2nd/10_Wiki/Topics/Frontend/단일 진실 공급원(Single Source of Truth).md

---
id: wiki-2026-0508-단일-진실-공급원-single-source-of-truth
title: 단일 진실 공급원(Single Source of Truth)
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [SSOT, Single Source of Truth, 진실의 단일 공급원]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [architecture, state-management, ssot, frontend]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: typescript
  framework: react-19
---

# 단일 진실 공급원(Single Source of Truth)

## 매 한 줄
> **"매 모든 derived state 의 single canonical origin"**. 매 1970s database normalization 에서 시작 — 매 modern frontend (Redux, TanStack Query, RSC) 의 핵심 원리. 매 duplication 의 elimination → 매 inconsistency bug 의 소멸.

## 매 핵심

### 매 정의
- 매 한 데이터 의 **하나의** authoritative location.
- 매 다른 모든 view/component 의 derive (read-only projection).
- 매 update 의 single entry point — 매 race / drift 의 prevention.

### 매 frontend 적용 layer
- **Server state SSOT**: 매 backend DB — 매 client 의 cache (TanStack Query, SWR).
- **URL state SSOT**: 매 query params / path — 매 shareable, refreshable.
- **Form state SSOT**: 매 RHF / Formik 의 form object — 매 controlled input 의 derive.
- **Global UI state SSOT**: 매 Zustand / Jotai store — 매 cross-component shared.

### 매 응용
1. TanStack Query — server SSOT mirror.
2. RSC (React Server Components) — server 의 SSOT 그대로 stream.
3. URL-driven state (nuqs, TanStack Router) — 매 shareable SSOT.
4. CRDT (Yjs, Automerge) — 매 distributed SSOT.

## 💻 패턴

### Server SSOT via TanStack Query
```typescript
// ❌ duplication — local state mirrors server
const [user, setUser] = useState<User | null>(null);
useEffect(() => { fetchUser().then(setUser); }, []);

// ✅ SSOT — server is truth, query is cached projection
const { data: user } = useQuery({
  queryKey: ['user', userId],
  queryFn: () => fetchUser(userId),
});
```

### URL as SSOT (nuqs)
```typescript
import { useQueryState, parseAsInteger } from 'nuqs';

function ProductList() {
  const [page, setPage] = useQueryState('page', parseAsInteger.withDefault(1));
  const [sort, setSort] = useQueryState('sort');
  // URL ?page=2&sort=price is the truth — refresh-safe, shareable
  return <List page={page} sort={sort} />;
}
```

### Derived state (no duplicate store)
```typescript
// ❌ duplicating filtered list in state
const [items, setItems] = useState(allItems);
const [filter, setFilter] = useState('');
useEffect(() => { setItems(allItems.filter(i => i.name.includes(filter))); }, [filter]);

// ✅ derive on render — items isn't state
const [filter, setFilter] = useState('');
const visible = useMemo(() => allItems.filter(i => i.name.includes(filter)), [filter]);
```

### Form SSOT via RHF
```typescript
const { register, watch, handleSubmit } = useForm<FormData>({
  defaultValues: { email: '', plan: 'free' },
});
const plan = watch('plan'); // derived from form SSOT
return (
  <form onSubmit={handleSubmit(submit)}>
    <input {...register('email')} />
    {plan === 'pro' && <input {...register('teamSize')} />}
  </form>
);
```

### Zustand store SSOT
```typescript
import { create } from 'zustand';

type CartStore = {
  items: CartItem[];
  add: (item: CartItem) => void;
  remove: (id: string) => void;
};

export const useCart = create<CartStore>((set) => ({
  items: [],
  add: (item) => set((s) => ({ items: [...s.items, item] })),
  remove: (id) => set((s) => ({ items: s.items.filter((i) => i.id !== id) })),
}));

// Total derived — not stored
const useCartTotal = () => useCart((s) => s.items.reduce((a, i) => a + i.price, 0));
```

### CRDT SSOT (collaborative)
```typescript
import * as Y from 'yjs';
import { WebrtcProvider } from 'y-webrtc';

const ydoc = new Y.Doc();
new WebrtcProvider('room-id', ydoc);
const ytext = ydoc.getText('content');

ytext.observe(() => {
  // single source — all peers converge
  render(ytext.toString());
});
```

## 매 결정 기준
| 데이터 종류 | SSOT location |
|---|---|
| 영속 entity (user, order) | Backend DB → TanStack Query cache |
| Shareable view state | URL params (nuqs) |
| Form draft | RHF / Formik form object |
| Cross-component UI | Zustand / Jotai |
| Component-local | useState (그 component 자체 가 SSOT) |
| Collaborative | CRDT (Yjs) |

**기본값**: 매 server 의 SSOT, 매 client 의 cache projection.

## 🔗 Graph
- 부모: [[State Management]]
- 변형: [[Event Sourcing]] · [[CQRS]]
- 응용: [[React Server Components — 경계 의식]] · [[CRDT]]
- Adjacent: [[단일 진실 공급원(Single Source of Truth) 구축]]

## 🤖 LLM 활용
**언제**: 매 state architecture design / data flow audit / bug-prone "two stores out of sync" 발견 시.
**언제 X**: 매 truly independent UI ephemeral state (hover, focus) — 매 local 이 자체 SSOT.

## ❌ 안티패턴
- **Mirror state**: 매 server 의 data 를 useState 로 복제. → 매 stale + 두 source 의 drift.
- **Multiple stores 의 same field**: Redux + Context + URL 모두 `selectedId` 보유. 매 update 의 race.
- **Premature derivation cache**: 매 derived value 를 별도 state 로 저장. 매 useMemo 충분.
- **localStorage 의 silent SSOT**: 매 sync 없는 cross-tab — 매 BroadcastChannel 또는 storage event 필요.

## 🧪 검증 / 중복
- Verified (Redux docs — "Three Principles", Dan Abramov; Martin Fowler — SSOT).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — SSOT FULL writeup |