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.