In a production environment, monitoring APIs primarily aims to ensure performance stability. This involves tracking response times, identifying bottlenecks, and addressing issues that may impact the overall user experience. By monitoring, teams can proactively address potential problems before they escalate, contributing to a reliable and efficient system.

Throttling or rate limiting in API integration controls the number of requests a client can make within a specified time frame. Testing should include scenarios of exceeding the allowed limits and verifying the system's response, ensuring graceful degradation instead of sudden failures. This ensures the application behaves appropriately under heavy usage conditions, preventing performance degradation or API abuse.

In API security, OAuth relies on refresh tokens to verify the authenticity of the access token. Refresh tokens play a crucial role in maintaining a secure and seamless authorization process.

The primary advantage of using cloud-based API testing tools for scalability and collaboration is on-demand resource allocation. Cloud platforms provide the flexibility to scale resources based on testing needs, ensuring optimal performance during peak periods. This scalability facilitates efficient collaboration among distributed teams, allowing them to access testing environments and resources as needed. While offline testing capabilities, in-house data security, and local execution speed are important considerations, they are not the primary advantages related to scalability and collaboration.

Concurrency in API performance testing refers to the capability of an API to handle multiple requests simultaneously. It assesses how well an API performs under concurrent loads, providing insights into its scalability and robustness under heavy usage. It's crucial for identifying bottlenecks and ensuring optimal performance.

CDNs distribute API requests across multiple servers, reducing the load on the origin server and improving overall performance by bringing data closer to end-users. This helps address performance bottlenecks in API responses.

In REST APIs, a PATCH request is used for partial updates to a resource. Unlike PUT, which updates the entire resource, PATCH is used to apply partial modifications. This is useful when you want to update only specific fields without affecting the rest of the resource.

API security testing commonly involves testing for Cross-Site Scripting (XSS) attacks, which can manipulate or steal sensitive information by injecting malicious scripts into the API responses.

When a server is temporarily unable to handle requests due to overloading or maintenance, it should return a 503 (Service Unavailable) status code. This informs clients that the server is temporarily unable to process the request and suggests trying again later. A 404 status code indicates that the requested resource is not found, and 302 is a temporary redirect. A 500 status code is a generic server error.

For age filtering, critical boundary values include the minimum and maximum age range. Testing values like 0, 1, 100, and 101 ensures the API correctly handles the entire range of possible inputs, covering the lower and upper limits.