Question 1

Edge caching on Cloudflare — how do you get 95%+ hit rate without stale data?

Accepted Answer

**Target Audience:** Intermediate — you know what a CDN does; Cloudflare-specific settings and terms are explained here.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Cloudflare product behaviour is taken from Cloudflare documentation and changelog entries cited inline, 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 exact words

Question 2

Token bucket vs sliding window vs fixed window — which rate limiter when?

Accepted Answer

**Target Audience:** Developers preparing for system design interviews. You know what an HTTP request is; no rate limiting background assumed.
**Difficulty:** Medium
**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. All behaviour shown comes from running the example; the **Ver

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

What is database replication?

Accepted Answer

**Target Audience:** Beginners to distributed data — you know what a database server is; no prior knowledge of replication 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 shown comes from running the example; the **Verified** section quotes the full outpu

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 an edge-native app on Cloudflare Workers + R2?

Accepted Answer

**Target Audience:** Beginners to edge and serverless design — you know HTTP, caching and a database; every Cloudflare product used is explained when it first appears.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Cloudflare limits and prices are from the Cloudflare developer docs (Workers limits, Workers pricing, R2 pricing, storage overview, Cache API, Durable Objects concepts, D1 overview), checked October 2026; they change, so recheck bef

Question 7

How does S3 multipart upload make 5TB uploads resumable and fast?

Accepted Answer

**Target Audience:** Developers preparing for system design interviews, and anyone who has watched a large upload fail at 97%. No prior S3 knowledge needed.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). S3 limits and dates are from AWS documentation and announcements, cited inline.

> **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 interviewe

Question 8

What is a message queue and why use one?

Accepted Answer

**Target Audience:** Beginners to system design who have built an API that does all its work inside the request.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Amazon SQS and Kafka facts are cited from AWS and Apache announcements and docs.

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

Question 9

What is a read/write quorum?

Accepted Answer

**Target Audience:** Beginners to distributed systems who know that databases can keep copies of data on several machines.
**Difficulty:** Medium
**Verified with:** Node 22.22, Python 3.12, Go 1.24 and Java 21 (all four print identical output). Cassandra consistency levels are cited from the DataStax Cassandra documentation; Dynamo techniques from the Dynamo paper (SOSP 2007) as summarised in course material.

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

Question 10

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

SYSTEM DESIGNDECODED.

Medium

Edge caching on Cloudflare — how do you get 95%+ hit rate without stale data?

Token bucket vs sliding window vs fixed window — which rate limiter when?

How would you design OpenTelemetry tracing for 200 microservices?

What is database replication?

What is a CDN and how does it work?

How would you design an edge-native app on Cloudflare Workers + R2?

How does S3 multipart upload make 5TB uploads resumable and fast?

What is a message queue and why use one?

Hard

How would you design a distributed cache?

How do you make a payment system reliable against retries?

How would you design a real-time chat system?

How would you design Twitter's timeline at scale?

How would you design a typeahead/autocomplete system?

Advanced feature flags — targeting, gradual rollout, and edge evaluation?

How would you design a rate limiter?

How would you design a URL shortener like TinyURL?