The primary role of the UserDetailsService in Spring Security is to load user details (including username, password, and roles) from a data store, typically a database. It's a fundamental component used for authentication and authorization, as it provides the necessary user information for the security framework to make access control decisions. The other options describe tasks related to authentication and authorization but are not the primary role of UserDetailsService.

The primary responsibility of an OAuth2 Authorization Server in a Spring Boot application is to authenticate users and grant access tokens to authorized clients. It does not store user passwords but rather validates user credentials and authorizes access to protected resources. It also does not manage user profiles and preferences, nor does it control database access; these are typically the tasks of the application's authentication system and database itself.

To optimize the performance of Spring Data JPA when dealing with large datasets, using PagingAndSortingRepository is recommended to read the datasets in chunks. This repository interface extends CrudRepository and provides methods for pagination and sorting, making it suitable for efficiently fetching and processing large datasets by breaking them into manageable chunks.

To customize access-denied behavior, you should implement a custom AccessDeniedHandler, which allows you to define how to handle access-denied situations. You then register this handler with Spring Security to apply it to secured methods.

In Spring Boot reactive programming, the map method is used to transform the items emitted by a Publisher. The map operation allows you to apply a function to each item emitted by a Publisher and produce a new Publisher with the transformed items. This is a common operation when working with reactive streams to perform data transformations.

In Spring Boot, the "@ConditionalOnProperty" annotation is used to conditionally apply an auto-configuration based on the value of a configuration property. This allows developers to customize the application's behavior depending on the configuration values, making it a powerful tool for managing application behavior in different environments or scenarios.

To implement dynamic DataSource routing, you should create custom routing logic based on specific conditions and request parameters. This allows you to switch between different DataSources dynamically. Using static configuration or a single global DataSource won't provide the required flexibility for dynamic routing. A connection pool is unrelated to DataSource routing.

Designing a Spring Cloud application to handle failovers and service unavailability with minimal impact on the user experience involves implementing Spring Cloud Circuit Breaker (e.g., Hystrix) to detect and gracefully handle service failures. Timeouts, fallback mechanisms, and retries with exponential backoff ensure robustness. Additionally, considering a global request rate limiter can prevent overloading services during failures, further enhancing user experience.

The spring.profiles.active property in Spring Boot's application properties or YAML file is used to specify the default active Spring profile. Profiles allow you to customize the application's configuration based on the environment (e.g., development, production) or other criteria. By setting this property, you determine which profile is active by default when your Spring Boot application starts. The other options are not the primary purpose of this property.

The TestRestTemplate is a test utility in Spring Boot for making HTTP requests to your application during integration tests. It can be auto-configured and is designed for use in integration testing scenarios.