System Design - Slack-like Chat System

In an OpenAI system design interview, the "Design a Slack-like Chat System" problem tests your ability to architect high-throughput, low-latency, and multi-tenant systems. Hello Interview +1 0 3

Problem Statement Overview

The primary objective is to design a scalable enterprise chat system capable of handling millions of users while maintaining real-time delivery and data consistency across devices. Codinginterview.com +1 5 6

Core Functional Requirements

A complete problem statement typically includes these requirements:

• Messaging: Support for one-on-one direct messages (DMs) and group channels (1:N).
• Real-time Delivery: Instant message updates using persistent connections (e.g., WebSockets).
• Organization: Features like message threads, private vs. public channels, and search functionality.
• Persistence: Durable message history that is searchable and consistently updated with unread markers.
• Presence: Real-time user status indicators (online, offline, away). Hello Interview +2

Non-Functional Requirements

At OpenAI, interviewers prioritize these architectural trade-offs: 7

• Scalability: Handling 100x or 1000x user spikes and managing the "fan-out" problem when one message must be delivered to thousands of recipients.
• Low Latency: Ensuring sub-second message delivery globally.
• High Availability: The system must remain operational even during regional failures.
• Consistency: Maintaining the same message order and "unread" status across mobile and web platforms. Hello Interview +2

Expected Interview Focus

Interviewers often use this prompt to dive deep into:

1. Fan-out Strategies: How do you efficiently notify 50,000 users in a single channel?
2. Storage Choices: Deciding between NoSQL (e.g., Cassandra) for fast writes and relational databases for metadata.
3. Concurrency: Handling many users typing or sending messages simultaneously without race conditions. LinkedIn +2

Would you like a high-level architectural diagram or a deep dive into the fan-out strategy for large channels?

[0] - Design Slack | Hello Interview [1] - Designing a real-time chat system like Slack or WhatsApp - LinkedIn [2] - OpenAI L5 Interview Guides & Questions (2026) [3] - 30+ Common OpenAI Interview Questions + Answers (by role) [4] - OpenAI System Design Interview (2026 Guide) - Exponent [5] - OpenAI System Design Interview Questions [6] - Slack-like Chat System - OpenAI Interview - DarkInterview [7] - OpenAI System Design Interviews (questions, process, prep)