You can configure your load balancer in different ways, depending on your goals and environment, as described in the following sections:
The most common way to deploy the load balancer is with a cluster or clusters of server instances. By default all the instances in a cluster have the same configuration and the same applications deployed to them. The load balancer distributes the workload between the server instances and requests fail over from an unhealthy instance to a healthy one. If you’ve configured HTTP session persistence, session information persists when the request is failed over.
If you have multiple clusters, requests can be load balanced across clusters but are only failed over between the instances in a single cluster. Use multiple clusters in a load balancer to easily enable rolling upgrades of applications. For more information, see Upgrading Applications Without Loss of Availability.
Requests cannot be load balanced across clusters and stand-alone instances.
It is also possible to configure your load balancer to use multiple stand-alone instances, and load balance and failover requests between them. However, in this configuration, you must manually ensure that the stand-alone instances have homogenous environments and the same applications deployed to them. Because clusters automatically maintain a homogenous environment, for most situations it is better and easier to use clusters.