API throttling is a technique to regulate API request rates, limiting the number of requests a client can make in a given time period. It's often used to prevent server overload, maintain a stable system, and ensure fair usage of API resources. Throttling can help balance resource allocation and avoid overloading the API server.

Effective error handling is essential in API development. Defining clear error codes and messages is crucial for both developers and consumers of the API to understand issues and take appropriate action. Additionally, logging errors for debugging and monitoring helps in diagnosing problems and maintaining the API's reliability. Returning stack traces in responses is generally not recommended, as it may expose sensitive information and create security risks.

Best practices for API testing and monitoring include load testing to ensure the API functions correctly under various conditions. Load testing evaluates how the API performs under different levels of user activity and traffic, helping to identify bottlenecks and potential performance issues. It's an important aspect of testing to ensure that the API can handle real-world usage.

During sudden spikes in traffic, it's essential to maintain service quality by using adaptive rate limits and dynamic throttling. Increasing rate limits may lead to resource exhaustion, and strict throttling can result in rejected requests, negatively affecting user experience. Adaptive rate limits and dynamic throttling allow the API to manage the increased load intelligently while preventing overload.

Using Relay as a GraphQL client is more beneficial when dealing with paginated lists and complex data requirements. Relay is specifically designed for these scenarios, making it a good choice for applications that require efficient data fetching and updates. Apollo (Options A, B, C) is a more general-purpose GraphQL client and may be a better fit for simpler applications or server-rendered pages.

The feature of Swagger that allows developers to design, build, and document APIs directly in the browser is the "Swagger Editor." It provides a browser-based interface for designing and documenting APIs, making it easier for developers to create and visualize API specifications. Swagger UI, on the other hand, is used for visualizing and interacting with already-documented APIs.

To keep the user interface updated in a social media application using GraphQL, you should use GraphQL subscriptions. Subscriptions enable the server to push real-time updates to clients over WebSocket connections, providing efficient and responsive real-time communication.

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.