From the scaladoc (it’s a private[spark] class so no way to find it outside the code):
Authority that decides whether tasks can commit output to HDFS. Uses a "first committer wins" policy. OutputCommitCoordinator is instantiated in both the drivers and executors. On executors, it is configured with a reference to the driver’s OutputCommitCoordinatorEndpoint, so requests to commit output will be forwarded to the driver’s OutputCommitCoordinator.
The most interesting piece is in…
This class was introduced in SPARK-4879; see that JIRA issue (and the associated pull requests) for an extensive design discussion.