Activation Groups and Threadsafe

I checked the archives on Activation Groups and although some
things are explained I still don't understand.
I checked the archives on Threadsafe and  some questions come
to mind.
Long, long, ago when I was but a boy I wrote a MRT program
on a S38. (MRT = multi requesting terminal).  It worked fine
but it was a bit of a monster, keeping  track of each users state
resetting variables etc, etc, etc.
I quickly came to a conclusion that SRT and the way the S38
managed jobs was the greatest thing since sliced bread was
invented and was thankful that, MRTs I will never have to 
endure again.
Then along came activation groups.
What exactly are AGs for.  I know I can scope file overrides
within AGs, but to what end.  A long time ago I wrote programs
that used 'shared file opens' to minimise ODPs and PAG sizes
and I had to be careful that my file cursor was explicitly positioned
for every IO (or sometimes very sneakily I used the position of
the cursor as input for the next program,  confused the heck out
of the contractors)  but I came to the conclusion that what I was doing
was complexity for my own egos sake and I could do the same job
using simple programs that opened ODPs as needed.  OK so the
machine needed to do a bit more work, but no one noticed the difference
and the programs were much simpler.

Am I now supposed to go back to the dark ages of the MRT.
Is that what activation groups are about.  Do you seriously expect
me to break my job up into multiple sub-jobs (AGs), when I can
simply invoke another job.
Do AGs address the issue of having thousands upon thousands of jobs
all the same and then allow all these similar jobs to live within 
only ONE job, that now has thousands upon thousands of AGs.
What sort of application needs such a thing.
Does anyone have examples where they use AGs in a production
environment where they could NOT have done the same job
using SRT type programs and the usual AS400 JOB setup.

What are threads, I assume it is like the MRT I wrote where I had
to keep each users data separate.  I understand JAVA uses threads,
is that because JAVA does not use the AS400 job structure and
so has to reinvent the AS400 wheel. 
(I am guessing, I am ignorant in JAVA)

Where is the simplicity of the AS400 going where I could 
write a program and have as many users call it as they want and
the computer keeps everything in its place, now I am supposed 
to be able to stick things within AGs (named or otherwise).

Now I have to consider whether a command is Threadsafe or
not.  I notice that DSPFFD is not threadsafe.  What does this mean,
am I now able to get a file description in one job then somehow
change the file (would you ever do this) on the fly and then use
DSPFFD in another job the gets back the old file layout. 
How can a DSPFFD give back corrupt data threadsafe or not
if the file layout has not changed.

Is ILE and AGs and Threads simply a way of redoing what the 
AS400 has already  done but in a fashion that is compatible with
the UNIX / C world.  If so then I pray that I retire before all
this comes to pass.

Frank Kolmann

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