Question 1

How does DNS resolution work?

Accepted Answer

**Target Audience:** Beginners to system design — you have used a browser; no networking background assumed.
**Difficulty:** Easy
**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 come from running the example; the **Verified** section quotes the full output.

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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

How would you design Google Drive?

Accepted Answer

**Target Audience:** Developers preparing for system design interviews. This page builds on the [Dropbox design](/interview-question/how-would-you-design-a-file-storage-service-like-dropbox) (block sync) and the [S3-like object storage design](/interview-question/how-would-you-design-s3-like-object-storage-572x) (blob storage) and focuses on what is different: an ID-based shared tree, permissions, the change feed and uploads.
**Difficulty:** Hard
**Verified with:** Node 22.22 and Python 3.12 (bo

Question 4

How would you design Nearby Friends?

Accepted Answer

**Target Audience:** Developers preparing for system design interviews. You know what a cache and a WebSocket are; the geo indexing side is covered on the sibling page about nearby drivers.
**Difficulty:** Hard
**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

Question 5

How would you design Google Maps?

Accepted Answer

**Target Audience:** Developers preparing for system design interviews. You know what a CDN and a graph are; shortest-path algorithms are explained from scratch.
**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. All behaviour shown comes from running the e

Question 6

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

Question 7

How would you design a hotel reservation system?

Accepted Answer

**Target Audience:** Backend engineers preparing for "design a booking system" rounds, and anyone who wants to see why "check, then insert" is the most expensive bug in reservations.
**Difficulty:** Hard
**Verified with:** Node 22.22 and Python 3.12 (identical output). The race and the fixes are simulated in-process; the SQL statements shown are the standard PostgreSQL and MySQL forms of the same logic.

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

Question 8

How would you design S3-like object storage?

Accepted Answer

**Target Audience:** Developers preparing for system design interviews. You know what a database, a disk and a load balancer are; no storage-systems background needed.
**Difficulty:** Hard
**Verified with:** Node 22.22, Python 3.12, Go 1.24 and Java 21 (all four print identical output). AWS facts are cited inline; the design is a generic object store, not a description of S3 internals, which AWS has published only in part.

> **How to read this doc:** Every idea is explained in plain language fi

Question 9

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 10

How would you design ad click event aggregation?

Accepted Answer

**Target Audience:** Intermediate engineers preparing for system design rounds — you know what a queue and a database are; every streaming term is defined when it first appears.
**Difficulty:** Hard
**Verified with:** Node 22.22, Python 3.12, Go 1.24 and Java 21 (all four print identical output). Traffic figures are stated assumptions. Apache Flink windowing behaviour (allowed lateness, late-data side outputs) is from the Flink windows documentation, checked October 2026.

> **How to read this d

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