How the HTTP Load Balancer Works

The load balancer attempts to evenly distribute the workload among multiple Application Server instances (either stand-alone or clustered), thereby increasing the overall throughput of the system.

The HTTP Load Balancer enables high availability of services deployed on Java EE Application Servers. While doing so, it fails over a session request to another server instance if the original servicing instance is detected to be unavailable or unhealthy to service a request. For HTTP session information to persist, you must be using the Cluster profile, have installed and set up the HADB, and configured HTTP session persistence. For more information, see Chapter 9, Configuring High Availability Session Persistence and Failover.

The load balancer does not handle URI/URLs that are greater than 8k.

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:

• Cookie Method: the load balancer plug-in uses a separate cookie to record the route information. The HTTP client (typically, the web browser) must support cookies to use the cookie based method. If the HTTP client is unable to accept cookies, the plug-in uses the following method.

• Explicit URL Rewriting: the sticky information is appended to the URL. This method works even if the HTTP client does not support cookies.

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.