code: @Synchronized
      public SegmentOutputStream getSegmentOutputStreamForKey(String routingKey) {
          if (currentSegments == null) {
              return null;
          }
          return writers.get(getSegmentForEvent(routingKey));
      }
docs: Selects which segment an event should be written to.
  
  @param routingKey The key that should be used to select from the segment that the event
  should go to.
  @return The SegmentOutputStream for the segment that has been selected or null if
  {@link #refreshSegmentEventWriters(Consumer)} needs to be called.