Overview
In a recent blog-post Clustering in JBoss AS7/EAP 6 we showed how basic clustering in the new EAP 6 and JBoss AS 7 can be used. The EAP 6 is basically an AS 7 with official RedHat-support. Our cluster we described in that post was small and simple. This post will cover much more complex cluster structures, how to build them and how we can utilize the new domain-mode for our clusters. There are multiple ways to build and manage bigger JBoss cluster environments. We will describe two ways to do so: One using separating techniques also applicable to older JBoss versions and the other way using an Infinispan feature called distribution.
Scalability vs. Availability
The main challenge when building a cluster is to make it both highly available and scalable.
Availability for a cluster means: If one node fails, all the sessions on that node will be seamlessly served by another node. This can be achieved through session-replication. Session-replication is preconfigured and enabled in the ha profile in the domain.xml. Flat replication means that all sessions are copied to all other nodes: If you have got four nodes with 1GB memory for each of them, your cluster can only use 1GB of memory because basically all nodes store copies from each other. I. e. your cluster will not have 4*1GB=4GB memory. If you would add more nodes to this cluster you would not get more memory, you will even lose some memory due to overhead for replication. But you will get more availability and more important more network traffic due to replication overhead (all changes need to be redistributed to all other nodes). Let us call this cluster topology full-replication.
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