Filed in by Brandon Woodward | November 30, 2009 12:52 pm
By Brandon Woodward, RHCE
In this article, I will walk you through creating a simple software load balancer using our Cloud Servers product. This is an entry-level job using simple readily available packages from any of the distributions repositories. I’ll be using Apache as our load balancer (Apache can be used as many things including a load balancer) in conjunction with the Apache module mod_proxy and mod_proxy_balancer. Both are available through CentOS and I’ll be using this as my base install.
The main thing that I want you to take out of this is that you can use Cloud Servers to scale horizontally. You can only scale a server vertically (i.e. more memory, processor) as a web head so far and eventually you won’t receive better performance from a faster server. This is when you need horizontal expansion and will need to add drone servers behind a smart host, each working a piece of workload.
We are going to use a total of 3 boxes to start out with:
• One 256M Cloud Server to be used as the load balancer
• Two Cloud Servers to be used as dumb web heads
The software for all three servers will be the same technically where they will be running the same packages. We’ll throw in only two software groups.
• Update your system to all the newest goodies:
• Apache and all its goodies, CentOS makes this ultra simple with the groupinstall feature:
• I will be installing links to a text based web browser in case we ever need to check that a particular web head is displaying the page it is supposed to be behind the load balancer (this is optional):
This is going to be the easiest part of the entire configuration. Since the webheads are really just drones, they’re only going to be doing grunt work and need no special configurations. That’s right, you don’t even open the httpd.conf file, just put a file called index.html in /var/www/html/index.html. In this file you can put any distinguishing characteristics you want. I put “It works, you are looking at WebHead #” where # is the numerical identifier of that particular webhead.
This is the tricky part of the operation. I’ll walk through each step and then bring it together at the end for you so you know what the end product should be. All of the configurations that we are going to go through should be placed at the bottom of /etc/httpd/conf/httpd.conf in a standard Virtual Host to work.
We are not working as a Forwarding Proxy (better known as an Open Proxy, and it’s bad news as it allows people to mask their identity by using your server to view web pages for them, it has its uses but not in this scenario). Turning off ProxyRequests will help avoid any unwanted traffic.
In this part of the Virtual Host we will be naming our web heads and declaring how we will be balancing. The BalanceMember directive is how you declare the webheads. Of course you can add as many as you would like, using these as templates. The ProxySet directive declares how you would like to balance. We’re going to use a “byrequest” balancing algorithm which is the same as a Round Robin, so for each new request you will get a new webhead. The order is sequential and although there are better and smarter algorithms out there, this is the easiest to configure and you need no knowledge of networking theory. All of this will be wrapped in <Proxy> tags, which is how Apache knows to send it to mod_proxy. The “balancer://mycluster” identifier is only an identifier although you could technically call it what you want as long as you put the “balancer://” prefix.
Keep in mind you will want to contact your webheads from the load balancer with their private IPs (this will keep your bandwidth charges down to a minimum by keeping all inter server communication between on the private network, where bandwidth is free).
This is a tool packaged with the mod_proxy_balancer tool and it allows you to make configurations from a gui tool through the web browser. It is viewable at “http://domain.com/balancer-manager.” Keep in mind that these changes die after you restart Apache. I won’t go over how to use this tool, but it is available to you.
This is the last part of the configuration and just adds the situations that will need to be proxied. We don’t want to proxy the balancer-manager, but we do want to proxy everything else.
If you have all this in your httpd.conf on your load balancer Cloud Server and start up Apache, you should be able to view your domain name that is properly pointed to your load balancer. When you hit refresh it should hop between your two webheads saying, “It works, you are looking at WebHead 1″ or “It works, you are looking at WebHead 2.” Congratulations, you are now balancing.
What I have done for you is combine all the things we’ve learned into a helpful packaged VirtualHost. You just need to trade out all the necessary values that are specific to your configuration like the domain name and the IP addresses to your webheads. Also, there are some security additions that are explained in the comments; everything is commented so you don’t have to refer back to this article to make changes later.
Source URL: http://www.rackspace.com/blog/simple-load-balancing-on-cloud-servers/
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