You might find this plugin nifty if you have multiple application servers processing requests. The Perlbal BackendHeaders plugin appends X-Backend headers with which backend served the request.

Update 06/26/09 Now on github perlbal-plugin-backendheaders

syris:~ victori$ curl -I http://fabulously40.com/questions HTTP/1.1 200 OK Server: nginx/0.7.52 Content-Type: text/html; charset=utf-8 Expires: Thu, 01 Jan 1970 00:00:00 GMT Content-Language: en X-Backend: 72.11.142.91:8880 X-Dilbert: If you have any trouble sounding condescending, find a Unix user to show you how it's done Content-Length: 48046
package Perlbal::Plugin::BackendHeaders;

use Perlbal;
use strict;
use warnings;

#
# Add $self->{service}->run_hook(‘modify_response_headers’, $self);
# To sub handle_response in BackendHTTP after Content-Length is set.
#
# LOAD BackendHeaders
# SET plugins        = backendheaders

sub load {
    my $class = shift;
    return 1;
}

sub unload {
    my $class = shift;
    return 1;
}

# called when we’re being added to a service
sub register {
    my ( $class, $svc ) = @_;

    my $modify_response_headers_hook = sub {
        my Perlbal::BackendHTTP $be  = shift;
        my Perlbal::HTTPHeaders $hds = $be->{res_headers};
        my Perlbal::Service $svc     = $be->{service};
        return 0 unless defined $hds && defined $svc;

        $hds->header( ‘X-Backend’, $be->{ipport} );

        return 0;
    };

    $svc->register_hook( ‘BackendHeaders’, ‘modify_response_headers’,
        $modify_response_headers_hook );
    return 1;
}

# called when we’re no longer active on a service
sub unregister {
    my ( $class, $svc ) = @_;
    $svc->unregister_hooks(‘BackendHeaders’);
    $svc->unregister_setters(‘BackendHeaders’);
    return 1;
}

1;