Question 1

How do you design for high availability?

Accepted Answer

**Target Audience:** Beginners to reliability engineering — you know what a server, a load balancer and a database are; every reliability term is defined as we go.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). All component availabilities in the run are stated assumptions, not vendor SLAs; AWS health-check defaults are from the Amazon Route 53 Developer Guide, checked in October 2026.

> **How to read this doc:** Every idea is explained in pl

Question 2

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 3

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 4

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 5

What is the circuit breaker pattern?

Accepted Answer

**Target Audience:** Beginners who have called one service from another and want to understand why one slow dependency can take a whole system down, and how a breaker prevents it.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Library defaults are cited from the Apache Camel Resilience4j configuration reference; the Hystrix status from the Netflix Hystrix README; the pattern history from Martin Fowler's "CircuitBreaker" article (2014).

> **Ho

Question 6

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 7

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 8

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

Question 9

How would you design a notification system?

Accepted Answer

**Target Audience:** Engineers preparing for a system design round; queues, retries and push notifications are explained from the start.
**Difficulty:** Hard
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Provider facts (FCM HTTP v1 migration, FCM payload size, SMS segment lengths, Gmail and Yahoo bulk sender rules) checked in October 2026 and cited inline.

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

Question 10

How do you make a payment system reliable against retries?

Accepted Answer

**Target Audience:** Engineers who know what an API and a database transaction are, and want to design the money path so that retries and crashes can never double-charge a customer.
**Difficulty:** Hard
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Stripe idempotency behaviour is cited from the Stripe API reference ("Idempotent requests"). Capacity numbers are worked arithmetic from stated assumptions, recomputed in Python.

> **How to read this doc:** Every idea i

SYSTEM DESIGNDECODED.

Medium

How do you design for high availability?

What is idempotency and why does it matter?

What is the difference between optimistic and pessimistic locking?

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

What is the circuit breaker pattern?

What is the difference between throttling and rate limiting?

What is the saga pattern for distributed transactions?

Hard

How would you design a distributed job scheduler?

How would you design a notification system?

How do you make a payment system reliable against retries?

How would you design a real-time chat system?

What are event sourcing and CQRS?

How would you design a typeahead/autocomplete system?

How would you design an analytics/metrics pipeline?

How would you design a distributed email service?