We have discussed what happens if one of the participants or the network fails during 2PC: if any of the prepare requests fail or time out, the coordinator aborts the transaction; if any of the commit or abort requests fail, the coordinator retries them indefinitely. However, it is less clear what happens if the coordinator crashes.
If the coordinator fails before sending the prepare requests, a participant can safely abort the transaction. But once the participant has received a prepare request and voted “yes,” it can no longer abort unilaterally—it must wait to hear back from the coordinator whether the transaction was committed or aborted. If the coordinator crashes or the network fails at this point, the participant can do nothing but wait. A participant’s transaction in this state is called in doubt or uncertain.
The situation is illustrated in Figure 9-10. In this particular example, the coordinator actually decided to commit, and database 2 received the commit request. However, the coordinator crashed before it could send the commit request to database 1, and so database 1 does not know whether to commit or abort. Even a timeout does not help here: if database 1 unilaterally aborts after a timeout, it will end up inconsistent with database 2, which has committed. Similarly, it is not safe to unilaterally commit, because another participant may have aborted.
Figure 9-10. The coordinator crashes after participants vote “yes.” Database 1 does not know whether to commit or abort.
Without hearing from the coordinator, the participant has no way of knowing whether to commit or abort. In principle, the participants could communicate among themselves to find out how each participant voted and come to some agreement, but that is not part of the 2PC protocol.
The only way 2PC can complete is by waiting for the coordinator to recover. This is why the coordinator must write its commit or abort decision to a transaction log on disk before sending commit or abort requests to participants: when the coordinator recovers, it determines the status of all in-doubt transactions by reading its transaction log. Any transactions that don’t have a commit record in the coordinator’s log are aborted. Thus, the commit point of 2PC comes down to a regular single-node atomic commit on the coordinator.