Question 1

How does DNS resolution work?

Accepted Answer

**Target Audience:** Beginners to system design — you have used a browser; 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 claims come from running the example; the **Verified** section quotes the full output.

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Question 2

How do you design for high availability?

Accepted Answer

**Target Audience:** Beginners to reliability engineering — you know what a server, a load balancer and a database are; every reliability term is defined as we go.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). All component availabilities in the run are stated assumptions, not vendor SLAs; AWS health-check defaults are from the Amazon Route 53 Developer Guide, checked in October 2026.

> **How to read this doc:** Every idea is explained in pl

Question 3

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 4

When would you choose SQL vs NoSQL?

Accepted Answer

**Target Audience:** Beginners to system design — you have used a spreadsheet or a database table; no database internals 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 output.

Question 5

What is the difference between horizontal and vertical scaling?

Accepted Answer

**Target Audience:** Beginners to system design — you know what a server and a database are, nothing more.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (the example prints the same table in both).

> **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. Every number below comes from actually running the example program; the **Verified** section quotes its f

Question 6

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 7

What are the three pillars of observability?

Accepted Answer

**Target Audience:** Beginners who have seen a dashboard or grepped a log file and want to understand what observability means, and candidates who need crisp vocabulary for an interview.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). The history of the "three pillars" phrase is from CrowdStrike's "Observability (Re)defined" article; the profiles signal status from the OpenTelemetry documentation and ClickHouse's write-up of the Alpha announcem

Question 8

What is a load balancer and what strategies does it use?

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, 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 numbers come from actually running the example; the **Verified** section quotes the f

Question 9

What is two-phase commit and its drawbacks?

Accepted Answer

**Target Audience:** Developers who know what a database transaction is and now meet data spread over several databases or services. No distributed-systems 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. All timings and counts come from running the examp

Question 10

Resumable browser uploads to S3 — presigned multipart with checksums?

Accepted Answer

**Target Audience:** Developers preparing for system design or full-stack interviews who know roughly what S3 is. The multipart mechanics themselves are covered on the [S3 multipart upload page](/interview-question/how-does-s3-multipart-upload-make-5tb-uploads-resumable-and-fast); this page is about doing it safely from a browser.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). S3 behaviour and limits are from AWS documentation, cited inline.

SYSTEM DESIGNDECODED.

Easy

How does DNS resolution work?

Medium

How do you design for high availability?

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

When would you choose SQL vs NoSQL?

What is the difference between horizontal and vertical scaling?

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

What are the three pillars of observability?

What is a load balancer and what strategies does it use?

What is two-phase commit and its drawbacks?

Hard

How would you design a distributed cache?

How do you make a payment system reliable against retries?

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

How would you design a typeahead/autocomplete system?

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

How would you design a rate limiter?

Outbox pattern vs polling for CDC — how do you avoid dual-write failure?

How would you design a distributed key-value store?