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

id, title, category, status, canonical_id, aliases, duplicate_of, source_trust_level, confidence_score, verification_status, tags, raw_sources, last_reinforced, github_commit, tech_stack

id	title	category	status	canonical_id	aliases	duplicate_of	source_trust_level	confidence_score	verification_status	tags	raw_sources	last_reinforced	github_commit	tech_stack
wiki-2026-0508-concurrent-rendering	Concurrent Rendering	10_Wiki/Topics	verified	self	React Concurrent Concurrent Mode React 18 Concurrent	none	A	0.9	applied	react concurrent rendering scheduler transitions		2026-05-10	pending	language framework typescript react

Concurrent Rendering

매 한 줄

"매 render 매 interruptible, prioritizable, abandonable". React 18 (2022) 에 stable. Fiber architecture (2017) 의 결실. 2026 현재 React 19+ 의 default; useTransition, useDeferredValue, Suspense, streaming SSR, React Compiler 매 모든 것 위에 빌드.

매 핵심

매 핵심 idea

• Interruptible — render 중 high-priority work (input, animation) 매 들어오면 yield.
• Concurrent (NOT parallel) — single-threaded JS 위에서 cooperative scheduling.
• Time-slicing — 5ms chunk 매 yield to browser (scheduler package).
• Multiple in-progress trees — current + work-in-progress; double buffering.
• Lane-based priority — sync, default, transition, idle lane.

매 hook / API

• useTransition — non-urgent update (filter, navigation).
• useDeferredValue — debounce-like, 매 lower priority value.
• startTransition — imperative version.
• Suspense — async boundary, fallback UI.
• use hook (React 19) — read promise during render.

매 응용

1. Search-as-you-type — input 매 sync, list filter 매 transition.
2. Tab switching — 매 instant feel, 매 expensive subtree 의 background render.
3. SSR streaming — 매 progressive shell + island hydration.
4. Route navigation — <Link> prefetch + transition.

💻 패턴

useTransition — heavy filter

function Search() {
  const [query, setQuery] = useState('');
  const [list, setList] = useState<Item[]>(allItems);
  const [isPending, startTransition] = useTransition();

  return (
    <>
      <input
        value={query}
        onChange={(e) => {
          setQuery(e.target.value);                    // urgent
          startTransition(() => {                       // non-urgent
            setList(allItems.filter(i => i.name.includes(e.target.value)));
          });
        }}
      />
      {isPending && <Spinner />}
      <List items={list} />
    </>
  );
}

useDeferredValue

function Page({ query }: { query: string }) {
  const deferred = useDeferredValue(query);
  return <ExpensiveResults query={deferred} />;
}

Suspense + use (React 19+)

function UserProfile({ userPromise }: { userPromise: Promise<User> }) {
  const user = use(userPromise);  // suspends if pending
  return <h1>{user.name}</h1>;
}

<Suspense fallback={<Skeleton />}>
  <UserProfile userPromise={fetchUser(id)} />
</Suspense>

Streaming SSR (Next.js 15 / React 19)

// app/page.tsx
export default async function Page() {
  return (
    <>
      <Header />
      <Suspense fallback={<FeedSkeleton />}>
        <Feed />  {/* streamed once data ready */}
      </Suspense>
    </>
  );
}

React Compiler (2026, automatic memoization)

// no useMemo / useCallback needed — compiler inserts
function Cart({ items }: { items: Item[] }) {
  const total = items.reduce((s, i) => s + i.price, 0);  // auto-memoized
  return <div>{total}</div>;
}

Selective hydration

// chat panel hydrates first if user clicks it,
// even if header is still loading
<Suspense fallback={<Sk />}>
  <Header />
</Suspense>
<Suspense fallback={<Sk />}>
  <Chat />
</Suspense>

Cancel stale render (transition supersession)

// older transition automatically discarded when new one starts
startTransition(() => setQuery('a'));   // discarded
startTransition(() => setQuery('ab'));  // wins

매 결정 기준

상황	Approach
Input + heavy derived UI	useTransition
External lib slow value	useDeferredValue
Async data	Suspense + use
SSR with slow data	streaming + Suspense islands
Manual memoization (legacy)	replace with React Compiler

기본값: React 19+ with Compiler; transitions for any non-urgent update.

🔗 Graph

• 부모: React · Rendering Pipeline
• 변형: Fiber_Architecture · Time_Slicing
• 응용: Streaming SSR
• Adjacent: Virtual DOM과 Reconciliation · Reconciliation · React Compiler

🤖 LLM 활용

언제: identify component 매 transition 으로 wrap 할 후보, code review 매 missed Suspense boundary. 언제 X: scheduler internals 의 deep debug (need devtools profiler).

❌ 안티패턴

• startTransition for urgent input: input lag.
• No Suspense fallback: 매 entire tree freeze 까지 falling back.
• Manual memoization with React Compiler: redundant + sometimes 더 느림.
• Async setState in transition without race control: stale data.
• Deferred value of huge object reference: 매 GC pressure.

🧪 검증 / 중복

• Verified (React 18 release notes 2022, React 19 docs 2026, Acdlite/Sebastian Markbåge talks).
• 신뢰도 A.

🕓 Changelog

날짜	변경
2026-05-08	Phase 1
2026-05-10	Manual cleanup — full content with React 19 + Compiler patterns