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A 'Process Session Daemon' for CGI applications

PSD - A 'Process Session Daemon' for CGI applications


PSD - A 'Process Session Daemon' for CGI applications


This document is not specific to any version of the PSD application.


This document describes the motivation and usage of the Process Session Daemon (PSD) server and related components.


After working for several years building Websites and writing CGI scripts in the mid 1990s I moved on to other projects. Coming back to this environment recently, I finally had the opportunity to start using mod_perl with the Apache 2.x Web server. After looking forward to gaining some experience in this area for many years, when I finally did start down this path it was with decidedly mixed reactions.

Using mod_perl to access the Apache API is truly a wonderful thing. Having the full power of Perl available to access and manipulate the internal server data structures, and the ability to create simple Perl modules that can alter the server flow is more than I ever imagined that mod_perl could do.

On the other hand, struggling with the CGI acceleration environment that mod_perl provides was a far less exhilerating experience. Trying to modify an existing Perl module hierarchy where several different applications use a module of the same name to store and manage separate 'global' data was an exercise in frustration. Only the first application to load in a given Web server instance would 'win'. The other apps would fail to work properly, unless they just happened to load first in a different Web server instance. Trying to rework the various modules failed due to the way mod_perl manipulates the variable name space.

Trying to mix'n'match Perl versions is another area where mod_perl as a CGI accelerator is lacking. A minimal Perl 5.8.0 was included with the Apache server we are using. While, in addition to a few of the Apache-related modiles, it did include LWP and an XML parser, it didn't begin to have the modules in our Perl 5.6.1 distribution that I've come to rely on in day-to-day programming.

So I could either 'downgrade' mod_perl to Perl 5.6.1 or start up a fairly major project to build a Perl 5.8.x distribution (along with the thousand or so CPAN modules that I like to have around) and upgrade mod_perl to match. Neither option was a good fit given our current project schedule.

Yet Another CGI Accelerator

It occurred to me that perhaps a monolithic, cumbersom process to embed a Perl-based CGI environment into the Web server was not an optimal solution, especially when this and application modules are replicated into each and every Web server subprocess/thread/instance.

Why not create a simple external component that could maintain a ``persistent session environment'' for CGI processes that did not rely on any particular version of Perl or, for that matter, on any particular programming language. And, unlike FastCGI, it should use simple HTTP protocols instead of requiring a set of custom interface libraries.

Starting with several of the POE modules it was possible in a few days to model a simple demo based on a programming pattern that I call ``Server / Controller / SubProcess''. This is a hugely useful pattern that consists of three components: a network server, a process manager, and a set of child processes that perform long-running tasks in the background.

Then, using mod_perl and an example from Chapter 7 of the the book ``Writing Apache Modules'' (by Stein, MacEachern, Pub by O'Reilly), it was trivial to create a simple SSL proxy/tunnel from Apache out to the ``Process Session Daemon'' (PSD). Did I mention how amazing mod_perl is for Apache API access?

The result is an HTTP server daemon that uses SSL connections to provide security for logins and manages child processes to provide a persistent, high-speed cache to accelerate CGI scripts. And, since we've validated the user during login, each child process can run with the user and group id of the given user.

Add to that a slight modification to the module (via a subclass that adapts the I/O streams in CGI scripts to the asychronous POE-based event-driven model) and adapting CGI scripts to run in this new enviroment is a breeze.

The PSD server can run on the same host as the Web server, or it can just as easily run on any other machine. This is great for environments where it is not appropriate to run a full Web server on a given machine, but access to a user's private data would still be useful. Multiple PSD servers can be started to accomodate various load balancing needs.

Given the PSD architecture, a user can have multiple concurrent session processes running with each one in a different 'realm' (for want of a better term). This is very nearly equivalent to the 'Basic Authentication Realms' that exist when using standard 'htaccess' protection but perhaps another term would be better here.

The PSD server is a rudimentary HTTP/HTTPS Web server, so proxying session requests through Apache is not necessary. However, the proxy setup does allow using the full power and efficiency of Apache. Most of a user's browser requests during a given session may not need to access private data. Up to 90% or more of the user's requests could simply access the Apache server (for images, static HTML, dynamic HTML w/JavaScript and Style Sheets, PHP parsing, etc.), while perhaps only 10% or less of the requests would need to be proxied to the PSD server for private data access and/or session state.

And, as an extra bonus, no recompiling Apache or Apache modules is required. Simple config file directives are used to glue all of the components together.

Architecture Diagram


The server command is used to start, stop and control the PSD server process.

Clients can be users running Web browsers, and/or scripts that understand the HTTP/HTTPS protocols.

The Apache::UriProxy or Apache2::UriProxy classes can be used to seamlessly integrate an existing Apache Web with the PSD server.


The PSD server depends upon the following.

The basic POE distribution from CPAN, with the addition of the related the POE::Component::Server::SimpleHTTP manpage and the POE::Component::SSLify manpage classes. The the Time::HiRes manpage and the Digest::MD5 manpage classes are also required.

An optional but suggested component includeds either the Apache::UriProxy class (for mod_perl 1.x) or the Apache2::UriProxy class (for mod_perl 1.99, aka 2.x) that selectively proxy http/https requests to the session daemon.


See the the Apache2::UriProxy manpage class for two alternative proxy mechanisms. It is possible to use either mod_proxy or mod_rewrite to proxy URIs to the PSD server instead of adding a Perl module to your Apache Web server.


Chris Cobb, <>


Copyright (c) 2006-2007 by Chris Cobb. All rights reserved. This module is free software; you can redistribute it and/or modify it under the same terms as Perl itself.