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

**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 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 the CAP theorem and how does it drive consistency vs availability tradeoffs?

Accepted Answer

# CAP Theorem: Consistency vs. Availability Tradeoffs (Beginner-Friendly Guide)

**Target Audience:** Junior & Senior Software Engineers preparing for System Design Interviews — no prior system design knowledge assumed.
**Difficulty:** Easy to Medium

> **How to read this doc:** Every concept is explained in plain language first. Right after, you'll see a callout like `📌 Interview term:` — that's the exact vocabulary a senior engineer would use for the same idea.

## 1. Why This Even Matters —

Question 3

When would you choose SQL vs NoSQL?

Accepted Answer

**Target Audience:** Beginners to system design — you have used a spreadsheet or a database table; no database internals 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 output.

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

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 6

How would you design an edge-native app on Cloudflare Workers + R2?

Accepted Answer

**Target Audience:** Beginners to edge and serverless design — you know HTTP, caching and a database; every Cloudflare product used is explained when it first appears.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Cloudflare limits and prices are from the Cloudflare developer docs (Workers limits, Workers pricing, R2 pricing, storage overview, Cache API, Durable Objects concepts, D1 overview), checked October 2026; they change, so recheck bef

Question 7

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 8

What is a read/write quorum?

Accepted Answer

**Target Audience:** Beginners to distributed systems who know that databases can keep copies of data on several machines.
**Difficulty:** Medium
**Verified with:** Node 22.22, Python 3.12, Go 1.24 and Java 21 (all four print identical output). Cassandra consistency levels are cited from the DataStax Cassandra documentation; Dynamo techniques from the Dynamo paper (SOSP 2007) as summarised in course material.

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

Question 9

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

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** searches — GET `/search?q=...`, cache lookup by normalized query hash, hit returns in <20ms, miss fetches search tier.
- **Service** caches query results — write-through on miss with TTL + jitter, LRU eviction, per-tenant isolation.
- **Service** invalidates — TTL expiry plus explicit invalidation on index

Question 10

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

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