@Override
  public boolean equals(Object o) {
    if (this == o) return true;
    if (o == null || getClass() != o.getClass()) return false;
    CheckpointId that = (CheckpointId) o;
    return millis == that.millis &&
        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