Linux HA – Heartbeat, ldirectord, apache, mysql, debian
Today, the internet is booming. Millions of people are viewing billions of pages. Some of the larger sites require robust hardware setups if the content they are serving is remotely complex, involving things like relational databases and dynamic content. We are going to look at a practical solution that is both cost effective and scalable to handle varying loads. High availability and load balancing is a technique that we can implement with apache and mysql using Heartbeat, a small, open source software package.
There are many ways this package can be set up. What we require is a number of physical servers, each behind a router. Our goals here are to have Fault Tolerance, as well as performance scalability.
Simple Linux HA – Scalable Performance
Our first configuration lacks some high availability features. It has a single point of failure, the load balancing machine Linux HA-1. Other possible points of failures are the router, and the network file storage. These two problems can be solved by looking elsewhere and are not necessarily in the scope of this article.
This is our original layout and has served quite well in many cases. It is simpler to set up and scales well as we add more machines.
The MySQL servers are set up in master-master replication mode. So a write on either of the machines will replicate to the other and vice-versa.
The performance here is achieved by using the ldirectord daemon running on Linux HA-1. It acts as both the apache server and the mysql server. It keeps track of open connections on each machine, and routes the next new connection to the machine (apache or mysql) to the least loaded host (an algorithm chosen in the ldirectord configuration).
An example on how to set this up in a how-to fashion can be found at our blog http://programminglinuxgames.blogspot.com/2010/09/load-balancing-web-servers-using.html
Note this example does not use heartbeat. Heartbeat is a useful tool that will give you the fault tolerance in this set up. Below we will discuss more
Linux HA – Fault Tolerance and Scalable Performance
The next logical step in this is to remove the single point of failure, the Linux HA-1 machine. What we can do is set up heartbeat on 2 machines, and allow one to take over if the other fails at any time.
In order for one to take over for another, we will have both of these machines using a “virtual” network interface. This is just a 2nd IP address, using the virtual interface supported in linux. The circle surrounding Linux HA-1 and Linux HA-2 represents this network interface. You must assign it a separate, new IP address, and have your router route all traffic to that virtual IP address.
More Information
http://www.linux-ha.org/wiki/Main_Page – Linux HA
http://www.ultramonkey.org/3/linux-ha.html – Ultramonkey (Heartbeat and Ldirectord)
http://en.wikipedia.org/wiki/Linux-HA – Wikipedia
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