# Medium

High-level design problems at **SDE-2** level. Focus: distributed systems, caching, replication, load balancing.

## Problems

| Problem                  | File                                                                                                                           | Key concepts                                                                                                                  |
| ------------------------ | ------------------------------------------------------------------------------------------------------------------------------ | ----------------------------------------------------------------------------------------------------------------------------- |
| **Twitter / News Feed**  | [twitter-news-feed.md](https://github.com/nishchalnishant/SystemDesign/blob/After-AI/hld-problems/medium/twitter-news-feed.md) | Timeline, fan-out, Redis; see also [twitter.md](https://nishchalnishant.gitbook.io/system-design/hld-problems/medium/twitter) |
| **Instagram**            | [instagram.md](https://nishchalnishant.gitbook.io/system-design/hld-problems/medium/instagram)                                 | Image storage, CDN, follower/feed                                                                                             |
| **YouTube**              | [youtube.md](https://nishchalnishant.gitbook.io/system-design/hld-problems/medium/youtube)                                     | Video transcoding, CDN, recommendations                                                                                       |
| **WhatsApp**             | [whatsapp.md](https://nishchalnishant.gitbook.io/system-design/hld-problems/medium/whatsapp)                                   | Chat, WebSockets, message queue                                                                                               |
| **Notification Service** | [notification-service.md](https://nishchalnishant.gitbook.io/system-design/hld-problems/medium/notification-service)           | Push, pub-sub, fan-out, queues                                                                                                |

Use the [HLD template](https://nishchalnishant.gitbook.io/system-design/5.-interview-templates/hld-template) and [trade-offs cheat sheet](https://nishchalnishant.gitbook.io/system-design/5.-interview-templates/trade-offs-cheat-sheet) when practicing.