Re: Externalizing I/O

On 10/28/2016 1:57 PM, Justin Taylor wrote:

> Sam:
> I'm trying to solve the problem of changing existing columns, not adding new columns.

Justin,

Buck gave a pretty clear answer.

Granted, it may be a bit more difficult changing an existing column in an old database. I think it means something like this:

* Find all the programs that don't use the field.
* Change them to a use new logical that does not contain the field to be changed. You will probably need more than one logical because of key requirements.
* Install as you go along.
* Create new logical(S) containing the field to be changed, and modify the program that use the field to be changed to use the new logical(s)
* Install as you go along.

At this point, no data or program logic has changed. Your system should continue as always.

* Make the changes to the field in the PF definition and the LF(s) containing the changed field.
* Modify the programs that use the changed field and test them.
* Finally, change the data and install the new LF(s) and programs. (All without worrying about those programs that don't use the field.)

As far as I know, using LFs to isolate data (or as Buck says, a projection) has been available since the beginning AS/400. Maybe even in the SYS/38 predecessor. But nearly all of us failed to use the capability, probably because we didn't understand the concept or it was easier not to.

In the concept of "Externalizing IO", this seems more practical to me. And probably more efficient than Getters and Setters.

Sam