Troubleshoot a Frontend Failure in a Client-Server Application

A frontend application in a client-server architecture has started failing. Explain how you would locate the failure across the browser, network, backend service, and its dependencies before choosing a fix. ### Constraints & Assumptions - The source does not give a particular error code or root cause. Begin by narrowing the symptom rather than assuming the backend is responsible. - Distinguish a blank or broken UI, a failed request, and a successful response rendered incorrectly. - Use observability and controlled reproduction; do not make destructive production changes merely to test a theory. ### Clarifying Questions to Ask - What does the user see, which action triggers it, and can it be reproduced? - Does it affect all users, a browser/version, a route, or a particular account or data shape? - When did it begin, and what code, configuration, or dependency changed near that point? ```hint Follow one failing interaction Connect the browser action to its actual request and response, then correlate that request with server evidence instead of searching unrelated logs. ``` ### What a Strong Answer Covers - Symptom classification and a reproducible failing case with an appropriate comparison case. - Browser console, assets, request timing, status, response body, and rendering checks. - Request correlation through backend and dependency traces when the evidence points there. - Hypothesis-driven isolation, mitigation, verification, and regression prevention. - Care with authentication, caching, and sensitive data in captured diagnostics. ### Follow-up Questions - How would your investigation differ if the API returned 200 but the page was still broken? - What if only a small subset of users saw the failure after a deployment?