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:

1. 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 9 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.
2. Apache httpd v2.5 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.
3. 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

• 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 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 https://httpd.apache.org/docs/trunk/mod/mod_proxy.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’;
 ?>

• After creating these files, try to load these files from your load balancer url i.e. http://lb.example.com/test.php for multiple times.
• To see, how your load balancer is performing and if you have set it up as explained earlier, you can visit the url http://lb.example.com/balancer-manager

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