this is supposed to be a three state flag  i use it for both detecting if we can stop checking for the host to schedule to and for storing default results  i stop checking in two cases  first when a wrong id is supplied or the host is found we can safely assume that there will not be any other hosts assosiated with specified volume  if there are such hosts sth is probably very wrong and my small optimization will not do any more harm we set it to false and return it  on the other hand a user may forget to supply or decide not to supply volume id and in this case all hosts should pass  in this case i set filtering result to true and use it as return value  if on the yet another hand it is set to none that means that we have a valid volume id and are in process of filtering  i did it with two variables at first  with self keep searching and self default search result  but this approach appeared to be excessively verbose  so i desided to use a three state flag instead 