maintenance page for VIP with different hosts point to different pools if available else redirect to http://$host/maintenance

Another point to simplify the configuration. use relative links in redirect instead of absolute if it is on the same Web service...

Replace :

HTTP::redirect "http://$host/maintenance"

By :

HTTP::redirect "/maintenance"

And with the auto refresh in the javascript, redirect to the previous URL for a better user experience..

when HTTP_REQUEST {
     Check if the URI is /maintenance
    switch [HTTP::uri] {
        "/maintenance" {
             sets the timer to return client to host URL 
            set stime 10
             Send an HTTP 200 response with a Javascript meta-refresh pointing to the host using a refresh time
            HTTP::respond 200 content \
                " \
                Sorry! This site is down for maintenance." "Content-Type" "text/html" 
            return
        }
    }

     If the respective pool is down, redirect to the maintenance page else 
     send the request to the respective pool 
    if { [ string length [HTTP::host] eq test.com ] && [ active_members test_PROD_V ] < 1 } { 
        log local0. $host 
        HTTP::redirect "http://$host/maintenance?uri=[HTTP::uri]" 
        return 
    } else {
        pool test_PROD_V
    } 
}

The following irule is one configured for some customers:

when HTTP_REQUEST {
    switch [HTTP::path] {
        "/maintenance/logo.png" {
            HTTP::respond 200 content [ifile get "logo.png"] "Content-Type" "image/png"
            return
        }
        "/maintenance/maintenance.html" {
            HTTP::respond 200 content [ifile get "maintenance.html"] "Content-Type" "text/html"
            return
        }
        "/maintenance/error.html" {
            HTTP::respond 200 content [ifile get "error.html"] "Content-Type" "text/html"
            return
        }
        default {
            set dpool [LB::server pool]
            if { $dpool equals "" } { HTTP::redirect "/maintenance/error.html" ; unset dpool; return}
            if { [active_members $dpool] == 0 } { HTTP::redirect "/maintenance/maintenance.html"; unset dpool; return}
            unset dpool
        }
    }
}

when HTTP_RESPONSE {
  if { [HTTP::status] eq "404" } { HTTP::redirect "/maintenance/error.html" }
}