Design a Slack-like chat system

Q: Design a Slack-like chat system

This question evaluates system-design competencies such as data modeling for channels and direct messages, scalability and sharding, fanout and delivery strategies, consistency and ordering guarantees, indexing and query patterns for message history, and low-latency architecture for real-time messaging, and it belongs to the System Design domain.

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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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Design a Slack-like chat system

Overview

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

Solution

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