Controls what transactions are applied in parallel when using --slave-parallel-threads. Possible values: "optimistic" tries to apply most transactional DML in parallel, and handles any conflicts with rollback and retry. "conservative" limits parallelism in an effort to avoid any conflicts. "aggressive" tries to maximise the parallelism, possibly at the cost of increased conflict rate. "minimal" only parallelizes the commit steps of transactions. "none" disables parallel apply completely.