How to Configure and Use Multiple Data Sources in Spring Boot with Spring Data JPA?

# Primary Data Source
spring.datasource.url=jdbc:h2:mem:default
spring.datasource.username=sa
spring.datasource.password=

# Additional Data Source
additional.datasource.url=jdbc:h2:mem:additional
additional.datasource.username=anotheruser
additional.datasource.password=anotherpassword

<code class="java">@Configuration
@EnableTransactionManagement
@EnableAutoConfiguration
@EnableJpaRepositories(
        entityManagerFactoryRef = "entityManagerFactory",
        transactionManagerRef = "transactionManager",
        basePackages = {"com.example.repository.primary"})
public class PrimaryDataSourceConfig {

    @Bean
    public EntityManagerFactory entityManagerFactory() {
        // Create and configure EntityManagerFactory manually
    }

    @Bean
    public PlatformTransactionManager transactionManager() {
        // Create and configure PlatformTransactionManager manually
    }
}</code>

<code class="java">@Configuration
@EnableTransactionManagement
@EnableJpaRepositories(
        entityManagerFactoryRef = "additionalEntityManagerFactory",
        transactionManagerRef = "additionalTransactionManager",
        basePackages = {"com.example.repository.additional"})
public class AdditionalDataSourceConfig {

    @Value("${additional.datasource.url}")
    private String url;

    @Value("${additional.datasource.username}")
    private String username;

    @Value("${additional.datasource.password}")
    private String password;

    @Bean
    public DataSource additionalDataSource() {
        // Create and configure DataSource manually
    }

    @Bean
    public EntityManagerFactory additionalEntityManagerFactory() {
        // Create and configure EntityManagerFactory manually
    }

    @Bean
    public PlatformTransactionManager additionalTransactionManager() {
        // Create and configure PlatformTransactionManager manually
    }
}</code>

<code class="java">@Service
@Transactional("transactionManager") // Use primary transaction manager
public class PrimaryService {

    // ...
}

@Service
@Transactional("additionalTransactionManager") // Use additional transaction manager
public class AdditionalService {

    // ...
}</code>