code: private String selectDataCenterForPlacements(String... placements) {
          // Get the set of data centers that have local access to all the placements.  Then pick one deterministically
          // (pick the one that sorts first alphabetically) and designate that one as the one to perform maintenance.
          Set<DataCenter> intersection = null;
          for (String placement : placements) {
              Set<DataCenter> dataCenters = Sets.newLinkedHashSet(_placementFactory.getDataCenters(placement));
              if (intersection == null) {
                  intersection = dataCenters;
              } else {
                  intersection.retainAll(dataCenters);
              }
          }
          if (intersection == null || intersection.isEmpty()) {
              return null;
          }
          return Ordering.natural().min(intersection).getName();
      }
docs: Returns a data center with local access to all specified placements using a deterministic algorithm that
  always picks the same data center given the same set of placements.  Returns {@code null} if no data center
  has access to all specified placements.