Why do I need two load balancers? High availability
The reason is actually quite simple; you are seeking performance and continuous uptime. You will have understood the damage of losing contact with your internet clients and stakeholders and what this will cost your company. Based on the results from the Ponemon Institute downtime costs as much as $5,000 per minute for website owners on average which converts into a staggering $300,000 per hour. Clearly it is important to take all the necessary steps to ensure business continuity, for this reason deciding to deploy your load balancers in High Availability or HA mode makes perfect sense.
How many load balancers should I actually have available?
It totally depends on the budget that you have available. If you are able to deploy 2 load balancers in HA mode you already have an advantage in that you can take either load balancer down for maintenance without disruption to the service you provides your website visitors. If your budget will extend to it having 2 load balancers supported by a development, a third unit would be ideal, as you can test configurations before applying them to the production units. Some companies have settled for a 5 load balancer deployment; 2 for production use, two for testing new configurations ready to be deployed and one spare for development work.
H/A deployments are going to be expensive aren’t they?
If you are used to Radware or F5 pricing you could have a point, but once again it depends what budget you have available. However if you look at solutions from manufacturers such as Kemp Technologies or loadbalancer.org you will be looking at a completely different set of numbers. Kemp have the edge over loadbalancer.org as they have their own hardware appliance platform, but if you prefer a Dell appliance or Cisco with Kemp Technologies Virtual Load Master onboard these are available too.
Is Stateful Failover important in H/A?
Absolutely, what stateful failover means is that the secondary backup load balancer keeps records of the sessions created by the primary load balancer. Should the backup unit need to take over from a failing primary unit stateful failover allows the backup unit to continue to support the active sessions without service outage for the users. Make sure if you are looking at the most cost effective load balancers like those from Kemp Technologies, Barracuda or Loadbalancer.org that this functionality is supported.
If I’m using H/A load balancers for Microsoft applications is there any help available from the vendors?
The first thing you need to do is visit http://technet.microsoft.com/en-us/exchange/gg176682.aspx to check the load balancer you are interested in is recognized by Microsoft as an approved platform. The installation wizard that the vendor provides to help the installer to configure their load balancer is vitally important particularly for inexperienced installers. A good example is the Kemp Technologies Exchange 2010 installation guide, it is held up by a number of Microsoft MVPs as a guide that is “As good as it could get.”