17.2.2. External Load Balancer Architecture

The HTTP-based JBoss services do not require the client to download anything. The client (for example, a web browser) sends in requests and receives responses directly over the wire using the HTTP protocol). In this case, an external load balancer is required to process all requests and dispatch them to server nodes in the cluster. The client only needs to know how to contact the load balancer; it has no knowledge of the JBoss Enterprise Application Platform instances behind the load balancer. The load balancer is logically part of the cluster, but we refer to it as “external” because it is not running in the same process as either the client or any of the JBoss Enterprise Application Platform instances. It can be implemented either in software or hardware. There are many vendors of hardware load balancers; the mod_jk Apache module is an excellent example of a software load balancer. An external load balancer implements its own mechanism for understanding the cluster configuration and provides its own load balancing and failover policies. The external load balancer clustering architecture is illustrated in Figure 17.3, “The external load balancer architecture for clustering”.

A potential problem with an external load balancer architecture is that the load balancer itself may be a single point of failure. It needs to be monitored closely to ensure high availability of the entire cluster's services.