When you have more than one Action Server, policies begin executing on the Primary Action Server (PAS), and continue to execute on the PAS until the ASPT for that server is reached. After the PAS reaches its ASPT (and only then), policies begin executing on the Standby Action Server (SAS). The SAS is used only when the PAS reaches its ASPT. As soon as there is at least one free policy slot on the PAS, new policies resume executing there. Figure 3.3 shows the list of Action Servers and their priority, which can be set in the OIS Client.
Figure 3.3 Action Server priority set by the OIS Client
As an example, if your ASPT is set to 50 for all Action Servers and the PAS is running 50 policies, the next policies that start will run on the SAS. However, if at any point one or more of the PAS's policies complete, the PAS would then be assigned policies again until it again reaches its ASPT.
There is no consideration given to the fact that the SAS might be idle when assigning policies. The PAS must reach its ASPT before policies "spillover" onto the SAS. Should you have more than one SAS, policies begin loading on the PAS, then spillover to the first SAS, and only then to the second SAS. Policies always attempt to load on the highest-ranking Action Server. (You can change the ranking order and role of Action Servers using the OIS Client.)
Regardless of how many Action Servers are available, policies will fill the PAS before spilling over to the SAS and will fill the first SAS before spilling over to the second SAS and so on. This is not a load balancing mechanism; it is much more helpful to think about the mechanism as spillover.
After all the Action Servers have reached their ASPT, any additional policies that are started will be queued. These queued policies will run as soon as a policy slot frees up on any of the Action Servers, but will always prefer the highest-ranking Action Server if more than one become free.
Policies Assigned to Specific Action Servers
The only time policy execution does not follow the standard spillover model is when a policy is set to Override Default Action Server Roles. This setting changes which Action Server acts as the PAS and SAS for the context of that specific policy. If a policy is configured to run only on one Action Server, the effect would be the same as running the policy in a single Action Server environment.
Exercise caution when using Override Default Action Server Roles. If a policy is set to run on a specific Action Server and that server has reached its ASPT, the policy must wait for a free policy slot regardless of how many other Action Servers have availability. If more than one Action Server is set using Override Default Action Server Roles, you can configure the rankings between the servers (and those will spill over according to the normal spill over rules), but the policy will run only on Action Servers in that list, even if others are free. Figure 3.4 shows a policy using the Override Default Action Server Roles feature.
Figure 3.4 A Policy that is overriding the default Action Server settings