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Slack System Design
Visual Problem Diagram
Scenario
Engineering ships a fix and 40 channels light up with @channel—each message must reach online members in < 100 ms while offline users replay 10K messages without gaps. Threads, reactions, and search index lag are different pipelines than the real-time fan-out path.
Design a team messaging product with workspaces, public/private channels, DMs, threads, and searchable history. Enterprise chat combines real-time fan-out with durable logs and async search indexing.
You should support channel membership, message delivery with ordering, presence, file sharing hooks, and full-text search. Be ready to explain per-channel sequencing, fan-out to members, and reconnect sync.
Constraints
Workspaces, channels, DMs, threads, messages, reactions, file attachments, search, presence
< 200 ms delivery for online users, 99.99% availability, durable history, search lag seconds OK
12M daily active users, billions messages/day, millions of channels, large enterprise workspaces
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