Question 1

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 2

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 3

What is backpressure and how do you handle it?

Accepted Answer

**Target Audience:** Beginners who have built a service or a data pipeline and want to understand why a fast producer and a slow consumer cause outages, and how systems push back.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (the strategy simulation prints identical output in both); the Node.js stream example ran on Node 22.22 and reads its own default `highWaterMark`. The earlier 16 KiB default is cited from the Node.js v22.0.0 release announcement, as reported in commun

Question 4

How would you design a news feed system?

Accepted Answer

**Target Audience:** Engineers preparing for a senior system design round. You know what a server, a database and a cache are; feed-specific ideas are defined as we go.
**Difficulty:** Hard
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Product facts are cited inline and were checked in October 2026.

> **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.

Question 5

How would you design a notification system?

Accepted Answer

**Target Audience:** Engineers preparing for a system design round; queues, retries and push notifications are explained from the start.
**Difficulty:** Hard
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Provider facts (FCM HTTP v1 migration, FCM payload size, SMS segment lengths, Gmail and Yahoo bulk sender rules) checked in October 2026 and cited inline.

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

Question 6

How would you design a typeahead/autocomplete system?

Accepted Answer

**Target Audience:** Engineers preparing for a system design round; you know what a server and a database are, and data structures like tries are explained from scratch.
**Difficulty:** Hard
**Verified with:** Node 22.22, Python 3.12, Go 1.24 and Java 21 (all four print identical output). The capacity figures are worked arithmetic from stated assumptions, not measurements of a real service.

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

Question 7

How would you design an analytics/metrics pipeline?

Accepted Answer

**Target Audience:** Engineers who have sent analytics events from an app and want to know how they become trustworthy dashboard numbers, and candidates preparing for a data-pipeline design round.
**Difficulty:** Hard
**Verified with:** Node 22.22 and Python 3.12 (both examples print identical output in both). The Lambda/Kappa history is from coverage of Jay Kreps's 2014 O'Reilly essay "Questioning the Lambda Architecture" (HPCwire, Kai Waehner); Redis HyperLogLog size and error (12 KB, 0.81%) f

Question 8

How would you design ad click event aggregation?

Accepted Answer

**Target Audience:** Intermediate engineers preparing for system design rounds — you know what a queue and a database are; every streaming term is defined when it first appears.
**Difficulty:** Hard
**Verified with:** Node 22.22, Python 3.12, Go 1.24 and Java 21 (all four print identical output). Traffic figures are stated assumptions. Apache Flink windowing behaviour (allowed lateness, late-data side outputs) is from the Flink windows documentation, checked October 2026.

> **How to read this d

Question 9

How would you design YouTube?

Accepted Answer

**Target Audience:** Engineers preparing for a system design round. You know what a server, a file and HTTP are; video encoding and streaming terms are defined as we go.
**Difficulty:** Hard
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Product and specification facts are cited inline, checked in October 2026.

> **How to read this doc:** Plain language first, then `📌 Interview term:` callouts. Every number in the capacity and player sections comes from the exampl

Question 10

How would you design a Web Crawler?

Accepted Answer

**Target Audience:** Engineers preparing for a system design round; no prior knowledge of crawling or search engines is assumed, and every term is defined when it first appears.
**Difficulty:** Hard
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). The robots.txt standard (RFC 9309) and search engine handling of `Crawl-delay` checked against the RFC and Google Search Central in October 2026.

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

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