Redirect to fallback vip based on number of live nodes

I'm pretty new to F5, can only do basic setups. Company has a new setup request and I need help on this.

 

 

We have 9.1.1 build 54.6

 

 

Scenario:

 

2 vips (primary website & fallback website) pointed to same 12 nodes.

 

Simple healthcheck (get page, check word on return) on all 12 nodes

 

 

Request:

 

If the number of node that failed healthcheck is more than 8, redirect traffic to fallback website; then when the go below 3, redirect back to primary website.

 

 

the simple health check is:

 

GET /healthcheck.jsp

 

if SERVERUP, it's ok

 

otherwise fail.

 

 

Any help is appreciated.