As far as I understand before two phase commit is even run a round trip communication to send the transactions to each site is needed. Each site excecutes their part of the transaction and when the coordinator gets a response from all sites then it runs two phase commit. This initiates the prepare phase, etc.
Why is it necessary to have the prepare phase be separate from the execution that precedes two phase commit? Is there a reason for not merging execution and the prepare phase, thus cutting out a round trip communication cost?
This is a followup to my previous question.