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

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 3

When do you use a read replica vs a cache?

Accepted Answer

**Target Audience:** Beginners to system design — you know what a database and a query are; replication and caching are explained here.
**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 claims (stale reads, counts) come from running the example; the **Verified** sec

Question 4

What is database replication?

Accepted Answer

**Target Audience:** Beginners to distributed data — you know what a database server is; no prior knowledge of replication 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 5

What is the difference between strong and eventual consistency?

Accepted Answer

**Target Audience:** Beginners to distributed systems — you know what a database and a replica copy are; everything else is defined as we go.
**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. Stale-read rates and timings come from the example program; the **Verified** section

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

How would you design a news feed system?

Accepted Answer

**Target Audience:** Engineers preparing for a senior system design round. You know what a server, a database and a cache are; feed-specific ideas are defined as we go.
**Difficulty:** Hard
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Product facts are cited inline and were checked in October 2026.

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

Question 8

How would you design a distributed cache?

Accepted Answer

**Target Audience:** Intermediate — you know what a cache is and what a hash function does; distributed-systems vocabulary 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. Slot numbers, key movements and node loads come from runn

Question 9

What are event sourcing and CQRS?

Accepted Answer

**Target Audience:** Back-end developers who have built CRUD services and keep hearing these two terms together. No prior knowledge of either 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 behaviour shown comes from running the example; the **Verified** section

Question 10

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

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