Question 1

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 2

How do you build a sliding-window rate limiter with Redis + Lua?

Accepted Answer

**Target Audience:** Backend developers preparing for system design interviews who have used Redis as a cache. No Lua knowledge assumed.
**Difficulty:** Hard
**Verified with:** Redis 7.0.15 (the Lua script, run with `redis-cli`), plus the simulation in 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 ex

Question 3

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 4

How would you design a rate limiter?

Accepted Answer

**Target Audience:** Developers preparing for system design interviews. You know what an API and a load balancer are; the algorithms are covered on sibling pages and only summarised here.
**Difficulty:** Hard
**Verified with:** Node 22.22, Python 3.12, Go 1.24 and Java 21 (all four print identical output). Memory figures were measured on Redis 7.0.15.

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

Question 5

How would you design a metrics monitoring and alerting system?

Accepted Answer

**Target Audience:** Engineers who use dashboards and alerts every day and want to understand how the system behind them is built, and candidates facing a "design Prometheus / Datadog" round.
**Difficulty:** Hard
**Verified with:** Node 22.22, Python 3.12, Go 1.24 and Java 21 for the alerting example (identical output), Node 22.22 and Python 3.12 for the compression example. Burn-rate thresholds are those of the Google SRE Workbook chapter "Alerting on SLOs" as reproduced by Last9, Pydantic Logf

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Medium

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

Hard

How do you build a sliding-window rate limiter with Redis + Lua?

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

How would you design a rate limiter?

How would you design a metrics monitoring and alerting system?