@Override
  public int hashCode() {
    final int prime = 31;
    final int result = prime * super.hashCode() + (this.runId != null ? this.runId.hashCode() : 0);
    return result;
  }
Id used to invalidate DrainMessages between runs. Ties to app.run.id from config.
samza-api/src/main/java/org/apache/samza/system/DrainMessage.java