Explain APIs and walk through how a request and its response work end to end

Explain what an API is and how one works when one piece of software asks another for data or an action. ```hint Follow one call Pick a single concrete request between two systems and narrate what each side does, from the moment the request is sent until the caller uses the reply. ``` ### Clarifying Questions - Should I pitch this to a non-technical stakeholder, or at the level I would use with the engineers on my team? - Should I focus on web APIs called over the internet, or also cover APIs inside a program or an operating system? - Would you like me to cover how a product team decides to offer an API to outside developers, or keep to the mechanics? ### What a Strong Answer Covers - A precise definition that separates the interface a caller relies on from the implementation behind it - A step-by-step walk through one request and response, including how the caller identifies itself and how the reply comes back - The parts of a web request and response, and what the common status code families mean - Failure and safety concerns: authentication, error handling, rate limits, timeouts, retries and versioning - Why APIs matter for product work, such as integrations, platforms and boundaries between teams ### Follow-up Questions - How do REST and GraphQL differ, and when would a team choose one over the other? - How would you decide whether your product should offer a public API, and what would you measure after launch? - A partner reports that your API is slow and unreliable. How would you work with engineering to find out why?