Sun Java System Application Server 9.1 High Availability Administration Guide

HTTP Load Balancing Algorithm

The Sun Java System Application Server load balancer, by default, uses a sticky round robin algorithm to load balance incoming HTTP and HTTPS requests.

When a new HTTP request is sent to the load balancer plug-in, it is forwarded to an application server instance based on a simple round robin scheme. If the request is for a session-based application, then this also includes a request for a new session. Subsequent requests from the same client for the same session-based application are considered assigned or sticky requests and are routed by the load balancer to the same instance. Hence, the name sticky round robin. Requests to a non session-based application and the first request for a session-based application are called unassigned requests. Stickiness is achieved by using cookies, or explicit URL rewriting. The load balancer determines the method of stickiness automatically.

The load balancer plug-in uses the following methods to determine session stickiness:

From the sticky information, the load balancer plug-in first determines the instance to which the request was previously forwarded. If that instance is found to be healthy, the load balancer plug-in forwards the request to that specific application server instance. Therefore, all requests for a given session are sent to the same application server instance.