Design a pipeline for collecting product and gaming events from clients and backend services. Events may arrive in batches and must be available for analytics and reporting.

Design a real-time messaging system. Users send text messages in 1-to-1 and group chats. Messages must be delivered in real time with at-least-once guarantee.

Design a file upload service for avatars, PDFs, and documents. A user uploads a file, the system validates it, stores it, and returns a URL for subsequent access.

Design an inventory reservation system for e-commerce. A user adds an item to the cart, and the system must temporarily reserve the stock to prevent overselling.

Design a notification service that can send email, push, and SMS. Notifications do not have to be instant, but they must not be lost.

An external payment provider sends webhook events for payments, refunds, and cancellations. You need to correctly handle retries and out-of-order events without breaking the order state.

Design a rate limiting system for an API Gateway. The system must throttle requests by IP and by API key. Limits: 100 req/min per IP, 1000 req/min per API key.

Design a search-as-you-type service for searching products or users. Suggestions must be returned very quickly on every keystroke.

Design a URL shortening system similar to bit.ly. A user enters a long URL and receives a short link. Redirects via the short link must be fast.

Design a pipeline for processing user-uploaded video: upload, transcoding into multiple quality levels, preview generation, and subsequent delivery via CDN.