Question about the sequence to call epoch_result function

Dear all,

In SumStats::SumStat, the epoch_result function will be called for each key once for each epoch. But what is the sequence to call these functions? Is there a way to control which key's function is called first?

Another question is about the epoch_finished function. Is this function called after all the epoch_result are called for this interval or before?

Thanks a lot.