Series · 10 parts · 78 min · Intermediate

Real-Time Chat at Scale

Build a chat app with WebSockets step by step: from a raw socket to auth, storage, delivery guarantees, presence, multi-node scaling, load testing and production.

What you'll build

Relay, a real-time chat service in Node.js, Socket.IO, Postgres and Redis that runs across many nodes and holds up under load.

Before you start

Comfortable with Node.js, TypeScript and basic React. Knowing some SQL and Redis helps.

Recommended firstScaling to 1M Requests

What you'll learn

How WebSockets work under HTTP, and when to use them over polling or SSE
Typed Socket.IO events, rooms and acknowledgements with a React client
Authenticating long-lived connections and authorizing every room join
Message storage with time-sortable IDs, idempotent writes and cursor pagination
Delivery guarantees, retries, catch-up after reconnect and read receipts
Presence and typing indicators in Redis without touching the database
Scaling out with the Redis adapter, sticky sessions and load balancers
Kernel and Node tuning, load testing with k6, and safe production deploys

(01)The parts

Your progress0/10 parts

Start at part 01

01WebSockets from First PrinciplesWhy chat needs a persistent connection, how the WebSocket handshake and frames work, and a minimal broadcast chat built on the raw ws package.8 min

02Your First Chat Server with Socket.IORebuild Relay on Socket.IO with fully typed events, conversation rooms, acknowledgements with timeouts, payload validation and a clean React hook.7 min

03Authenticating WebSocket ConnectionsVerify JWTs during the Socket.IO handshake, authorize every room join, handle tokens that expire on hours-long connections and block cross-site WebSocket hijacking.7 min

04Storing Messages: Schema, IDs and PaginationDesign a Postgres schema for chat, pick time-sortable message IDs, make writes idempotent with client IDs and page through history with keyset cursors.7 min

05Delivery Guarantees: Acks, Retries and Read ReceiptsMake Relay reliable on flaky networks with a client outbox, safe retries, catch-up after reconnects, and delivered and read receipts with unread counts.7 min

06Presence and Typing Indicators with RedisTrack who is online across tabs and devices with Redis heartbeats, survive crashed nodes, debounce flapping, and ship typing indicators that never touch the database.9 min

07Scaling Out: Multiple Nodes with the Redis AdapterRun Relay on many servers: share broadcasts through the Socket.IO Redis adapter, configure sticky sessions, Nginx and ALB correctly, and understand the real cost of fan-out.8 min

08Tuning for a Million Concurrent ConnectionsMeasure memory per socket, raise file descriptor and kernel limits, avoid ephemeral port exhaustion, cut ping and compression overhead, and survive reconnect storms.9 min

09Load Testing WebSockets with k6Drive thousands of real Socket.IO connections with k6, measure connect time and end-to-end message latency, set thresholds, and scale tests across load generators.7 min

10Production Hardening: Backpressure, Deploys and PushRate limit sockets, handle slow consumers, drain connections gracefully on deploy, add metrics and health checks, reach offline users with push, and lock down security.9 min

(10)Let's talk

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