public String getOldestOffset() {
      return oldestOffset;
    }
@return The oldest offset that still exists in the stream for the
        partition given. If a partition has two messages with offsets 0
        and 1, respectively, then this method would return 0 for the
        oldest offset. This offset is useful when one wishes to read all
        messages in a stream from the very beginning. A null value means
        the stream is empty.
samza-api/src/main/java/org/apache/samza/system/SystemStreamMetadata.java