Question 1

When would you use WebSockets vs polling vs SSE?

Accepted Answer

**Target Audience:** Beginners to system design who have built a web page that calls an API; no networking background assumed.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Protocol facts are cited from RFC 6455, RFC 8441, MDN and the AWS load balancer docs.

> **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. Numbers come from r

Question 2

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 3

How would you design a feature flag system (LaunchDarkly-style) at scale?

Accepted Answer

**Target Audience:** Beginners to system design — you know what a web service and a database are; every flag-system term is defined as it appears.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). The fleet, flag counts and latencies are stated assumptions; LaunchDarkly behaviour quoted below is from LaunchDarkly documentation (architecture overview and SDK contributor guide), checked October 2026.

> **How to read this doc:** Every idea is expla

Question 4

Advanced feature flags — targeting, gradual rollout, and edge evaluation?

Accepted Answer

**Target Audience:** Intermediate engineers who know what a feature flag is (see the [feature flag system design](/interview-question/how-would-you-design-a-feature-flag-system-launchdarkly-style-at-scale)) and want the evaluation internals; every term is still defined.
**Difficulty:** Hard
**Verified with:** Node 22.22, Python 3.12, Go 1.24 and Java 21 for the bucketing example (identical output); Node and Python for the edge example. LaunchDarkly bucketing facts are from the LaunchDarkly perce

Question 5

How would you design a distributed email service?

Accepted Answer

**Target Audience:** Backend engineers preparing for a "design Gmail" round, and anyone curious how a message actually gets from one mailbox to another.
**Difficulty:** Hard
**Verified with:** Node 22.22 and Python 3.12 (identical output for both examples). SMTP reply classes are from RFC 5321; the Postfix default `maximal_queue_lifetime` of 5 days is from the Postfix queue manager documentation; the bulk-sender rules (more than 5,000 messages a day to Gmail: SPF, DKIM and DMARC, one-click unsub

Question 6

How would you design a payment system?

Accepted Answer

**Target Audience:** Backend engineers preparing for system design rounds at fintech, marketplace and e-commerce companies, and anyone who wants to understand where money actually lives inside a payment system.
**Difficulty:** Hard
**Verified with:** Node 22.22 and Python 3.12 (identical output). Stripe behaviour (amounts in minor units, a 7-day authorization window for most online card payments, shortened to 5 days for Visa online merchant-initiated transactions from April 2024) is from the Str

SYSTEM DESIGNDECODED.

Medium