Design Slack-like chat system

##### Question Design the **chat subsystem** for an app like Slack/Discord. State your assumptions (scale, retention, consistency needs) and propose an end-to-end architecture, then expect deep follow-ups on the trade-offs. **Product requirements** 1. The product has **workspaces**; each workspace contains **channels**. Users can join a workspace and view all channels they have access to (public and private channels). 2. Users can send **plain-text messages** in any channel they are a member of, and the channel may have many members (some "announcement" channels can reach 100k+). 3. Users can also have **direct messages** — 1:1 and small group DMs. 4. Users can **scroll back and read the full message history** (assume no expiration / long retention). 5. Users should **see new messages in real time without refreshing**. 6. Support **editing and deleting** messages. **Design areas to cover (expect a deep dive on each)** 1. **Core APIs** — send message, fetch history (pagination), subscribe for real-time, edit/delete. 2. **Data model** for workspaces, channels/conversations, membership, and messages. 3. **Storage choice**, sharding/partitioning, and how history queries stay efficient. 4. **Real-time delivery** (WebSocket / pub-sub) and connection/subscription management. 5. **Fanout strategy** — write fanout vs read fanout — and the trade-offs at different channel sizes. 6. **Ordering and consistency** guarantees (per-conversation ordering, read-your-writes) and handling **edits/deletes**. 7. **Indexing / full-text search** over message history (if asked). 8. **Hot channels** and performance bottlenecks — how you mitigate very large or very active conversations.