# System Design Coach

> Expert system design coach covering scalability, distributed systems, and interview preparation based on Alex Xu's System Design Interview books (Vol 1 & 2). Use when designing large-scale systems, preparing for system design interviews, or needing architecture guidance for specific components like rate limiters, chat systems, payment systems, etc.

- Skill: `nafisrayan/system-design-coach` (Agent Skill, multi-file: 422 files)
- Install (CLI): `npx skillmds@latest add nafisrayan/system-design-coach`
- Raw SKILL.md: https://api.skillmd.com/api/skills/nafisrayan/system-design-coach/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- License: MIT
- Author: NafisRayan (https://skillmd.com/u/nafisrayan)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/nafisrayan/system-design-coach

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# System Design Pro

Expert system design coach grounded in Alex Xu's *System Design Interview* books (Vol 1 & 2). Covers the full spectrum from foundational concepts to real-world system designs.

## When to Use This Skill

- Preparing for system design interviews
- Designing or reviewing large-scale distributed systems
- Performing back-of-the-envelope estimations for capacity planning
- Learning about specific system design patterns (rate limiting, consistent hashing, etc.)
- Architecting real-world systems like chat, payment, search, or video platforms

## Core Workflow

1. **Understand the Problem** — Clarify requirements, ask clarifying questions, establish design scope and constraints
2. **Estimate Scale** — Perform back-of-the-envelope calculations for storage, bandwidth, QPS, and memory
3. **Propose High-Level Design** — Present architecture diagram, identify core components, and get agreement on the approach
4. **Deep Dive** — Walk through detailed component design, data models, API design, and key algorithms
5. **Address Trade-offs** — Discuss bottlenecks, failure modes, scaling strategies, and operational concerns
6. **Wrap Up** — Summarize decisions, identify future improvements, and discuss monitoring/alerting

> This workflow follows the 4-step framework from Chapter 3. Always start with requirements before jumping to solutions.

## Reference Guide

Load the relevant chapter based on the system being designed or the concept being studied:

### Foundational Concepts

| Topic | Reference | Load When |
|-------|-----------|-----------|
| Scaling Fundamentals | `01. Scaling/Readme.md` | Learning about vertical vs horizontal scaling, load balancers, CDNs, caching, database replication/sharding |
| Back-of-the-Envelope Estimation | `02. Back Of the Envelope Estimation/Readme.md` | Estimating QPS, storage, bandwidth, or memory for any system |
| System Design Framework | `03. System Design Framework/Readme.md` | Understanding the 4-step interview framework or structuring any design session |

### Core Building Blocks

| Topic | Reference | Load When |
|-------|-----------|-----------|
| Rate Limiter | `04. Rate Limiter/Readme.md` | Designing API rate limiting, token bucket, sliding window algorithms |
| Consistent Hashing | `05. Consistent Hashing/Readme.md` | Distributing data across nodes, virtual nodes, hash rings |
| Key-Value Store | `06. Key-Value Store/Readme.md` | Designing distributed KV stores, CAP theorem, replication strategies |
| Unique ID Generator | `07. Unique-Id Generator/Readme.md` | Generating distributed unique IDs, Snowflake, UUID strategies |

### Web & Content Systems

| Topic | Reference | Load When |
|-------|-----------|-----------|
| URL Shortener | `08. URL Shortener/Readme.md` | Designing URL shortening services, hash functions, base62 encoding |
| Web Crawler | `09. Web Crawler/Readme.md` | Designing web crawlers, BFS traversal, politeness, deduplication |
| News Feed System | `11. News Feed System/Readme.md` | Designing social media feeds, fan-out strategies, ranking |
| Search Autocomplete | `13. Search Autocomplete/Readme.md` | Designing typeahead/autocomplete, trie data structures, prefix matching |
| YouTube | `14. Youtube/Readme.md` | Designing video streaming platforms, transcoding, CDN delivery |
| Google Drive | `15. Google Drive/Readme.md` | Designing file storage/sync, block-level sync, conflict resolution |

### Communication & Real-Time Systems

| Topic | Reference | Load When |
|-------|-----------|-----------|
| Notification System | `10. Notification System/Readme.md` | Designing push notifications, SMS, email delivery pipelines |
| Chat System | `12. Chat System/Readme.md` | Designing messaging platforms, WebSocket, online presence |
| Distributed Message Queue | `19. Distributed Message Queue/Readme.md` | Designing message brokers, producer-consumer patterns, ordering guarantees |

### Location & Geo Systems

| Topic | Reference | Load When |
|-------|-----------|-----------|
| Proximity Service | `16. Proximity Service/Readme.md` | Designing location-based search, geohashing, quadtrees |
| Nearby Friends | `17. Nearby Friends/Readme.md` | Designing real-time location sharing, WebSocket pub/sub |
| Google Maps | `18. Google Maps/Readme.md` | Designing navigation, map tile serving, shortest path algorithms |

### Infrastructure & Monitoring

| Topic | Reference | Load When |
|-------|-----------|-----------|
| Metrics Monitoring & Alerting | `20. Metrics Monitoring and Alerting System/Readme.md` | Designing monitoring pipelines, time-series databases, alerting rules |
| Ad Click Event Aggregation | `21. Ad Click Event Aggregation/Readme.md` | Designing real-time event aggregation, MapReduce, stream processing |
| S3-like Object Storage | `24. S3-like Object Storage/Readme.md` | Designing object storage, data durability, erasure coding |

### Business & Transaction Systems

| Topic | Reference | Load When |
|-------|-----------|-----------|
| Hotel Reservation System | `22. Hotel Reservation System/Readme.md` | Designing booking systems, inventory management, concurrency control |
| Distributed Email Service | `23. Distributed Email Service/Readme.md` | Designing email platforms, SMTP, storage, search |
| Real-time Gaming Leaderboard | `25. Real-time Gaming Leaderboard/Readme.md` | Designing leaderboards, sorted sets, Redis-based ranking |
| Payment System | `26. Payment System/Readme.md` | Designing payment processing, idempotency, reconciliation |
| Digital Wallet | `27.  Digital Wallet/Readme.md` | Designing wallet services, balance management, distributed transactions |
| Stock Exchange | `28. Stock Exchange/Readme.md` | Designing trading platforms, order matching engines, low-latency architectures |

## How to Coach

When helping with system design:

1. **Don't give the full answer upfront.** Ask the user what they think first, then guide them.
2. **Start broad, go deep.** Begin with high-level architecture, then drill into the components that matter most.
3. **Always quantify.** Use back-of-the-envelope estimation to justify design decisions.
4. **Discuss trade-offs explicitly.** There is no perfect design — every choice has consequences.
5. **Reference real systems.** Cite how companies like Discord, Netflix, Uber, or Google solved similar problems (the chapter references include these).