Round Robin Load Balancing

When to use Round Robin DNS load balancing

When you have decided that you need to deploy load balancers to improve the performance of your network servers you will also need to decide which method of server performance selection works best. The load balancers will need to be configured to use a load balancing technique that takes the decision from one server in the cluster to decide where to direct traffic to a dedicated load balancer that sits between the users and the servers and acts as a kind of traffic cop monitoring server performance and directing traffic to the content servers that offer the best response.

Deploying load balancers allows you to use a load balancing technique that splits the DNS server role from the content server and in this way means that you can optimize your network performance. Placing the DNS server role on the load balancer instead of the content server gives you a double advantage in that each server is dedicated to its role and therefore the risks of task overload are greatly reduced.

How does round robin content load balancing actually work?

Essentially this is a simple mechanism in which the content access request is responded to by the load balance in a rotational basis, the first request grants access to the first available content server giving its IP address and the second to the second server IP address and so on. The moment a server IP address has been given its IP address is moved to the back of the list of available IP addresses and gradually it moves back to the top of the list and becomes available again. The frequency that it returns to the top depends on the number of available servers in the round robin server cluster being used. A good way to think of this is a method of server allocation on a continuous looping fashion.

What are the best server load balancing environments  for using DNS round robin?

Geographically distributed web servers are best served by applying  DNS load balancing round robin server content distribution. As an example a company can have a single domain name and four absolutely identical company home pages on four physical servers based in Europe, Asia, North America and Africa. Each time a request for access comes in the first one will be sent to the European server the second to the one in Asia and then the third request goes to the North American server, as each IP address is given out it is automatically placed at the back of the list.

Simple to implement but maybe not the best form of load balancing

DNS round robin load balancing has one major advantage, it is extremely simple to implement, but it needs to be understood that it does have a number of potentially important drawbacks. These come from the very DNS hierarchy that it uses to perform its load balancing. Load balancers use smart techniques to measure and respond to TTL (Time To Live) times they will try to maintain a connection with a server to complete a user session using caching and TTL even if the server in question is failing and about to be unable to continue to service the network users.

This problem can result in unpredictability and even corrupt the DNS tables of all but the most advanced network load balancers. This means that servers that have failed continue to receive requests for providing content to users despite the fact that they are down and therefore no longer available. KEMP Technologies LoadMasters are able to deal with this problem perfectly by using the combination of stateful failover and Layer 7 application health checking  these load balancers can isolate and remove failed servers from the DNS round robin server list with little or no disruption to the users.

In summary round robin DNS techniques in combination with load balancing and server clustering methods are easy to implement, not difficult to maintain and can produce more than satisfactory results in certain situations, however implementing this method of load balancing is simple but by no means the most sophisticated and advanced and effective method of load balancing today.