For a long while MySQL only had single threaded replication, so all transactions are applied one at a time in a serial order. This meant ordering and correctness of application works (the data on the replica still matches the data on the primary), then they began to add parallel replication. This causes out of order problems (thread 1 applies before thread 2 leading to either a data mismatch or a conflict). There's some more discussion here about group commit and LOGICAL_CLOCK replication: dev.mysql.com/doc/refman/8.0/en/replication-optio…