Now, mod_fastcgi is configured and running. FastCGI supports connection via UNIX sockets or TCP/IP networking. This is useful to spread load among various backends. For example, php will be severed from 192.168.1.10 and python / ruby on rails will be severed from 192.168.1.11. This is only possible with mod_fastcgi.

Required utilities

You can spawn FastCGI processes using a dispatcher script or using spawn-fcgi utility, which is used to spawn remote FastCGI processes. spawn-fcgi included with lighttpd web server. You can grab source code from lighttpd.net or simply install it using lighttpd as follows (you need EPEL repo enabled under RHEL / CentOS / Fedora Linux):
# yum install lighttpd-fastcgi
# cp /usr/bin/spawn-fcgi /tmp
# yum remove lighttpd-fastcgi
# mv /tmp/spawn-fcgi /usr/bin/spawn-fcgi
lighttpd-fastcgi is FastCGI module and spawning helper for lighttpd and PHP configuration.

How do spawning php as TCP/IP remote app?

Use /usr/bin/spawn-fcgi as follows, enter:
# /usr/bin/spawn-fcgi -f /usr/bin/php-cgi -a 192.168.1.10 -p 9000 -P /var/run/php-cgi.fastcgi.pid -u apache -g apache
You can also jail php, using following syntax (make sure /var/run/ and /usr/bin/php-cgi exists inside jail directory):
# /usr/bin/spawn-fcgi -c /httpdjail -a 192.168.1.10 -p 9000 -P /var/run/php-cgi.fastcgi.pid -u apache -g apache -- /usr/bin/php-cgi
Where,

• -f /usr/bin/php-cgi: Filename of the fcgi-application
• -a 192.168.1.10 : Bind to ip address
• -p 9000 : Bind to tcp-port
• -P /var/run/php-cgi.fastcgi.pid: Name of PID-file for spawed process
• -c /httpdjail : Chroot to directory (security feature)
• -u apache : Change to user-id (security feature – drop root user privileges to apache user)
• -g apache : Change to group-id (security feature – drop root group privileges to apache group)

Configure Apache 2 mod_fastcgi connect to external PHP fcgi application

Above command will run php fcgi on 192.168.1.10:9000. Here is our sample setup:

1. 192.168.1.10 port 9000 : PHP FastCGI server
2. 192.168.1.11 port 9000 : Python or Ruby on rails cgi process
3. 202.54.1.20 port 80 : Apache 2 running mod_fastcgi (DocumentRoot set to /webroot/http)

Open your httpd.conf on 202.54.1.20, enter:
# vi /etc/httpd/conf/httpd.conf
Locate your domain VirtualHost configuration and append following two directives:

AddHandler php5-fastcgi .php
FastCgiExternalServer /webroot/http -host 192.168.1.10:9000

Here is complete snippet from one my box:

<virtualHost nixcraft.com:80>
    ServerAdmin webmaster@nixcraft.com
    DocumentRoot /webroot/http
    ServerName nixcraft.com
    ErrorLog logs/nixcraft.com-error_log
    CustomLog logs/nixcraft.com-access_log common
    AddHandler php5-fastcgi .php
    FastCgiExternalServer /webroot/http -host 192.168.1.10:9000
</virtualHost>

Save and close the file. Restart httpd:
# service httpd restart
Make sure iptables is configured to allow communication between public and private fastcgi server.

How do I configure PHP FastCGI via UNIX sockets?

UNIX sockets are faster as compare to TCP/IP sockets. However, they do not support remote spawning. Create /tmp/php.socket as follows:
# /usr/bin/spawn-fcgi -f /usr/bin/php-cgi -s /tmp/php.socket -u apache -g apache
Add following configuration to your httpd.conf virtual host:

AddHandler php5-fastcgi .php
FastCgiExternalServer /webroot/http -socket /tmp/php.socket

Save and close the file. Restart httpd, type:
# service httpd restart

mod_fastcgi has lots of other options. Please refer to Apache and mod_fastcgi documentation for further information.

A note about mod_fastcgi limitation

You can not load balance between multiple php backend. You need to use lighttpd or nginx or other reverse proxy software.