Any participants with prepared but uncommitted transactions are then stuck in doubt. Since the coordinator’s log is on the application server’s local disk, that server must be restarted, and the coordinator library must read the log to recover the commit/abort outcome of each transaction. Only then can the coordinator use the database driver’s XA callbacks to ask participants to commit or abort, as appropriate. The database server cannot contact the coordinator directly, since all communication must go via its client library.
