What is Clustering
Clustering is the technique of linking many computers together to act
like a single computer. Generally, a PHP cluster is composed of
multiple webservers running PHP individually which then is load-balanced
either by a hardware or a software based load balancer. Common examples
of hardware load balancer could be F5 BigIP, Nortel’s (Alteon),
AceDirector AD3 etc and software load balancer are Apache httpd server,
Nginx etc.
Requirements for PHP Cluster
To set a PHP cluster, following are required:
- Three Linux servers – One server as a load balancer and other
two as the PHP application nodes. They can run any Linux OS. We would be
using Debian 6 and CentOS 64bits edition in these three environments in
two separate examples. We will refer to load balancer node as LB and
the application nodes as s1 and s2 respectively from now.
- Apache httpd v2.2 with mod_rewrite, mod_proxy – Apache httpd
should be installed on all the three servers with the support of
mod_rewrite, mod_proxy, mod_proxy_http modules.
- PHP – PHP should be installed on the two Linux servers acting
as application nodes (s1 and s2). You can refer my earlier post – The
Perfect LAMP Stack to setup the application nodes. We will use PHP only
for testing our cluster setup.
Theory of Operation
The setup will make load balancer (LB) to act as the reverse proxy in
front of the application nodes (A1 and A2). When a client will send the
request to our server, the LB will pass it to the application nodes
based on a predefined method as set by us in the Apache httpd
configuration.
How to cluster with PHP Debian
After installing the above mentioned applications, to setup the PHP
clusters, now we only require to create Apache httpd configuration on LB
server to make it act like a reverse proxy.
Open the VirtualHost file default in /etc/apache2/sites-available
- ServerName will have your URL or ip for the load balancer (LB) server.
ServerName lb.example.com
- DocumentRoot will be your path to publicly available documents.
DocumentRoot /var/www/
- ProxyRequests off will prevent your load balancer to act as a forward proxy server.
Note: When you compile mod_proxy with Apache httpd, it can allow
you to use the server to act either as a forward proxy or reverse proxy
server. The above setting is just used for prevention.
ProxyRequests Off
- <Proxy> directive will allow you to enforce rules upon the
proxied content. Here you can set what constraint you want to apply on
the proxied content for e.g. you can only allow few ip range to access
the load balancer. In our case we are going to allow from all as it is
the load balancer in front of public.
<Proxy *>
Order deny,allow
Allow from all
</Proxy>
- ProxyPass directive will configure the url mapping from the
reverse proxy server to the application nodes. You can also configure
which url should be translated and sent to backend and which should not.
In the following settings, we have configured that when the
/balancer-manager is requested, it should not be sent to the backend
whereas all other requests should be forwarded there. The
balancer://mycluster/ defines our clusters where we will transmit the
load and which we will be configuring in next step. The stickysession
directive is used here for the sessions which I will explain later in
some other post. nofailover directive is related to sessions.
ProxyPass /balancer-manager !
ProxyPass / balancer://mycluster/ stickysession=PHPSESSID $
ProxyPassReverse / http://s1.example.com/
ProxyPassReverse / http://s2.example.com/
- Once again, we would be defining the <Proxy> directive,
but this time we will be using it to enhouse our balancer
configurations. The BalancerMember directive is used to add the add the
application nodes to the cluster and as of our configuration, we are
going to add our two application nodes a1 and a2. The route directive
value will be appended to the session id. The ProxySet directive is used
to define the additional balancer configuration parameters. In our
case, we are going to define the load balancing method which we are
going to use in the lbmethod parameter and that is byrequests. The other
available methods are bytraffic and bybusyness. To know more about load
balancing methods, please refer to http://httpd.apache.org/docs/2.1/mod/mod_proxy_balancer.html
<Proxy balancer://mycluster>
BalancerMember http://s1.example.com route=s1
BalancerMember http://s2.example.com route=s2
ProxySet lbmethod=byrequests
</Proxy>
- Next, we have defined the settings of the balancer-manager which
allows us to view the dynamic update of balancer members i.e. the
application nodes. You would be requiring to enable mod_status module in
Apache httpd to use the balancer-manager
<Location /balancer-manager>
SetHandler balancer-manager
Order deny,allow
Allow from all
</Location>
Note: We have explained the very basic settings here to setup the
Apache httpd to act as a reverse proxy load balancer. You can fine tune
the settings according to your needs and to know more about the
available options, please refer to http://httpd.apache.org/docs/current/mod/mod_proxy.html
- The complete settings which we have explained earlier can be seen below:
<VirtualHost *:80>
ServerName lb.example.com
DocumentRoot /var/www/
ProxyRequests Off
<Proxy *>
Order deny,allow
Allow from all
</Proxy>
ProxyPass /balancer-manager !
ProxyPass / balancer://mycluster/ stickysession=PHPSESSID $
ProxyPassReverse / http://s1.example.com/
ProxyPassReverse / http://s2.example.com/
<Proxy balancer://mycluster>
BalancerMember http://s1.example.com route=s1
BalancerMember http://s2.example.com route=s2
ProxySet lbmethod=byrequests
</Proxy>
<Location /balancer-manager>
SetHandler balancer-manager
Order deny,allow
Allow from all
</Location>
</VirtualHost>
- Save the default VirtualHost after making the above settings and
restart the server. That will complete our task of creating Apache
httpd as reverse proxy load balancer server.
Testing PHP Cluster
Now, to test your PHP cluster setup, create a file test.php in your
application nodes s1 and s2. The source code for the file would be very
simple and as below:
For s1 node,
<?php
echo 'Hi, I am from Server 1';
?>
For s2 node,
<?php
echo 'Hi, I am from Server 2';
?>
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