OpenID Connect handles user authentication and authorization securely by using identity tokens for authentication and OAuth tokens for authorization. This separation of concerns enhances security. Identity tokens contain user information, while OAuth tokens grant access permissions. The combination of both ensures a secure and standardized authentication and authorization process.

Partner APIs play a crucial role in a business ecosystem by enabling collaboration and data exchange with external organizations, such as partners, suppliers, or customers. They allow businesses to extend their services and reach beyond their internal boundaries, fostering strategic partnerships and creating new revenue opportunities.

OpenID Connect extends OAuth 2.0 by adding JSON Web Tokens (JWT) to enable Clients to verify the identity of the End-User based on the authentication performed by an Authorization Server. JWTs are a compact, self-contained format for representing claims between two parties, making them suitable for securely transmitting identity and other information between the Client, Authorization Server, and Resource Server.

To optimize web application performance, utilizing caching for frequently requested data through the Web API is crucial. This approach reduces the need for repeated API calls, improving responsiveness and load times. Synchronous API calls can block the application, increasing latency. Increasing payload size might slow down requests, and adding unnecessary authentication layers can have the opposite effect.

In Node.js and Express, the res.json() method is used to send a JSON response to the client. It serializes a JavaScript object into JSON format and sends it as the HTTP response, allowing for structured data interchange between the server and client.

The primary purpose of HTTP (Hypertext Transfer Protocol) in Web APIs is to request and transmit data between clients and servers. It defines the methods for clients to request resources from servers and the format for the exchange of information. It plays a fundamental role in enabling communication and data transfer in web-based applications.

REST (Representational State Transfer) is an architectural style for designing networked applications. It emphasizes a stateless client-server interaction where resources are identified by URIs and can be manipulated using standard HTTP methods.

Implementing both real-time and historical monitoring for APIs is essential. Real-time monitoring helps in detecting issues as they occur, allowing for immediate responses. Historical monitoring, on the other hand, provides valuable data for analyzing long-term trends, identifying patterns, and improving overall API performance.

The purpose of API documentation is to guide developers on how to use the API effectively. It should include information on endpoints, parameters, request and response formats, and example use cases. Providing such detailed documentation is essential for developers to understand and integrate with the API.

HTTP status codes in the range "200-299" indicate successful API requests. These codes signify that the request was received, understood, and successfully processed.