Tuesday, September 18, 2007

Separation of...

You hear a lot about the merits of "separation of X and Y", for many values of X and Y. Content and presentation. Church and State. Running people and scissors. I thought I would just touch on this subject briefly before delving into more detail on the PL/SQL APIs for web development, like the HTP and HTF packages.

In web development generally, it's conventional wisdom that you want to separate content (HTML) and presentation (CSS). It's useful to understand how that works if your HTML is generated from PL/SQL. Essentially you're abstracting out certain parts of the markup so that maintenance is easier down the road.

Instead of:

<p style="border: 1px solid black;">

you're writing:

<p class="thing_that_deserves_solid_black_border">

filling in, of course, a semantically meaningful name for the class. It's self-documenting, smaller file size (assuming you don't use ridiculously long names), and the definition of how the classes look goes into a CSS file. The CSS in turn can be a static file, generated by mod_plsql the same as the HTML page, or even produced via some other language like Perl, so there's more flexibility in making global changes.

The same notion applies in pages generated from PL/SQL, but the simplicity of HTP.P() makes it easy to overlook. The Java world is hot on the "model/view/controller" architecture. How does that play out in PL/SQL?

Basically, it just means that you have separate procedures for manipulating data, and rendering the data as a web page. Something as simple as:

create or replace procedure generate_some page as

The conundrum is that you know in your head that HTP.P() is just stuffing the strings in a big data structure anyway, so why not code something like:

create or replace procedure generate_some page as
for item in (select * from some_table)
end loop;

But, what if you want to write stored procedures that can be called from Java, Perl, VB, or some other environment? In that case, they'll have all their own printing logic, and it will be convenient to have stored procedures that purely work on the data and don't do any HTP.P() calls of their own. You'll want to centralize things like cursor declarations, type declarations, and constants in packages, where otherwise you might feel fine with putting those inside a particular procedure, if only that procedure uses them.