Question 1

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 2

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 3

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 4

What is a write-ahead log (WAL)?

Accepted Answer

**Target Audience:** Beginners to database internals — you know a database stores data on disk and that a server can crash; everything else is defined as we go.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). PostgreSQL settings were read with `SHOW` on a local PostgreSQL 16.15 server; SQLite behaviour was checked with SQLite 3.45.1 from Python.

> **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 I

Question 5

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 6

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

Question 7

What is backpressure and how do you handle it?

Accepted Answer

**Target Audience:** Beginners who have built a service or a data pipeline and want to understand why a fast producer and a slow consumer cause outages, and how systems push back.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (the strategy simulation prints identical output in both); the Node.js stream example ran on Node 22.22 and reads its own default `highWaterMark`. The earlier 16 KiB default is cited from the Node.js v22.0.0 release announcement, as reported in commun

Question 8

What is leader election in distributed systems?

Accepted Answer

**Target Audience:** Engineers who know what a server, a network and a replica are; consensus is introduced from scratch, and every term is defined as it appears.
**Difficulty:** Hard
**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. Election times, vote counts and failure beh

Question 9

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 10

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

Accepted Answer

**Target Audience:** Back-end developers who publish events from services that also own a database. You know what a database transaction and a message broker are; no prior knowledge of CDC assumed.
**Difficulty:** Hard
**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 counts come from running t

SYSTEM DESIGNDECODED.

Medium