Question 1

What is the difference between REST and GraphQL?

Accepted Answer

**Target Audience:** Beginners to system design — you have called an HTTP API; no GraphQL experience 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. Request counts and byte sizes come from running the example; the **Verified** section quotes the full output.

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Question 2

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 3

What is idempotency and why does it matter?

Accepted Answer

**Target Audience:** Beginners who have built or called a web API and want to understand why retries are dangerous and how real systems make them safe.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). HTTP method semantics are cited from RFC 9110; Stripe behaviour from the Stripe API reference ("Idempotent requests"); the Idempotency-Key header from the IETF httpapi working-group Internet-Draft.

> **How to read this doc:** Every idea is explain

Question 4

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 5

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 6

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 7

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.

Question 8

What are the trade-offs of microservices vs a monolith?

Accepted Answer

**Target Audience:** Beginners to architecture — you have built a web application; every term is defined the first time it is used.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Hop latencies, availabilities and memory sizes are stated assumptions. The case studies are cited to their published coverage (InfoQ, July 2018, on Segment; DevClass, May 2023, on Prime Video), checked October 2026.

> **How to read this doc:** Plain language first, t

Question 9

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 10

What is the circuit breaker pattern?

Accepted Answer

**Target Audience:** Beginners who have called one service from another and want to understand why one slow dependency can take a whole system down, and how a breaker prevents it.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Library defaults are cited from the Apache Camel Resilience4j configuration reference; the Hystrix status from the Netflix Hystrix README; the pattern history from Martin Fowler's "CircuitBreaker" article (2014).

> **Ho

SYSTEM DESIGNDECODED.

Medium

What is the difference between REST and GraphQL?

What is the difference between horizontal and vertical scaling?

What is idempotency and why does it matter?

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

What is an API gateway?

What is two-phase commit and its drawbacks?

Resumable browser uploads to S3 — presigned multipart with checksums?

What are the trade-offs of microservices vs a monolith?

Hard

How would you design a notification system?

How do you make a payment system reliable against retries?

How would you design Mint.com (personal finance aggregator)?

How do you build a sliding-window rate limiter with Redis + Lua?

How would you design a rate limiter?

How would you design an analytics/metrics pipeline?

How would you design AI inference infra with GPU queue and KV-cache?

How does CDC with Debezium keep search and cache in sync with Postgres?