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The two-phase commit protocol is designed to handle system and media failures in such a way that data integrity is preserved across all the participating database servers. The two-phase commit protocol performs an automatic recovery if a failure occurs.
The coordinator implements the commit handling in two phases. It first sends the prepare request to each of the participants. Once it receives a successful response from all the participants, the coordinator marks the transaction as prepared to complete. Then it sends the commit request to all the participants.
Advantages of Two Phase commit protocolThe data is consistent and always available. The database is always synchronized. All the database gets an update at once or none of them gets the update.
With two-phase commit, each system durably stores its updates before the transaction commits, so it can commit the transaction when it recovers.
No, they are not instructed to roll back because in the original poster's scenario, some of the nodes have already committed. What happens is when the crashed node becomes available, the transaction coordinator tells it to commit again.
Because the node responded positively in the "prepare" phase, it is required to be able to "commit", even when it comes back from a crash.
Summarizing everyone's answers:
One cannot use normal databases with distributed transactions. The database must explicitly support a transaction coordinator.
The nodes are not instructed to roll back because some of the nodes have already committed. What happens is that when the crashed node comes back, the transaction coordinator tells it to finish the commit.
No. Point 4 is incorrect. Each node records in stable storage that it was able to commit or rollback the transaction, so that it will be able to do as commanded even across crashes. When the crashed node comes back up, it must realize that it has a transaction in pre-commit state, reinstate any relevant locks or other controls, and then attempt to contact the coordinator site to collect the status of the transaction.
The problems only occur if the crashed node never comes back up (then everything else thinks the transaction was OK, or will be when the crashed node comes back).
Two phase commit isn't foolproof and is just designed to work in the 99% of the time cases.
"The protocol assumes that there is stable storage at each node with a write-ahead log, that no node crashes forever, that the data in the write-ahead log is never lost or corrupted in a crash, and that any two nodes can communicate with each other."
http://en.wikipedia.org/wiki/Two-phase_commit_protocol
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