Slack

3 design problems known to be asked at Slack.

Medium2

Mediumlow level design design patterns oop

Mediator Pattern: Turn a Mesh of Dependencies Into a Star

When every object holds a reference to every other, change becomes impossible — an O(N²) tangle where one edit ripples everywhere. The Mediator pulls the wiring into one place so components can stay gloriously ignorant of each other.

Asked at Google, Microsoft, Meta +3

Aug 8, 2026·15 min read

Mediumhigh level design distributed systems networking

Real-Time Updates: Polling, Long Polling, SSE, and WebSockets

HTTP can't push — the server only answers when asked. Here's the ladder of techniques that fake or fix that, each one solving the last one's flaw, and the fan-out problem that breaks all of them at scale.

Asked at Slack, Discord, WhatsApp +3

Aug 8, 2026·16 min read

Hard1

Hardsystem design interview distributed systems message queues realtime

Design a Chat System

WhatsApp-scale messaging, built end to end: persistent WebSocket gateways, a user→server registry, store-and-forward for offline devices, the one-tick / two-tick / blue-tick delivery state machine, message ordering, and group fan-out. Drive the delivery protocol yourself, then take it apart.

Asked at Meta, WhatsApp, Slack +3

Aug 9, 2026·14 min read