System Design: Slack

System DesignOpenAILast reported May 2026High Frequency
Reported
38× across candidate reports
First seen
January 2025
Last reported
May 2026
Reported outcome
mixed

Problem Overview

Design a Slack-like messaging platform. Core requirements: (1) Direct messages (1:1) and group channels; (2) Create new group chats and add users to a group; (3) Send and receive messages in real-time; (4) Send notifications to groups or individual users — push notifications when offline, unread-message alerts when online; (5)…

  • 12 candidate-reported follow-ups — the exact probes interviewers asked, with the trigger for each
  • The rest of the problem statement — full requirements, constraints, and edge cases
  • Approach and trade-offs — what passing candidates did, and the mistakes that sink people
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Verified Source
Every question is reconstructed from multiple independent candidate reports. Verbatim follow-ups, not invented ones.
Codex Fact-Checked
Technical claims, formulas, and scale numbers are reviewed against primary sources.
Interviewer Follow-ups
The exact follow-ups reported by candidates, with the trigger that prompts each one — plus the mistakes that sink people.
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