A data-collection app uses API Gateway, Lambda, and S3. Users upload files and then wait while a long validation runs; they must refresh the dashboard to see results. Some users upload many large files and repeatedly refresh. The developer must change the app so the dashboard receives immediate validation results for a file without reloading the whole page. Which solution is the most operationally efficient?

The most operationally efficient solution is using API Gateway WebSocket API. This allows for a persistent, full-duplex communication channel between the client and the server. When a file upload completes and validation finishes, the backend can directly push the status update to the specific client's WebSocket connection, enabling real-time updates without polling or page refreshes. This is serverless, highly scalable, and cost-effective. Launching an EC2 instance for a WebSocket server adds operational overhead and cost for managing the instance. Email notifications are not real-time updates to a dashboard. Using DynamoDB Streams with SNS push notifications to the browser is possible, but API Gateway WebSocket API is a more direct and purpose-built solution for real-time client-server communication in a serverless architecture, making it more operationally efficient.