iRule to send http status 400 code when pool is offline for a web service call

The fallback server (in http profile) should be able to handle this.

However, the redirect solution leaves much to be improved. You can implement an irule to read from an ifile to avoid any redirect, so the end-user can simply do a page reload to check if the site back up again.

This has been such an FAQ and use case that it warrants a feature request to F5.

Try this untested iRule.

when HTTP_REQUEST {
         if { [active_members [LB::server pool]] == 0 } {
        HTTP::respond 400 content "Bad Request page\
           400 - The request could not be understood by the server due to malformed syntax. \
           " "Content-Type" "text/html" 
            return
    }
}

You probably want to add some cache control directives to prevent the 400 page from being cached by any means.

Scratch that first irule. It was untested and has flaws. I agree with Jie about the no-cache. Give this one a try.

when HTTP_REQUEST {  
    if { [active_members [LB::server pool]] == 0 } { 
        HTTP::respond 400 content "  
          
         
            Bad Request page
         
         
            400 - The request could not be understood by the server due to malformed syntax.
         
       
        " Cache-Control No-Cache Pragma No-Cache"
    } 
 }

HTTP Status 400 pages shouldn't be cached, if clients are acting appropriately. You know, since all web clients always response exactly according to RFCs right? Right??

Still...not a terrible idea to force said behavior. Good call, Jie. ;)