Question 1

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 2

How do you handle asynchronism, message queues and backpressure at scale?

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** triggers async work — POST returns 202, job enqueued (email, thumbnail, analytics), polled or webhook on done.
- **Service** consumes — workers pull with prefetch, ack on success, DLQ on poison, retry with backoff.
- **Service** handles backpressure — bounded queues, shed/slow producers, scale consumers on

Question 3

What is database connection pooling?

Accepted Answer

**Target Audience:** Backend beginners — you have written code that queries a database; no knowledge of 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 outpu

Question 4

What is the publish-subscribe pattern?

Accepted Answer

**Target Audience:** Beginners to system design who have written services that call each other; no messaging background assumed.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Redis and Kafka behaviour is cited from their documentation.

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

Question 5

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 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

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 8

What is the difference between throttling and rate limiting?

Accepted Answer

**Target Audience:** Developers preparing for system design or backend interviews. No prior knowledge of rate limiting 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 behaviour shown comes from running the example; the **Verified** section quotes the full outpu

Question 9

What are blue-green and canary deployments?

Accepted Answer

**Target Audience:** Beginners to release engineering — you know what a server, a load balancer and a deploy are; every term is defined as we go.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Kubernetes defaults are from the Kubernetes Deployment API reference; the canary definition is from the Google SRE Workbook, chapter 16, checked in October 2026.

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

Question 10

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

SYSTEM DESIGNDECODED.

Medium

What is idempotency and why does it matter?

How do you handle asynchronism, message queues and backpressure at scale?

What is database connection pooling?

What is the publish-subscribe pattern?

How would you design OpenTelemetry tracing for 200 microservices?

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

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

What is the difference between throttling and rate limiting?

Hard

How would you design a distributed job scheduler?

How would you design a notification system?

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

How would you design a URL shortener like TinyURL?

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

Exactly-once processing — can Kafka or SQS guarantee it?

How would you design a distributed email service?

How would you design a distributed message queue?