Another important aspect of concurrency troubleshooting concerns replicated environments.

When troubleshooting replication, you need to remember that a master server is always multithreaded, whereas the slave runs all updates in a single thread.^[8] This affects performance and consistency during replication, regardless of the binary log formats or options you use.

The majority of replication issues involve data inconsistency, meaning that the data on the slave is different from that on the master. In most cases, MySQL replication takes care of data consistency, but sometimes you can still experience issues, especially if you use statement-based replication. This section focuses on consistency, with a few words at the end about how performance can be affected by replication.