Design a scalable and reliable backend service that handles high-volume network I/O (e.g., ingesting and serving large files or streaming data) for millions of users. Define functional and non-functional requirements; propose APIs and data models; estimate capacity and perform back-of-the-envelope calculations; design the high-level architecture (load balancing, stateless services, storage, caching, queues, CDN). Explain strategies for horizontal scaling, performance optimization, and cost controls. Address security and privacy (authentication, authorization, encryption in transit/at rest, rate limiting, multi-tenant isolation). Detail failure handling, retries, idempotency, consistency choices, and disaster recovery. Describe deployment, observability (logging/metrics/tracing), incident response, and rollback. Identify corner cases (e.g., partial uploads, duplicate requests, slow clients, network partitions) and how you would test them.