The primary advantage of using reactive programming in Spring Boot applications is improved developer productivity. Reactive programming enables developers to write more concise and expressive code for handling asynchronous and event-driven scenarios. It simplifies complex, non-blocking operations, making it easier to work with asynchronous data streams and events, leading to more efficient and maintainable code.

In this scenario, you would use unit testing with mocked dependencies to isolate and test individual components of the microservice. This helps ensure that each component functions correctly in isolation. Integration testing with real external services can introduce complexity and is not suitable for ensuring the correctness of interactions among components. Manual testing and ignoring component interactions are not effective strategies.

In Spring Boot's reactive programming model, you can efficiently handle streaming of large result sets from a database by using the Flux API provided by Project Reactor. The Flux API allows you to work with reactive streams, which are ideal for handling asynchronous and potentially large datasets. It provides methods for transforming, filtering, and processing data in a non-blocking manner, making it suitable for scenarios where traditional synchronous JDBC calls may not perform efficiently.

In Spring Boot, the application.yaml file is used to define configuration properties in YAML format. YAML is a human-readable data format often preferred for configuration in Spring Boot. While Spring Boot also supports .properties files, they use a different format. Options 3 and 4 are variations of option 1 and do not represent valid Spring Boot configuration file names.

Spring Cloud primarily facilitates building microservices by providing essential tools for service discovery, load balancing, configuration management, and more. It simplifies the development of microservices-based applications.

The Refresh Token's purpose in OAuth2 is to enable the client to obtain a new access token without requiring the user to reauthenticate. It helps maintain the session's continuity by ensuring that the client can access protected resources even after the initial access token expires. The other options are not the primary purposes of the Refresh Token.

The @WebMvcTest annotation in Spring Boot includes, by default, only the beans annotated with @Controller, @ControllerAdvice, @JsonComponent, and Converter beans. It does not include @Component, @Service, and @Repository beans.

The @Autowired annotation is used to inject a bean dependency into a Spring component. It enables automatic injection of dependencies by type. When a bean of the required type is available in the Spring context, it will be injected into the annotated field or constructor parameter. This annotation simplifies the process of wiring components together in a Spring application.

To handle multiple database types in a Spring Boot project, you can configure multiple DataSource beans, each for a different database type. This allows you to specify which data source to use for each repository. It's a flexible and maintainable approach to supporting both relational and NoSQL databases in a single project.

Groups in Bean Validation allow you to perform partial validation by defining custom groups for different validation scenarios. By categorizing your validation rules into these groups, you can selectively apply validation based on the specific use case or context in your Spring Boot application. It's a powerful feature for fine-tuning validation logic.