Preparing for GUFE
If you've followed along either of the last two articles
in the Dabbling with Live Databases mini-series, you're
probably prepared for GUFE already. GUFE requires that your
host system is installed with Perl, the DBI module,
and the DBD module for the particular database system you
are using. For instance, on a Windows machine with Microsoft
Access, you would need the DBD::ODBC module; on a Un*x
system with MySQL you would need the DBD::MySQL module.
If none of this makes sense, it is very important that you
backtrack to the first two installments in this mini-series.
Caveat: if GUFE seems to stumble, especially if it fails to
provide a list of available tables to view, you
probably do not have the latest DBI and DBD modules
installed. Be sure that you do, and install them if necessary,
because older versions of these modules do not support
the ability to request a list of available tables.
Let's Walk GUFE
The bulk of this article is a walkthrough of the GUFE Perl
script. In doing so, we'll touch on a variety of
Perl issues, from those encountered last month to some
from much earlier in the Perl You Need to Know series. While
we will highlight segments of the GUFE script, you may
wish to consult the entire unmodified
code. Some of the code lines are quite long and may
need to be wrapped on this web page --
the unmodified reference code
contains the original line lengths.
First, it is worth noting that GUFE relies on the web
template technique that we saw in Part 4 of the Perl You
Need to Know. Briefly, this means that GUFE
"inserts" its output into a pre-existing HTML
document. This relieves GUFE of the need to recreate
the entire web page on each call, and provides the
web designer with much greater flexibility in designing
the surrounding elements of the page. Simply put, GUFE
dumps its output in place of a <!--FRONTEND-->
comment found within the
source of the HTML file.
Taking an overview of GUFE we can see that it is quite
simple. GUFE is built mostly of Perl subroutines, each
of which performs a focused action. Once the connection
to the database has been established, GUFE translates CGI
queries (which are created by the browser, usually based
on the settings of form fields) into SQL statements. These
SQL statements are passed to the database, and GUFE
retrieves the results, formatting them for the screen. Various
results, such as field labels, are hyperlinked with rigged
CGI queries that will, when clicked, intentionally cause
GUFE to generate certain SQL queries (for instance, the
field labels are rigged with sort parameters which in turn
are translated by GUFE into SQL statements with sorting
The easiest way to understand GUFE is from the inside out.
We'll walk through GUFE in several stages, starting,
of course, from the top.
#setup database parameters
#prepare for output to browser
GUFE begins by including our two crucial modules, CGI for
interaction with the web browser, and DBI for interaction
with the database.
For convenience, we assign several connection parameters
for the database: the DBD protocol ("ODBC"
in the case of Microsoft Access), the name of the database,
and a username/password if needed to connect to the
database. Some systems are case sensitive (Un*x), and some
are not (Windows); it's best to pay attention to case.
For example, the DBD protocol for MySQL must be all lowercase
"mysql" -- you can find the proper syntax
from the DBD module documentation for your database.
Standard CGI practice leads us to establish a handle for
this instance of the CGI object,
we output a standard HTML header to prepare the browser
for incoming data.