code: protected String lookupServerURL(DataBinder binder) {
          _logger.info("STANDARD lookupServerURL: servers=" + _servers + ", selector='" + _selector + '\'');
          if (_servers != null) {
              if (_selector != null) {
                  return _servers.get(binder.get(_selector));
              } else if (_servers.size() == 1) {
                  return _servers.values().iterator().next();
              } else {
                  return null;
              }
          } else {
              return null;
          }
      }
docs: Looks up server URL based on request parameters in the data binder.
  
  The default implementation uses a servers looup map and a selector parameter. You may choose not to use a selector if the
  lookup map contains only 1 entry.
  
  @return the URL to a remote server, or null if lookup fails