MySQL semi-synchronous is a plugin mechanism on top of asynchronous replication that can offer better durability and even consistency. It helps in high availability solutions, but can in itself reduce availability. In this article, we will look at some basics and follow up to present scenarios requiring higher-level intervention to ensure availability and avoid split-brains.
As a quick recap, semi-synchronous replication is a mechanism where a commit on the primary does not apply the change onto the internal table data and does not respond to the user until the changelog is guaranteed to have been persisted (though not necessarily applied) on a preconfigured number of replicas. We limit our discussion to MySQL 5.7 or equivalent.