It refers to the database used in data insertion, update and deletion. It only supports single primary database for now.
It refers to the database used in data query. It supports multiple replica databases.
It refers to the operation to asynchronously replicate data from the primary database to the replica database. Because of the asynchrony of primary-replica synchronization, there may be short-time data inconsistency between them.
Through this strategy, queries separated to different replica databases.