Magic code check in RetryFailedPartitionJob

[woksin]

I don't understand this SuccessfulSteps check:

 /// <inheritdoc/>
  public override async Task OnCompleted()
    {
        if (State.Progress.SuccessfulSteps == 1)
        {
            var observer = GrainFactory.GetGrain<IObserver>(Request.ObserverKey);
            await observer.FailedPartitionRecovered(Request.Key);
        }
    }