code: public static KeySnapshot globalSnapshot(long timeTolerance){
      KeySnapshot res = _cache;
      final long t = System.currentTimeMillis();
      if(res == null || (t - res.timestamp) > timeTolerance)
        res = new KeySnapshot((new GlobalUKeySetTask().doAllNodes()._res));
      else if(t - res.timestamp > _updateInterval)
        H2O.submitTask(new H2O.H2OCountedCompleter() {
          @Override
          public void compute2() {
            new GlobalUKeySetTask().doAllNodes();
          }
        });
      return res;
    }
docs: Cache-enabled call to get global key snapshot.
  User can provide time tolerance to indicate a how old the snapshot can be.
  @param timeTolerance - tolerated age of the cache in millis.
  If the last snapshot is bellow this value, cached version will be returned immediately.
  Otherwise new snapshot must be obtained by from all nodes.
  @return KeySnapshot containing user keys from all the nodes.