how to select a different virtual server's current pool?

Hi,

you could stick this in the irule (add in your other logic as needed)...This will check if the app_default has any active members if not use the shunt pool. So come upgrade time you just disable the app_default pool members. You could also apply this on the app VS.

if {[active_members app_default] >= 1}{
  pool app_default
  } else {
  pool app_shunt
  }

Another way without using separate pools is to create priority groups within the app_default pool, add the app_default servers in the higher priority group and the shunt ones in the other. In a steady state keep the shunt ones disabled/offline and when it's maintenence time enable the shunt pool members and disable the default pool members and drain the connections off as needed...

cheers