Monitoring one node to control the status of another. Feasible?

There's a neat way to do this with iRules, and you can actually monitors to both pools.

• Create a pool for your application servers and apply an appropriate monitor. Attach this pool to your VIP.

• Create a separate pool for the 5th server and apply an appropriate monitor, but don't attach it to a VIP.

• Apply an iRule like this to your VIP:

  when HTTP_REQUEST {
      if { [active_members "server5_pool"] < 1 } {
           your 5th server monitor has failed - do something here.
      }
  }
  

So then if the 5th server monitor fails, the iRule can take some action, like presenting a maintenance page, or redirecting to a different URL.