RE: Separation of Presentation, BL, and I/O Tiers

> From: David Morris
>
> With a Mediator, you can use one area if you want (I use a user space)
> that serves many files. The Mediator maintains information about the
> files it is serving and the space where it is loading data.

How do you handle multiple concurrent cursors, then?  I assume you partition
the user space with your own linked list of pointers.  Once you've started
doing that, you then have to deal with fragmentation as cursors open and
close, right?  So this is the real difficult part - managing heap space
within your user space.  One way would be to have a fixed management
structure in your user space which included a pointer to the actual
(variably sized) data space.  Sort of a Cursor Control Block.


> With a Mediator, you can detect the recursive call (you can store a
> stack level by open file) and allocate another piece of memory.

If you're managing multiple cursors as I indicated in the previous
paragraph, this makes perfect sense.  You simply set aside another chunk of
your user space for another set of records.


> Mediator does not help much here, The best way to do this is to
> register
> an exit and a close routine or use a scope message, either one imposes
> a new requirement on the caller. Ideally you could call the Mediator to
> register the cleanup routine but IBMs APIs and scope messages do
> not support a level. The Mediator does simplify the cleanup effort
> because all of the allocation is done in a single module.

It's the same issue with the CCB layout I identified in the first section.
At some point, you're going to have to free up all those blocks.  The good
news is that a properly written caller would tell the Mediator to free up
individual blocks by issuing a "close cursor" command.  The other way is to
make sure there's an exit procedure identified that does a "close ALL
cursors".

Cripes, we're getting closer and closer to my early days of writing
operating system code.  After being hidden from our view for nearly 20 years
by RPG and OS/400, these old concepts are coming back into vogue <grin>.

Joe