When building APIs with ASP.NET Core, developers can use Swagger/OpenAPI to define the routes, controllers, and actions. Swagger is a popular tool for documenting and defining APIs in a machine-readable format, making it easier to create and consume web APIs in ASP.NET Core.

When selecting an API Gateway for a microservices architecture, you must consider various factors, including security and authentication options to protect your APIs, performance and scalability features for handling traffic, API versioning to manage changes, and documentation support to make APIs easily discoverable. Additionally, logging and monitoring capabilities are crucial for troubleshooting and maintaining the health of your services.

Optimizing the performance of an API involves maximizing its speed and efficiency. This includes reducing response times, minimizing resource usage, and ensuring the API can handle a high volume of requests while delivering results quickly and reliably. Performance is a key consideration for successful APIs.

In this scenario, OAuth 2.0 is used for user authentication to obtain an access token securely. JWT is then used for authorization by including user claims in the token, specifying what the user is allowed to access within the SPA. This separation of concerns allows for a secure and flexible way to manage user authentication and authorization.

The statelessness of RESTful APIs simplifies server-side scalability since each request from a client contains all necessary information, and servers don't need to maintain session state. However, it may require client-side state management for complex interactions. This is important to understand in the context of designing efficient, scalable systems.

Exposing API keys can lead to unauthorized access to API resources, which is a significant security risk. It can result in data breaches, unauthorized actions, and potential financial or reputational damage. Properly securing API keys is essential to mitigate these risks.

Flask uses the @app.route() decorator to associate a function with a specific "URL Endpoint." When a client makes a request to the specified URL, the associated function is executed, allowing you to define routes and their corresponding actions in Flask applications.

gRPC uses "Protocol Buffers" to define services and message types. Protocol Buffers is a language-agnostic data serialization format, and it aids in generating client and server code in multiple programming languages, making gRPC efficient and cross-compatible.

The primary purpose of Role-Based Access Control (RBAC) in APIs is to restrict access to certain resources and functionality based on the roles or permissions assigned to users. RBAC is used to enhance security by ensuring that only authorized users with specific roles can access particular API endpoints or perform specific actions, reducing the risk of unauthorized access.

RESTful APIs leverage caching through HTTP headers, which offers fine-grained control over caching strategies and enhances performance. This approach is more efficient and flexible compared to SOAP APIs, which typically rely on server-side caching. Understanding these differences is vital when optimizing API performance.