@Override
  public int compareTo(CheckpointId that) {
    if (this.millis != that.millis) return Long.compare(this.millis, that.millis);
    else return Long.compare(this.nanoId, that.nanoId);
  }
Serialization of {@link CheckpointId} as part of task checkpoints, in conjunction with {@link #deserialize(String)}.
@return the String representation of this {@link CheckpointId}.
samza-api/src/main/java/org/apache/samza/checkpoint/CheckpointId.java