Clustering and load balancing are two different things, although they usually are used together.
The IS cluster enables the automatic failover of services and processes between the clustered IS. So if one IS goes down another takes over. You need to code you flows and proccesses to be able to failed over , details are in the webMethods documentation. You can use IS cluster without a loadbalancer if you don’t have clients which need to access the servers.
You use a load balancer if
- you want to distribute the load of client requests between different servers
- have the client always connecting to an available IS automatically
you can use a loadbalancer against non clustered IS if the server side is completely stateless
It’s quite common to have an IS cluster together with a loadbalancer, so the cluster takes care of failover of services or processes and the loadbalancers guarantees the accessibility to clients.
If failover of running components is not needed you may also use equally setup IS sharing a common queue to broker and a loadbalancer to just distribute workload.