Question 1

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 2

Head vs tail sampling in OpenTelemetry — when does each break?

Accepted Answer

**Target Audience:** Engineers who have set up tracing and now have to choose a sampling strategy, and candidates who want to discuss sampling trade-offs with numbers rather than slogans.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Tail sampling processor settings (`decision_wait` default 30 s described as time since the first span, `num_traces` default 50000) from the Grafana Alloy reference for `otelcol.processor.tail_sampling` (a wrapper

Question 3

Why prefer stateless services and how do you handle sessions?

Accepted Answer

**Target Audience:** Beginners to system design — you have built a web app with a login, nothing more.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (the example prints the same output, byte for byte, including the token).

> **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 output shown was produced by actually running the example; the **Verified**

Question 4

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 5

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 6

How would you design S3-like object storage?

Accepted Answer

**Target Audience:** Developers preparing for system design interviews. You know what a database, a disk and a load balancer are; no storage-systems background needed.
**Difficulty:** Hard
**Verified with:** Node 22.22, Python 3.12, Go 1.24 and Java 21 (all four print identical output). AWS facts are cited inline; the design is a generic object store, not a description of S3 internals, which AWS has published only in part.

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

SYSTEM DESIGNDECODED.

Medium