What is the process for linking two MongoDB databases in Spring Boot?

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To link two MongoDB databases in Spring Boot, you can follow these steps:

1. Add the MongoDB Driver Dependency to your pom.xml or build.gradle file.
2. Configure your application.properties file or application.yml file with the required properties for both databases.
3. Create two separate configurations for each database. For example, you can create two configurations named mongoConfig1 and mongoConfig2.
4. In each configuration, create a Bean of MongoClient type using the MongoClients.create() method and pass the configuration properties for each database respectively.
5. Create two MongoDB templates, one for each database, by using the appropriate MongoClient bean in each template. For example, you can create two templates named mongoTemplate1 and mongoTemplate2.
6. Use the appropriate MongoDB template in your MongoDB repository interface by annotating the collection name with the @Document annotation, and passing the corresponding template name as a value to the annotation.

Here is an example of how to link two MongoDB databases in Spring Boot:

application.properties:

spring.data.mongodb.first.uri=mongodb://localhost/test1
spring.data.mongodb.first.database=test1

spring.data.mongodb.second.uri=mongodb://localhost/test2
spring.data.mongodb.second.database=test2

MongoConfig1.java:

@Configuration
@EnableMongoRepositories(basePackages = "com.example.demo.repository.test1",
        mongoTemplateRef = "mongoTemplate1")
public class MongoConfig1 {

    @Bean(name = "mongoClient1")
    public MongoClient mongoClient() {
        return MongoClients.create("mongodb://localhost/test1");
    }

    @Bean(name = "mongoTemplate1")
    public MongoTemplate mongoTemplate() {
        return new MongoTemplate(mongoClient(), "test1");
    }
}

MongoConfig2.java:

@Configuration
@EnableMongoRepositories(basePackages = "com.example.demo.repository.test2",
        mongoTemplateRef = "mongoTemplate2")
public class MongoConfig2 {

    @Bean(name = "mongoClient2")
    public MongoClient mongoClient() {
        return MongoClients.create("mongodb://localhost/test2");
    }

    @Bean(name = "mongoTemplate2")
    public MongoTemplate mongoTemplate() {
        return new MongoTemplate(mongoClient(), "test2");
    }
}

UserRepository.java:

@Repository
@Document(collection = "user", templateRef = "mongoTemplate1")
public interface UserRepository extends MongoRepository<User, String> {
    // define your repository methods here
}

ProductRepository.java:

@Repository
@Document(collection = "product", templateRef = "mongoTemplate2")
public interface ProductRepository extends MongoRepository<Product, String> {
    // define your repository methods here
}

By following these steps, you can successfully link two MongoDB databases in Spring Boot.