A Public API is accessible to anyone and does not require special permissions for use, while a Private API has restricted access and is typically used within an organization or by specific authorized users. Public APIs may have usage limitations or require authentication, while Private APIs are usually for internal or controlled use.

Developers may choose to create a Web API instead of a traditional web application to enable integration with other systems. Web APIs allow different software applications to communicate and interact, making them ideal for data exchange and third-party integrations. Providing a graphical user interface, offering a mobile app, or improving website performance may be goals but are not primary reasons for creating a Web API.

When implementing API key rotation, it's crucial to consider security best practices. API keys should be changed periodically to reduce the risk of unauthorized access. Sharing keys with everyone or storing them in public places should be avoided. Storing keys securely, managing access, and ensuring that compromised keys can be revoked are important aspects of API key rotation.

GraphQL allows for more flexibility in querying data compared to REST because it enables clients to specify the structure and fields of the response, allowing them to request only the data they require. In contrast, REST often returns fixed data structures, and clients must make multiple requests to retrieve the needed data.

Load testing an API involves assessing its performance under a specific load. This test measures how well the API can handle a high volume of concurrent requests or transactions, helping identify performance bottlenecks and ensuring the API's reliability under heavy usage.

Optimizing the performance of APIs is essential for ensuring responsiveness. Minimizing database queries and employing caching mechanisms can significantly reduce response times. Caching allows you to store frequently accessed data in memory, reducing the need for repeated database queries. This is a recommended practice for performance optimization.

The primary purpose of API authentication is to ensure data privacy and security. It verifies the identity of the user or application requesting access to an API, preventing unauthorized access and protecting sensitive data. This security measure is vital in modern web development to safeguard user information and maintain the integrity of the API.

In this situation, OAuth 2.0 is employed to securely expose user data to third-party apps. OAuth acts as a resource server, authenticating and authorizing these apps, and issuing access tokens. It ensures controlled access to user data without exposing sensitive information directly. JWTs can also be used to include user data in the token if necessary.

The primary data format used in GraphQL APIs is JSON (JavaScript Object Notation). JSON is a lightweight and human-readable format, making it well-suited for data exchange between clients and GraphQL servers. GraphQL can also work with other data formats, but JSON is the most common.

Automating API tests offers several benefits, including faster test execution. Automated tests can be run more frequently and consistently than manual tests, helping to identify issues early in the development process and saving time and resources. Automated testing can also provide more comprehensive test coverage.