Question 1

What is the difference between a reverse proxy and a forward proxy?

Accepted Answer

**Target Audience:** Beginners to system design — you know a browser talks to a server; no networking background assumed.
**Difficulty:** Easy
**Verified with:** Node 22.22 and Python 3.12 (both print identical output).

> **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. Behaviour shown comes from running the example; the **Verified** section quotes the full output.

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Question 2

What is an API gateway?

Accepted Answer

**Target Audience:** Beginners to system design — you know what an HTTP API is; no microservices background assumed.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output).

> **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. Behaviour and latency numbers come from running the example; the **Verified** section quotes the full output.

Question 3

How would you design OpenTelemetry tracing for 200 microservices?

Accepted Answer

**Target Audience:** Engineers who know what a trace is (see the pillars doc) and want to design tracing for a real, polyglot microservice fleet, and candidates preparing for an observability design round.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Header format from the W3C Trace Context Recommendation; Collector component behaviour from the loadbalancing exporter README on pkg.go.dev and Grafana Alloy documentation of the tail sampling p

Question 4

When would you use WebSockets vs polling vs SSE?

Accepted Answer

**Target Audience:** Beginners to system design who have built a web page that calls an API; no networking background assumed.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Protocol facts are cited from RFC 6455, RFC 8441, MDN and the AWS load balancer docs.

> **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the words an interviewer expects. Numbers come from r

Question 5

What is a CDN and how does it work?

Accepted Answer

**Target Audience:** Beginners to system design — you know what a web server is; no networking background assumed.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output).

> **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. Hit ratios and fetch counts come from running the example; the **Verified** section quotes the full output.

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Question 6

How would you design a distributed cache?

Accepted Answer

**Target Audience:** Intermediate — you know what a cache is and what a hash function does; distributed-systems vocabulary is defined as it appears.
**Difficulty:** Hard
**Verified with:** Node 22.22, Python 3.12, Go 1.24 and Java 21 (all four print identical output).

> **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. Slot numbers, key movements and node loads come from runn

Question 7

How do you scale WebSockets to 1M connections with Redis Pub/Sub?

Accepted Answer

**Target Audience:** Engineers who have run a single WebSocket server and now need many. Read [When would you use WebSockets vs polling vs SSE?](/interview-question/when-would-you-use-websockets-vs-polling-vs-sse) and [WebSocket presence, sticky sessions, and resume — do you need sticky LB?](/interview-question/websocket-presence-sticky-sessions-and-resume-do-you-need-sticky-lb) first if those are new.
**Difficulty:** Hard
**Verified with:** Node 22.22, Python 3.12, Go 1.24 and Java 21 (all four

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