With the reactive programming model in Spring Boot, "Reactive Data Repositories" allow for "asynchronous" database interaction. Reactive Data Repositories, part of Spring Data's reactive support, enable non-blocking database access by providing a reactive API for interacting with databases. This allows applications to efficiently work with data streams and handle concurrent requests without blocking threads.

In this scenario, you should use two separate transactions for reading and writing. The read transaction fetches the data, and the write transaction modifies the data and commits the changes to the database. This approach ensures that changes are committed while maintaining the integrity of the database. Using read-only transactions or in-memory databases for testing would not fulfill the requirement.

The @Qualifier annotation in Spring is used to specify the exact bean to be injected when there are multiple candidates of the same type. This helps resolve ambiguity in cases where there are multiple beans of the same type that could be injected. By using @Qualifier with the bean's name, you can explicitly indicate which bean should be injected, ensuring that the correct one is selected. It's particularly useful when you have multiple beans of the same type and need to specify which one should be used for injection.

Eureka is the Spring Cloud component primarily used for service discovery. It allows microservices to find and communicate with each other dynamically.

Circular dependencies can be resolved in Spring Boot by using the @Lazy annotation to delay the initialization of beans involved in the circular dependency. This allows Spring to break the circular reference. The other options do not effectively resolve circular dependencies and may lead to issues.

In Spring Boot, the @EnableCaching annotation is used to enable caching capability in the application. It allows you to use caching annotations like @Cacheable and @CacheEvict to control caching behavior. The other options are related to caching but not used for enabling caching at the application level.

In Spring Security, the class primarily responsible for holding the authenticated user's details is UserDetails. It represents user information, including username, password, authorities, and account status. SecurityContext is used to hold the security context, and the other options do not typically hold user details.

You can specify a default value for a request parameter in a Spring Boot controller method using the @RequestParam annotation with the defaultValue attribute. This attribute allows you to provide a default value that will be used if the parameter is not present in the request. Setting the default value in the application.properties file is not the correct approach, and creating a custom annotation is not a standard way to specify default values for request parameters.

In Mockito, you can simulate the throwing of an exception by a method using doThrow(Exception.class).when(mockedObject).methodCall(). This sets up the behavior that when methodCall is invoked on mockedObject, it will throw the specified exception.

In JUnit, Parameterized tests allow you to run the same test method multiple times with different sets of input arguments. This is useful for testing the same logic with various input values and ensuring that it behaves correctly for all cases.

In Spring Boot, the @Test annotation is predominantly used to indicate that a method is a JUnit test method. It marks the method as a test to be run by the JUnit framework.

The @FetchType.EAGER annotation in Spring Data JPA can be used to eagerly fetch the associated entities from the database. When an entity is loaded, all its associations marked with FetchType.EAGER are fetched immediately along with the main entity, reducing the number of database queries. However, you should use this option judiciously as it can lead to performance issues if overused.