicate with the BL. 

I started working towards this second goal and wrote an initial IDL file
for the communication, which I attached. The basic idea is that the BL
implements an interface through which the PL can return errors and
values. The PL implements a callback interface for whenever it returns an
error, so that the BL can either stop or continue a run when an error
occurs.

This is just a first start to work with, and I already have a number of
questions about the design:

1. How will we distinguish between different BL processes submitted to
the PL? There are two ways I could think of to do it:

a. Have a different BlReporter submitted for each run. The PL would then
somehow have to keep track of which BlReporter object is which.

b. Have an id number assigned to each submitted process that would be
given to a generic BlReporter (or we could have a general BL interface
that gave specific BlReporters based on id numbers). In this case the PL
and BL would have to track these id numbers.

2. How can the stream class return the stop or continue messages back to
the node? I'm not sure how we can achieve this kind of callback with the
stream setup, but this is probably just me missing something.

Anyways, this is just something to get started with. I'm excited to see
this start getting hashed out and seeing the parts be more integrated!
Talk with you all soon.

Brad