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 distributed systems detect node failures?

Accepted Answer

**Target Audience:** Beginners who know a cluster is several servers working together and want to understand how the cluster notices when one of them stops, and why it sometimes gets it wrong.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). The FLP result is from Fischer, Lynch and Paterson (JACM, 1985); Kubernetes lease behaviour from the Kubernetes node-status documentation; memberlist and Lifeguard from HashiCorp's "Making Gossip More Robust

Question 3

What is consistent hashing?

Accepted Answer

**Target Audience:** Beginners to system design who know what a hash function and a cache are; no distributed-systems background assumed.
**Difficulty:** Medium
**Verified with:** Node 22.22, Python 3.12, Go 1.24 and Java 21 (all four print identical output). History is cited from Karger et al., STOC 1997; system usage from the Dynamo paper (SOSP 2007) and the Apache Cassandra documentation.

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

Question 4

What is the CAP theorem?

Accepted Answer

**Target Audience:** Beginners to distributed systems — you know a database stores data and that servers talk over a network; 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. The numbers in the worked run come from the example program; th

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

What is two-phase commit and its drawbacks?

Accepted Answer

**Target Audience:** Developers who know what a database transaction is and now meet data spread over several databases or services. No distributed-systems background 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 timings and counts come from running the examp

Question 7

What is the difference between optimistic and pessimistic locking?

Accepted Answer

**Target Audience:** Beginners to databases and concurrency — you know what a row, a transaction and an `UPDATE` are; locking is built up from there.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). PostgreSQL defaults quoted below were read with `SHOW` on a local PostgreSQL 16.15 server; the SQL patterns are shown for illustration and were not executed in this run.

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

Question 8

How would you design a rate limiter (token bucket vs leaky bucket)?

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** calls API — each request checks limiter (user_id / IP / API key) against budget (e.g., 100/min), 429 if over.
- **Service** refills — token bucket adds tokens at fixed rate, allows bursts up to capacity; leaky bucket drains at constant rate (smooth).
- **Service** windows — fixed vs sliding window counters

Question 9

What is the saga pattern for distributed transactions?

Accepted Answer

**Target Audience:** Developers who build or interview for microservice back ends. You know what a database transaction is; no distributed-systems background 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 **V

Question 10

How would you design a distributed job scheduler?

Accepted Answer

**Target Audience:** Engineers preparing for a system design round. You know what a database, a queue and a server are; leases, fencing and misfire policies are explained from zero.
**Difficulty:** Hard
**Verified with:** Node 22 and Python 3.12 (identical output). Kubernetes CronJob and PostgreSQL facts are cited inline, checked in October 2026.

> **How to read this doc:** Plain language first, then `📌 Interview term:` callouts with the vocabulary an interviewer expects. Every number in the d

SYSTEM DESIGNDECODED.

Medium

What is idempotency and why does it matter?

How do distributed systems detect node failures?

What is consistent hashing?

What is the CAP theorem?

How would you design OpenTelemetry tracing for 200 microservices?

What is two-phase commit and its drawbacks?

What is the difference between optimistic and pessimistic locking?

How would you design a rate limiter (token bucket vs leaky bucket)?

Hard

How would you design a distributed job scheduler?

What is leader election in distributed systems?

How would you design a distributed cache?

How would you design a distributed key-value store / query cache for search?

How would you design a Web Crawler at scale?

How would you design a distributed key-value store?

How would you design a distributed email service?

How would you design a distributed message queue?