2nd/10_Wiki/Topics/Architecture/Hydration.md

---
id: wiki-2026-0508-hydration
title: Hydration
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [Client Hydration, SSR Hydration]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [web, ssr, react, frontend]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: typescript
  framework: react
---

# Hydration

## 매 한 줄
> **"매 server 가 render 한 static HTML 에 client JS 가 event handler / state 를 부여하는 과정"**. 매 SSR 의 SEO + first paint 이점 + SPA 의 interactivity 의 결합. 2026 시점 매 trend 는 매 less hydration — Astro Islands, React Server Components, Qwik resumability 가 매 full-page hydration 을 대체.

## 매 핵심

### 매 단계
1. **Server**: render to HTML string (`renderToString` / `renderToPipeableStream`).
2. **Client**: HTML parsing → first paint (TTFB / FCP).
3. **Hydration**: JS download → React/framework 가 component tree 의 event listener attach + state restore.
4. **Interactive**: TTI 도달.

### 매 cost
- 매 download + parse + execute JS — 매 모바일 mid-tier 에서 무거움.
- Hydration mismatch — 매 server vs client output 차이 시 warning + re-render.
- 매 unused interactivity 도 매 hydrate (Wasted hydration).

### 매 modern alternatives
- **Selective / progressive hydration** (React 18+): Suspense 단위로 lazy hydrate.
- **Islands architecture** (Astro, Fresh): 매 interactive island 만 ship + hydrate.
- **Resumability** (Qwik): hydration 자체 제거 — 매 server 에서 serialize 한 closure 를 매 event 시점에 lazy resume.
- **React Server Components**: 매 server-only component 는 ship X — 매 client component 만 hydrate.

### 매 응용
1. Next.js / Remix App Router (RSC + selective hydration).
2. Astro (islands).
3. Qwik / QwikCity (resumability).
4. SvelteKit, SolidStart (fine-grained reactivity, hydration cheaper).

## 💻 패턴

### React 19 SSR + hydrateRoot
```ts
// server.ts
import { renderToPipeableStream } from "react-dom/server";
import App from "./App";
const stream = renderToPipeableStream(<App/>, {
  bootstrapScripts: ["/client.js"],
  onShellReady() { stream.pipe(res); },
});

// client.ts
import { hydrateRoot } from "react-dom/client";
import App from "./App";
hydrateRoot(document, <App/>);
```

### Suspense for selective hydration
```tsx
<Suspense fallback={<Skeleton/>}>
  <HeavyChart/>{/* 매 이 boundary 만 lazy hydrate */}
</Suspense>
```

### Astro Islands
```astro
---
import Counter from "../components/Counter.svelte";
---
<h1>Static</h1>
<Counter client:visible /> {/* 매 viewport 진입 시 hydrate */}
```

### Qwik resumability
```tsx
import { component$, useSignal } from "@builder.io/qwik";
export default component$(() => {
  const c = useSignal(0);
  // 매 click 전에는 JS 미실행 — 매 lazy resume
  return <button onClick$={() => c.value++}>{c.value}</button>;
});
```

### Avoiding hydration mismatch
```tsx
// 매 client-only value (Date.now, window) 는 useEffect 에서
function Now() {
  const [t, setT] = useState<string | null>(null);
  useEffect(() => setT(new Date().toISOString()), []);
  return <span>{t ?? ""}</span>; // 매 server: empty, client: 실제값
}
```

### React Server Components (Next.js)
```tsx
// app/page.tsx — server component, 매 hydrate X
export default async function Page() {
  const data = await db.posts.findMany();
  return <ClientWidget data={data}/>; // 매 ClientWidget 만 hydrate
}
```

### Streaming SSR (Node)
```ts
const stream = renderToPipeableStream(<App/>, {
  onShellReady() { res.setHeader("content-type", "text/html"); stream.pipe(res); },
  onError(err) { console.error(err); },
});
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| Mostly static + 적은 island | Astro / 11ty + islands |
| Full app, dynamic data | Next.js App Router (RSC) |
| Extreme TTI 우선 | Qwik (resumability) |
| Fine-grained reactivity | Svelte / Solid |
| 매 SEO 불필요 SPA | CSR (no hydration) |

**기본값**: Next.js 15 App Router (RSC + selective hydration). 매 mostly-static 사이트 는 Astro.

## 🔗 Graph
- 부모: [[SSR]] · [[Web Architecture]]
- 변형: [[Selective Hydration]] · [[Islands Architecture]] · [[Resumability]]
- 응용: [[Next.js]] · [[Astro]] · [[Qwik]] · [[Remix]]
- Adjacent: [[Fiber_Architecture|Fiber Architecture]] · [[Streaming SSR]] · [[React Server Components]] · [[Core Web Vitals Optimization (INP, LCP 개선)|Core Web Vitals]]

## 🤖 LLM 활용
**언제**: TTI / LCP / INP 진단, framework 선택, hydration mismatch 디버깅.
**언제 X**: 매 SEO + first paint 둘 다 불필요한 internal tool — 매 CSR SPA 면 충분.

## ❌ 안티패턴
- **Hydration mismatch (Date.now, Math.random)**: 매 server-client output 불일치 → re-render.
- **Hydrating entire static page**: 매 island 분리 안 함 → JS 무겁게.
- **Heavy useEffect in root**: 매 hydration 직후 main thread block.
- **Conditional render based on `typeof window`**: 매 mismatch 의 흔한 원인.
- **Hydrating below-the-fold immediately**: 매 selective + visibility 활용.

## 🧪 검증 / 중복
- Verified (React 19 docs, Next.js 15 docs, Astro docs, Qwik docs, Addy Osmani *Hydration* 2024).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — hydration + islands + RSC + resumability 정리 |