|Using Elastic Load Balancing / Elastic Load Balancing Overview|
This section describes the terminology and concepts you need for understanding and using the Elastic Load Balancing service.
When a new load balancer is created, Eucalyptus will assign a unique DNS A record to the load balancer. After the load balancer has been launched and configured, the IP address of its interface will be added to the DNS name. If more than one cluster is specified, there can be more than one IP addresses mapped to the DNS name.
The load balancers in one cluster will distribute traffic to the instances only in the same cluster. Application users can query the application service using the DNS name and the Eucalyptus DNS service will respond to the query with the list of IP addresses on a round-robin basis.
Eucalyptus generates a DNS name automatically (e.g., lb001.euca-cloud.example.com). Typically there will also be a CNAME record that maps from the meaningful domain name (e.g., service.example.com) to the automatically-generated DNS name. The CNAME records can be serviced anywhere on Internet.
In order to route traffic to healthy instances, and prevent traffic from being routed to unhealthy instances, the Elastic Load Balancing service routinely checks the health of your service instances. The health check uses metrics such as latency, RequestCount and HTTP response code counts to ensure that service instances are responding appropriately to user traffic.
Load balancers are the core of the Elastic Load Balancing service. Load balancers are special instances created from a Eucalyptus-provided VM image, and are managed by Eucalyptus. Instances off the image forward packets to your service instances using load-balancing software to ensure no instances are overloaded.
There may be a brief delay between the time that the first service instance is registered to a load balancer and the time the load balancer becomes operational. This is due to the virtual machine instantiation, and will not adversely affect the operation of your load balancer.
Each balancer VM will service only one load balancer. Launching unnecessary load balancers may be detrimental to your cloud's performance.
A service instance is an instance created from an image in your cloud. These are the instances that serve your application and users. The Elastic Load Balancing service monitors the health of these instances and routes traffic only to healthy instances.