GlassFish Clustering Concepts.

You can have many Domains. - To Manage Separate Applications in a hosting environment.

A Domain has DAS. (Domain Administration Server)

A Domain can have many instances running. Each instance runs in its own JVM.
Each instance will belong to only one domain.
An instance can be clustered or stand-alone.
For instances to be running in a domain, it MUST have a Node Agent.
A Node Agent is for creating and managing instances, and also acts as a watchdog,..fail over stuff.

First Create a Cluster.
Then Create a Node Agent and start it.
Next create an Instance, and note: when creating it you have to specify the node agent that is running. Optionally you can
specify the cluster that you created, if you don't specify it, then it will be taken as stand alone instance.
start the instance you created.
start the cluster if it is not automatically started.

when deploying you can specify the target..to a cluster.

Note : You can share libraries across a cluster.