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Thu, Jun 4, 20267:00 PM - 8:00 PM PDTSession ended

Slack & Real-Time Chat

Message ordering, fan-out, and realtime delivery at scale

About this session

A 90-minute live deep-dive into designing a Slack-like chat system: strict message ordering in group channels, real-time notification fan-out to 100M+ users over WebSocket and push, message deletion with audit/retention, and keeping the experience smooth under load. Great prep for chat/messaging interviews at OpenAI, Meta, and Google.

Course Syllabus

Course Overview

Format:
Live Zoom, 90 minutes (75 min teaching + 15 min Q&A)
Audience:
Mid-level to Staff+ engineers prepping for chat/messaging interviews
Prerequisite:
Familiarity with WebSockets, message queues, and basic distributed systems

What You Will Learn

  1. Guarantee strict message ordering in group channels when messages arrive concurrently from distributed devices.
  2. Design real-time notification fan-out for 100M+ users across online (WebSocket) and offline (push) delivery.
  3. Support message deletion that propagates to all clients while honoring audit logs and retention.
  4. Manage WebSocket connection state and reconnection without overwhelming the backend.
  5. Reason about read fan-out for large channels versus per-user inboxes.

What This Course Is NOT

  • Not a UI/frontend course — we focus on the backend and delivery path
  • Not voice/video calling — text messaging and presence only

Pre-Class Preparation (24 hours before)

Read the problem framing only (10 min):

Design a Slack-like chat application that supports channels and direct messages, delivers messages in real time to online clients and via push to offline clients, preserves message ordering within a channel, and scales notification fan-out to 100M+ users without overwhelming the backend.

Think about (don't research yet):

  • How do you assign an ordering to messages sent concurrently from different devices?
  • Where does a message go when the recipient is offline?
  • What's the cost of fanning out one message to a 50,000-member channel?

Come to class with a first-pass ordering scheme and a fan-out approach in mind.

Lecture: author of showoffer

Tony

ShowOffer Coach

Engineering Manager: Over 12 years of industry experience with 6 years in engineering leadership, building and scaling tech infrastructure teams that deliver end-to-end large-scale distributed systems.

Reviews

5.0(1)

  • Reviewer 1CZA

    Jun 6, 2026

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