Question 1

What is the publish-subscribe pattern?

Accepted Answer

**Target Audience:** Beginners to system design who have written services that call each other; no messaging background assumed.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (both print identical output). Redis and Kafka behaviour is cited from their documentation.

> **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the words an interviewer expects. Behaviour claims come from the example; the

Question 2

Token bucket vs sliding window vs fixed window — which rate limiter when?

Accepted Answer

**Target Audience:** Developers preparing for system design interviews. You know what an HTTP request is; no rate limiting background assumed.
**Difficulty:** Medium
**Verified with:** 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 expects. All behaviour shown comes from running the example; the **Ver

Question 3

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 4

Why prefer stateless services and how do you handle sessions?

Accepted Answer

**Target Audience:** Beginners to system design — you have built a web app with a login, nothing more.
**Difficulty:** Medium
**Verified with:** Node 22.22 and Python 3.12 (the example prints the same output, byte for byte, including the token).

> **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 output shown was produced by actually running the example; the **Verified**

Question 5

How does caching work end-to-end (where, what, when) — cache-aside vs write-through?

Accepted Answer

# How Does Caching Work End-to-End? Where, What, and When (Cache-Aside vs. Write-Through) **Target Audience:** Junior & Senior Software Engineers preparing for System Design Interviews — assumes basic familiarity with the idea of a cache. **Difficulty:** Medium > This doc builds on the Caching guide (eviction policies, LRU) and the

Question 6

How does caching improve system performance?

Accepted Answer

**Target Audience:** Beginners to system design — you know what a database query is; no caching 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. Hit ratios, latencies and counts come from running the example; the **Verified** section quotes the full outpu

Question 7

What is a Bloom filter and when is it useful?

Accepted Answer

**Target Audience:** Beginners to system design — you know what a hash function and a set are; the probability is worked through step by step.
**Difficulty:** Medium
**Verified with:** 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 expects. Error rates and sizes come from running the example; the **Ve

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

How would you design a distributed cache?

Accepted Answer

**Target Audience:** Intermediate — you know what a cache is and what a hash function does; distributed-systems vocabulary is defined as it appears.
**Difficulty:** Hard
**Verified with:** 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 expects. Slot numbers, key movements and node loads come from runn

Question 10

How do you scale WebSockets to 1M connections with Redis Pub/Sub?

Accepted Answer

**Target Audience:** Engineers who have run a single WebSocket server and now need many. Read [When would you use WebSockets vs polling vs SSE?](/interview-question/when-would-you-use-websockets-vs-polling-vs-sse) and [WebSocket presence, sticky sessions, and resume — do you need sticky LB?](/interview-question/websocket-presence-sticky-sessions-and-resume-do-you-need-sticky-lb) first if those are new.
**Difficulty:** Hard
**Verified with:** Node 22.22, Python 3.12, Go 1.24 and Java 21 (all four

SYSTEM DESIGNDECODED.

Medium

What is the publish-subscribe pattern?

Token bucket vs sliding window vs fixed window — which rate limiter when?

When do you use a read replica vs a cache?

Why prefer stateless services and how do you handle sessions?

How does caching work end-to-end (where, what, when) — cache-aside vs write-through?

How does caching improve system performance?

What is a Bloom filter and when is it useful?

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

Hard

How would you design a distributed cache?

How do you scale WebSockets to 1M connections with Redis Pub/Sub?

How would you design a 'nearby drivers' / proximity search?

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

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

How would you design a rate limiter?

How does CDC with Debezium keep search and cache in sync with Postgres?

How would you design Twitter's timeline and search? (Facebook feed)