Q: How would you design an analytics/metrics pipeline?

**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

Q: How would you design a URL shortener like TinyURL?

**Target Audience:** Engineers who know the basic shortener design (codes, a table keyed by code) and want to go deep on scaling reads; caching terms are defined as they come up. **Difficulty:** Hard **Verified with:** Node 22.22, Python 3.12, Go 1.24 and Java 21 (all four print identical output). HTTP caching rules for redirects checked against RFC 9110 in October 2026. > **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you als

Question 1

Kafka vs SQS — how do you choose for an event-driven system in 2025?

Accepted Answer

**Target Audience:** Beginners to messaging — you know what a server and a database are; queues, logs, offsets and consumer groups are defined as we go.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Product limits are from the AWS SQS Developer Guide and the Apache Kafka release announcements, checked in October 2026.

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

Question 2

How would you design a feature flag system (LaunchDarkly-style) at scale?

Accepted Answer

**Target Audience:** Beginners to system design — you know what a web service and a database are; every flag-system term is defined as it appears.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). The fleet, flag counts and latencies are stated assumptions; LaunchDarkly behaviour quoted below is from LaunchDarkly documentation (architecture overview and SDK contributor guide), checked October 2026.

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

Question 3

How would you design a URL shortener like TinyURL? (Pastebin / Bit.ly)

Accepted Answer

## Step 1: Use Cases, Constraints, Assumptions, Back-of-Envelope

### Use cases
We'll scope to handle only the core flows that expose the bottlenecks; everything else is deferred.

- **User** creates a paste / short URL — POST long URL or text, get back `https://sho.rt/Ab3x9K` (302 redirect for URL, render for paste).
- **User** reads a paste/short URL — GET `/Ab3x9K`, cache-hit serves in <10ms, miss hits DB then populates cache.
- **Service** generates short code — deterministic Base62 of auto-

Question 4

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 5

How would you design a URL shortener like TinyURL?

Accepted Answer

**Target Audience:** Engineers who know the basic shortener design (codes, a table keyed by code) and want to go deep on scaling reads; caching terms are defined as they come up.
**Difficulty:** Hard
**Verified with:** Node 22.22, Python 3.12, Go 1.24 and Java 21 (all four print identical output). HTTP caching rules for redirects checked against RFC 9110 in October 2026.

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

Question 6

How would you design Amazon's category sales ranking?

Accepted Answer

## Step 1: Use Cases, Constraints, Assumptions, Back-of-Envelope

### Use cases
We'll scope to handle only the core flows that expose the bottlenecks; everything else is deferred.

- **Service** ingests orders — order events (item, category, qty) into durable log (Kinesis/Kafka).
- **Service** aggregates rank — sliding window (hourly/daily) count per category, sorted set, rank = position in sorted order.
- **User** reads rank — GET `/category/{id}/rank?item_id=...`, <50ms p95, stale <1m acceptab

SYSTEM DESIGNDECODED.

Medium