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.

Continuous Integration (CI) in the context of API testing primarily focuses on the frequent integration of code changes into a shared repository. This ensures that the API tests are executed regularly, validating the code changes and detecting integration issues early in the development process.

Testing request transformation features in an API Gateway is crucial to ensure that the gateway can properly handle and transform incoming requests. This includes validating and modifying request parameters, headers, or payload to meet the requirements of the backend services. It plays a vital role in data integration and interoperability, allowing the API Gateway to act as a mediator between clients and backend services by adapting requests appropriately. Thorough testing ensures the reliability of this functionality.

Enhancing the security of a mobile application using OAuth can be achieved by implementing Refresh Tokens. Refresh Tokens help in obtaining new access tokens without requiring the user to re-authenticate, thereby improving the overall security of the API.

Frequent changes in API specifications can significantly increase the complexity of API test automation. Test scripts may need frequent updates to adapt to changes in the API, impacting the stability of the automation suite.