How do you use MyBatis with multiple data sources?

Supporting multiple databases in a single MyBatis application generally means configuring a separate SqlSessionFactory (and corresponding DataSource) per database, with mappers explicitly associated with the correct factory rather than assuming a single, application-wide default.

@Configuration
@MapperScan(basePackages = "com.example.mapper.primary", sqlSessionFactoryRef = "primarySqlSessionFactory")
public class PrimaryDataSourceConfig {
    @Bean
    @Primary
    public DataSource primaryDataSource() { /* ... */ }

    @Bean
    public SqlSessionFactory primarySqlSessionFactory(DataSource primaryDataSource) throws Exception {
        SqlSessionFactoryBean factoryBean = new SqlSessionFactoryBean();
        factoryBean.setDataSource(primaryDataSource);
        return factoryBean.getObject();
    }
}

In a Spring Boot application, this typically means defining separate @Configuration classes for each data source, each with its own DataSource, SqlSessionFactory, and @MapperScan pointed at a distinct base package — the sqlSessionFactoryRef attribute is what ties a specific package of mapper interfaces to the correct factory, so mappers for database A aren't accidentally wired against database B's connection.

Transaction management also needs care in a multi-data-source setup: each data source typically needs its own PlatformTransactionManager bean, and operations spanning both databases within a single logical transaction generally require a distributed transaction manager (like JTA) rather than Spring's simpler, single-resource @Transactional handling, since a plain local transaction manager can't atomically coordinate commits across two entirely separate database connections.