A blog about Application Express and Oracle related topics.

Category Archives: Perl

Showing diagrams in Apex, with Graphviz and OHS

This Apex forum thread reminded me that I mentioned how to integrate Graphviz in Apex to somebody at last year’s OOW. Since the next OOW is right around the corner, it’s high time to finally publish how this can be done.

Setting up an Output Filter in Oracle HTTP Server

The Apache-based OHS allows to specify output filters, to post-process data. Such filters can be written in shell script or Perl. My example uses the latter.

In dads.conf, I entered

LoadModule perl_module “${ORACLE_HOME}/ohs/modules/”
PerlSwitches -I/home/cneumuel/perl
PerlModule CN::DotToSvgFilter

at the top level to load mod_perl, add my local Perl directory to Perl’s path and register my filter module. Then I added

PerlOutputFilterHandler CN::DotToSvgFilter

within the DAD’s <location> tags. This line tells OHS that all output from the DAD has to be post processed by my module.

In /home/cneumuel/perl/CN, I created the file

package CN::DotToSvgFilter;
use strict;
use warnings;
use IPC::Open2;
use base qw(Apache2::Filter);
use Apache2::RequestRec ();
use APR::Table ();
use Apache2::Const -compile => qw(OK);
use constant BUFF_LEN => 4096;
sub handler : FilterRequestHandler {
    my $f = shift;
    my $r = $f->r;
    if ($r->content_type() =~ /^text\/dot;/) {
        # convert dot to svg
        my $pid = open2(*Reader, *Writer, “/home/cneumuel/usr/bin/dot -Tsvg”) or die “can not run dot”;
        while ($f->read(my $buffer, BUFF_LEN)) {
            print Writer $buffer;
        close Writer;
        my $got = do { local $/; <Reader> };
        close Reader;
        waitpid $pid, 0;
        $r->headers_out->set(‘Content-Length’, length($got));
    } else {
        # pass through
        while ($f->read(my $buffer, BUFF_LEN)) {
    return Apache2::Const::OK;
This script checks if the content type is “text/dot”. That’s not an official mime type, we just use that internally to tell the filter when content has to be post-processed. If we get “text/dot”, it filters the input with /home/cneumuel/usr/bin/dot, which is called with a parameter that specifies svg as the output format. Otherwise, the script just passes the input data through.

After a restart of OHS, the filter is active.

Creating a sample Apex application that uses the filter

I created a new application with a blank page. On that page, I defined a HTML region “Org Chart” with the following region source:

< object
    type=”image/svg+xml” >
< /object >

I also created an Ajax callback on the page, called “org_chart_graph”:

— set mime type – converted by apache
sys.owa_util.mime_header (
    ccontent_type => ‘text/dot’,
    bclose_header => true );
— open graph
sys.htp.p(‘digraph orgchart {‘);
— empoyee nodes
for i in ( select empno,
             from emp )
              ‘ [label=”‘||
              replace(i.ename, ‘”‘, ‘\”‘)||
end loop;
— edges, via mgr
for i in ( select empno,
             from emp
            where mgr is not null )
    sys.htp.p(‘e’||i.empno||’ -> e’||i.mgr||’;’);
end loop;
— close graph
Finished! Here is the result:
Note that this is just a simple graph, Graphviz has many advanced options (clickable nodes and edges, custom shapes, etc.) and different layout algorithms that are just a few lines of code away. The gallery has some quite impressive examples.