Check for persistence entry before invoking HTTP::respond 200 content

You can use [HTTP::cookie exists ] to see if a persistence cookie was passed from the client to BIG-IP. By default, the name of the cookie is "BIGipServer" - so if your pool is named "http_pool" the cookie name is "BIGipServerhttp_pool" (You'll need to check your cookie persistence profile to see if the persistence cookie is named something other than the default.)

For example:

when HTTP_REQUEST {
    if { ![HTTP::cookie exists "BIGipServerhttp_pool"] } {
        if { [active_members [LB::server http_pool]] == 0 } {
            HTTP::respond 200 content "The application is currently unavailable"
        }
    }
}