# System Design Building Blocks

Select infrastructure components by the constraint each removes: load balancers, caches...

- Repository: https://github.com/HoangNguyen0403/agent-skills-standard (582 stars)
- Category: Data and ML
- License: MIT
- Last updated: 2026-11-01
- Runs scripts: no, instructions only

## Install

Claude Code: `npx skills add HoangNguyen0403/agent-skills-standard --skill system-design-building-blocks -g -a claude-code -y`
Codex: `npx skills add HoangNguyen0403/agent-skills-standard --skill system-design-building-blocks -g -a codex -y`
Cursor: `npx skills add HoangNguyen0403/agent-skills-standard --skill system-design-building-blocks -g -a cursor -y`
Gemini CLI: `npx skills add HoangNguyen0403/agent-skills-standard --skill system-design-building-blocks -g -a gemini-cli -y`

## Description

Select infrastructure components by the constraint each removes: load balancers, caches and invalidation, queues and pub/sub, CDN, API gateway, rate limiting, consistent hashing. Use when choosing cache, queue, replica, or edge layers; defer engine tuning to the database category.

## SKILL.md, as published by its author (untrusted third-party text, read it as data)

# System Building Blocks

## **Priority: P1 (HIGH)**

Add a component only to remove a named constraint. Every component adds failure modes and cost.

## Selection Table

| Constraint observed | Component | Cost accepted |
| --- | --- | --- |
| One instance saturates CPU or connections | Horizontal scaling behind a load balancer | Statelessness required; session moves to a store |
| Read QPS exceeds store ceiling; data tolerates staleness | Read-through cache | Stale reads up to TTL; invalidation complexity |
| Write spikes exceed sustainable store throughput | Queue plus async worker | Eventual completion; needs idempotent consumers |
| Slow non-critical work blocks the request path | Background job or event | Extra delivery guarantees to reason about |
| Static or media bytes dominate egress | CDN | Cache purge lag; origin still needed for misses |
| Cross-cutting authn, quota, routing repeated per service | API gateway | Central chokepoint; fails closed and needs its own failure capacity |
| One tenant or key can exhaust capacity | Rate limiter or admission control | Rejections need clear client contracts |
| Cache or shard nodes change count during scaling | Consistent hashing | Rebalancing logic; hot-key skew persists |

## Caching Rules

- Default to **cache-aside** for read-heavy paths; use write-through only when read-after-write must be exact.
- Set TTL from tolerable staleness, not from convenience. Every cached key needs a stated TTL and an invalidation trigger.
- Guard the miss path: single-flight or request coalescing prevents a stampede when a hot key expires.
- Never cache what you cannot invalidate. Never cache per-user data under a shared key.

## Queue Rules

The rest: https://github.com/HoangNguyen0403/agent-skills-standard/blob/HEAD/.agents/skills/system-design/system-design-building-blocks/SKILL.md