How do you achieve multiple DB connections in a Spring Boot app?

Question

Accepted Answer

Multiple database connections in a Spring Boot app are set up by defining separate DataSource, EntityManagerFactory, and TransactionManager beans for each database, configured in distinct @Configuration classes.

Key Points:
• Each database gets its own @Configuration class with @Bean methods for its DataSource, EntityManagerFactory, and TransactionManager.
• @Primary marks one DataSource as the default when Spring needs to pick automatically.
• @Qualifier is used on injection points in repositories/services to specify exactly which DataSource or EntityManagerFactory to use.
• Repositories for each database are usually placed in separate packages, scanned via @EnableJpaRepositories(basePackages = ..., entityManagerFactoryRef = ...).
• Connection pool settings (e.g. HikariCP) should be tuned independently for each database based on its expected load.

Example:
An application that reads product data from a legacy Oracle database while writing orders to a newer PostgreSQL database defines two full sets of DataSource/EntityManagerFactory/TransactionManager beans, one per database, and wires each repository to the correct one with @Qualifier.

Code Example:
```
@Configuration
@EnableJpaRepositories(
    basePackages = "com.example.orders.repo",
    entityManagerFactoryRef = "ordersEntityManagerFactory",
    transactionManagerRef = "ordersTransactionManager")
public class OrdersDbConfig {

@Primary
    @Bean
    public DataSource ordersDataSource() {
        return DataSourceBuilder.create().url("jdbc:postgresql://.../orders").build();
    }
}
```

Interview Tip:
A concise interview answer is:

"I create a separate @Configuration class per database, each defining its own DataSource, EntityManagerFactory, and TransactionManager beans, and use @EnableJpaRepositories to point each repository package at the right factory. @Qualifier disambiguates injection where Spring can't infer which bean to use."