URL path rewrite?

Try this, assuming pool node34 is defined with your nodes 3 and 4 as members, and pool node12 is defined with your nodes 1 and 2 as members:

when HTTP_REQUEST {
if { [string tolower [HTTP::host]] equals "www.example.com" } {
   if { [string tolower [HTTP::uri]] equals "/external" } {
     pool node34
   }
   else {
    pool node12
   }
}

Also keep in mind this isn't redirection, only pool selection based on the client's request. No redirection should be needed here.

Sorry that I didn't make the question accurate enough.

http://www.example.com/external -> node3 and node4 (and user sees http://www.example.com/external/... in the browser URL address bar)

In this case, node3/node4 don't know the uri /external/. I've to strip the /external/ part and pass the rest of uri to node3 and node4, then add it back in response to client. Is it possible? Thanks a lot.

Sure, just a simple change necessary to hide the /external from the nodes:

when HTTP_REQUEST {
if { [string tolower [HTTP::host]] equals "www.example.com" } {
   if { [string tolower [HTTP::uri]] equals "/external" } {
     http::uri "/"
     pool node34
   }
   else {
    pool node12
   }
}

Thanks, but seems I also have to rewrite the uri in HTTP response so that user's browser will see http://www.example.com/external/... no matter what content is returned. I tried to do following but seems didn't work.

when HTTP_RESPONSE {

STREAM::disable if {[HTTP::header value Content-Type] contains "text"}{ STREAM::expression {@www.example.com/@www.example.com/external/@} STREAM::enable } }

Thanks.