Design a Slack-like messaging system

Design a Slack-like real-time team messaging system.

Requirements:

• Users can:
  • Create and join workspaces.
  • Create public and private channels within a workspace.
  • Send messages in channels and direct messages (DMs).
  • See message history and search messages by keyword.
• Messages should appear in near real-time to all online participants in a channel or DM.
• Support presence indicators (online/away) at a basic level.
• System should support:
  • 10 million registered users.
  • 1 million daily active users.
  • Peak 200k concurrent connections.

Tasks:

1. Clarify and state any additional assumptions you need (e.g., file sharing scope, message size limits, retention policy).
2. Propose a high-level architecture:
   • How clients connect (e.g., WebSockets, polling).
   • Major backend services and their responsibilities (e.g., gateway, messaging service, user service, search service).
   • Data storage for users, workspaces, channels, and messages.
3. Design the data model for core entities:
   • Users, workspaces, channels, memberships, and messages.
   • Explain how you would partition/shard message data.
4. Describe how real-time messaging works end-to-end:
   • From a user sending a message to it being delivered and persisted.
   • Handling fan-out to many subscribers in a busy channel.
5. Discuss scalability:
   • How to handle 200k+ concurrent connections.
   • Horizontal scaling strategies for message processing.
   • Handling hot channels with very high message throughput.
6. Address reliability and consistency:
   • Ensuring messages are not lost (at-least-once vs exactly-once semantics).
   • Ordering guarantees within a channel.
7. Describe how you would implement search over message history:
   • Indexing strategy.
   • High-level read/write path for search.
8. Mention key trade-offs and potential future extensions (e.g., threading, reactions, file attachments).