What is contract testing and why is it important for APIs?

Contract testing is a testing method that verifies the interaction between services or components through their API, ensuring that both parties (the provider and the consumer) adhere to a pre-agreed contract (a set of expectations and requirements).

This is important for APIs because:

• It ensures data and behavior consistency between services.
• It helps identify contract violations early, reducing the risk of integration errors.
• It simplifies independent development and deployment of services, as each service knows what to expect from the other.

Example: if the frontend expects the API to return a "userId" field as a number, but the backend changes this field to a string, contract testing will detect the mismatch before it causes bugs in production.