Design a Slack-like chat system

Q: Design a Slack-like chat system

This is a System Design interview question from Discord for Software Engineer roles. View the full question and solution on PracHub.

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Question

System Design: Slack-like Messaging (Channels + DMs)

Design a Slack-like messaging backend that supports:

Public/private channels with many members
1:1 and small group direct messages
Sending and receiving messages with low latency
Message history (scrollback)

Interview focus areas (expect deep follow-ups):

Data model for channels, membership, messages
Fanout strategy (write fanout vs read fanout)
Indexing / querying for message history and (optional) search
Database choice and sharding/partitioning
Ordering guarantees and handling edits/deletes
Hot channels and performance bottlenecks

State your assumptions (scale, retention, consistency needs) and propose an end-to-end architecture.

Design a Slack-like chat system

System Design: Slack-like Messaging (Channels + DMs)

Solution

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