Active standby selection of a server through Irule.

why you need an iRule for that? Priority group activation will achieve it.

Priority-based member activation is a feature that allows you to categorize pool members into priority groups, so that pool members in higher priority groups accept traffic before pool members in lower priority groups. The priority-based member activation feature has two configuration settings:

For the priority group activation setting, you specify the minimum number of members that must remain available in each priority group in order for traffic to remain confined to that group. The allowed value for this setting ranges from 0 to 65535. Setting this value to 0 disables the feature (equivalent to using the default value of Disabled). Priority group When you enable priority group activation, you also specify a priority group for each member when you add that member to the pool. Retaining the default priority group value of 0 for a pool member means that the pool member is in the lowest priority group and only receives traffic when all pool members in higher priority groups are unavailable.

If the number of available members assigned to the highest priority group drops below the number that you specify, the BIG-IP system distributes traffic to the next highest priority group, and so on.