Booking.com uses MySQL parallel replication extensively with thousands of servers replicating. The presentation summarized MySQL and MariaDB parallel replication features including: 1) MySQL 5.6 uses schema-based parallel replication but transactions commit out of order. 2) MariaDB 10.0 introduced out-of-order parallel replication using write domains that can cause gaps. 3) MariaDB 10.1 includes five parallel modes including optimistic replication to reduce deadlocks during parallel execution. Long transactions and intermediate masters can limit parallelism.