The load balancer periodically fetches a list of all active application servers of an SAP system from its message server via HTTP. This list includes the host names as well as a static value indicating the capacity of each server (see below). This capacity is separated by stacks, i.e. a dual-stack instance has an ABAP capacity and a Java capacity.

When an instance is shutting down or scheduled for shutdown, the capacity of the instance will be set to 0. This means that stateful requests for existing sessions can still be sent to this instance but the instance should be taken out of load balancing, because it will not accept any new logons.

Servers will be dynamically added and removed from the list whenever the SAP message server detects a new server starting up or a running server shutting down (or crashing), respectively.

It is required that a load-balancing device removes crashed servers from its internal server list immediately for the following reasons:

• The message server may take up to ten minutes to detect a server crash. Therefore, the server list may not reflect the current state of the servers

• The load balancer should only update the list periodically (e.g. every several minutes)