Design Slack
Hard · System Design · about 45 minutes
Asked at Slack, Salesforce, Microsoft, Atlassian, Zoom, OpenAI, ServiceNow, Workday.
Design a team messaging app like Slack for ~20M daily users. Workspaces have public and private channels (up to 100k members), DMs and threads. Messages must reach every member's open clients within 500ms, unread state must sync across devices, and the full history must be searchable — without one workspace's data ever reaching another.