Directing traffic to a node based on a specific uri only if the node is up

You could do something like this.

when HTTP_REQUEST {
if {[HTTP::uri] starts_with "/someuri" } {
    if { [active_members other_pool] >= 1 } {
        pool other_pool
    } 

} }

This will check for the existence of a specific URI and if it matches then check that your secondary pool is available, based on health monitors, if it is up then the traffic for this uri will be passed to it. If the pool is down, or the traffic is for other uri's then it will follow the default logic of the VIP