20.3. Load Balancing Policies

Both the JBoss client-side interceptor (stub) and load balancer use load balancing policies to determine to which server node a new request should be sent. In this section, let us go over the load balancing policies available in JBoss Enterprise Application Platform.

20.3.1. Client-side interceptor architecture

In JBoss Enterprise Application Platform 5, the following load balancing options are available when the client-side interceptor architecture is used. The client-side stub maintains a list of all nodes providing the target service; the job of the load balance policy is to pick a node from this list for each request. Each policy has two implementation classes, one meant for use by legacy services like EJB2 that use the legacy detached invoker architecture, and the other meant for services like EJB3 that use AOP-based invocations.
  • Round-Robin: each call is dispatched to a new node, proceeding sequentially through the list of nodes. The first target node is randomly selected from the list. Implemented by org.jboss.ha.framework.interfaces.RoundRobin (legacy) and org.jboss.ha.client.loadbalance.RoundRobin (EJB3).
  • Random-Robin: for each call the target node is randomly selected from the list. Implemented by org.jboss.ha.framework.interfaces.RandomRobin (legacy) and org.jboss.ha.client.loadbalance.RandomRobin (EJB3).
  • First Available: one of the available target nodes is elected as the main target and is thereafter used for every call; this elected member is randomly chosen from the list of members in the cluster. When the list of target nodes changes (because a node starts or dies), the policy will choose a new target node unless the currently elected node is still available. Each client-side proxy elects its own target node independently of the other proxies, so if a particular client downloads two proxies for the same target service (for example, an EJB), each proxy will independently pick its target. This is an example of a policy that provides “session affinity” or “sticky sessions”, since the target node does not change once established. Implemented by org.jboss.ha.framework.interfaces.FirstAvailable (legacy) and org.jboss.ha.client.loadbalance.aop.FirstAvailable (EJB3).
  • First Available Identical All Proxies: has the same behavior as the "First Available" policy but the elected target node is shared by all proxies in the same client-side VM that are associated with the same target service. So if a particular client downloads two proxies for the same target service (e.g. an EJB), each proxy will use the same target. Implemented by org.jboss.ha.framework.interfaces.FirstAvailableIdenticalAllProxies (legacy) and org.jboss.ha.client.loadbalance.aop.FirstAvailableIdenticalAllProxies (EJB3).
Each of the above is an implementation of the org.jboss.ha.framework.interfaces.LoadBalancePolicy interface; users are free to write their own implementation of this simple interface if they need some special behavior. In later sections we will see how to configure the load balance policies used by different services.