UnderTheScale

Latest episode

Now live

WebSockets Redis Pub/Sub Presence System

How WhatsApp's Typing Indicator Actually Works

That "typing…" bubble hides a serious distributed systems challenge. Sub-100ms latency, 2 billion users, Redis TTLs, cross-gateway fan-out, reconnect storms — all for three animated dots.

8 min read Interactive diagram HLD + LLD + DB schema Tradeoffs

Read deep dive →

Architecture overview

Alice → Gateway A → Redis

Redis → Gateway B → Bob

Key: presence:{conv}:{user}
TTL: 3 seconds · auto-expiry

WS connection TTL key Pub/Sub Shard by conv_id Jitter backoff DB schema LLD walkthrough

Coming next

Episode roadmap

Get notified →

Coming soon

Episode 02 · Realtime Systems

Slack/Teams Active-Inactive Status

How Slack tracks online/away for 20M+ concurrent users. Heartbeats, fanout storms, and why "active" isn't just one WebSocket ping.

WebSocketsHeartbeatPresence

Coming soon

Episode 03 · Scheduling

Google Calendar Free Slot Detection

How Calendar finds mutual free windows across time zones and calendars. Interval trees, aggregation, and smart scheduling at scale.

Interval treesSchedulingTime zones

Coming soon

Episode 04 · Streaming

YouTube Live Low Latency

From 30-second satellite delay to sub-3-second streams. CMAF, chunked encoding, CDN edge routing, and ABR algorithms.

CMAFCDNABR

Coming soon

Episode 05 · Realtime Systems

Instagram Seen Receipts

How Instagram delivers read receipts instantly without triggering a DB write per message open. Event batching and cursor mechanics.

Read receiptsRedisEvent sourcing

Coming soon

Episode 06 · Geo / Realtime

Uber Driver Live Tracking

Real-time GPS from driver to rider every second. Location interpolation, geo-indexing, and heading prediction on live maps.

GPSGeo-indexWebSockets

Coming soon

Episode 07 · Notifications

Push Notifications at Scale

APNs, FCM, and the fan-out queue architecture behind 500M device deliveries in seconds. Priority routing and delivery guarantees.

APNsFCMFan-out

Live now

1 of 40 episodes live · more coming soon

01 WhatsApp Typing Indicator Live

Browse by topic

All Realtime Systems AI Infrastructure Payments Streaming Notifications Databases Recommendations

What you get in every episode

Not just text. An experience.

⚡

01 · Architecture

Animated packet flows

Watch events travel across nodes in real time. Click any component to inspect its role.

🗃️

02 · Data Model

Interactive DB schema

Every table, every key. Why it's in Redis and not Postgres. What gets stored vs what expires.

🔍

03 · Design

HLD + LLD walkthrough

High-level system view, then a step-by-step sequence of exact method calls and state transitions.

⚖️

04 · Decisions

Tradeoff radar charts

Every design decision has a cost. Pros, cons, and when the right answer changes.

The engineering behind everydayinternet features.

The engineering behind everyday
internet features.