Describe a scenario where you encountered a CORS error in a React application with a microservice architecture. Detail the steps you took to diagnose and resolve the issue.
React JS interview question for Advanced practice.
Answer
In a scenario with a React frontend at https://app.example.com, an authentication service at https://auth.example.com, and a data service at https://api.example.com, a CORS error occurred. After logging in, a request to the data service with an Authorization header was blocked. Diagnosis: 1. I used the browser's Network tab and saw the browser first sent a preflight OPTIONS request to https://api.example.com. 2. This preflight request was failing. Inspecting its response headers revealed that Access-Control-Allow-Headers was missing, so the browser couldn't verify if the Authorization header was permitted. Resolution: The backend architecture used an API Gateway as the single entry point for all services. The resolution was to configure the CORS policy at the gateway level. We updated the gateway to handle OPTIONS requests and respond with the necessary headers for all proxied routes: - Access-Control-Allow-Origin: https://app.example.com - Access-Control-Allow-Methods: GET, POST, OPTIONS, PUT - Access-Control-Allow-Headers: Content-Type, Authorization (This was the key fix) - Access-Control-Allow-Credentials: true This solved the root cause by creating a single, consistent CORS policy at the edge of the system, ensuring all services handled cross-origin requests correctly.
Explanation
Many CORS issues in microservice architectures stem from inconsistent configurations across services. An API Gateway helps enforce a single, consistent policy.