@Override
  public String toString() {
    return String.format("%s%s%s", millis, SEPARATOR, 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