Question 1

Why prefer stateless services and how do you handle sessions?

Accepted Answer

**Target Audience:** Beginners to system design — you have built a web app with a login, nothing more.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (the example prints the same output, byte for byte, including the token).

> **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact words an interviewer expects. All output shown was produced by actually running the example; the **Verified**

Question 2

WebSocket presence, sticky sessions, and resume — do you need sticky LB?

Accepted Answer

**Target Audience:** Engineers who know what a WebSocket is (see [When would you use WebSockets vs polling vs SSE?](/interview-question/when-would-you-use-websockets-vs-polling-vs-sse)) and want to run them on more than one server.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Socket.IO and AWS facts are cited from their documentation.

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview

Question 3

What are blue-green and canary deployments?

Accepted Answer

**Target Audience:** Beginners to release engineering — you know what a server, a load balancer and a deploy are; every term is defined as we go.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Kubernetes defaults are from the Kubernetes Deployment API reference; the canary definition is from the Google SRE Workbook, chapter 16, checked in October 2026.

> **How to read this doc:** Every idea is explained in plain language first, then tagged wi

SYSTEM DESIGNDECODED.

Medium

Why prefer stateless services and how do you handle sessions?

WebSocket presence, sticky sessions, and resume — do you need sticky LB?

What are blue-green and canary deployments?