2nd/10_Wiki/Topics/Architecture/상태 머신 (State Machine) 모델링 및 Redux 액션_리듀서 설계.md

---
id: wiki-2026-0508-상태-머신-state-machine-모델링-및-redux-
title: 상태 머신 (State Machine) 모델링 및 Redux 액션/리듀서 설계
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [State Machine + Redux, FSM Redux Design, XState + Redux]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [state-machine, redux, xstate, frontend, architecture]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: typescript
  framework: redux-toolkit
---

# 상태 머신 (State Machine) 모델링 및 Redux 액션/리듀서 설계

## 매 한 줄
> **"매 impossible state 의 unrepresentable 화"**. 매 finite state machine (FSM) 는 explicit state + event + transition 의 enumerate. Redux reducer 는 매 (state, action) → state 의 pure function — 매 FSM 와 isomorphic. 2026 년 매 XState 5 + RTK 의 combine 패턴 의 standard.

## 매 핵심

### 매 FSM 5-tuple
- **States (Q)**: enumerable set — `idle | loading | success | error`.
- **Events (Σ)**: external triggers — `FETCH | RESOLVE | REJECT`.
- **Transitions (δ)**: `Q × Σ → Q` — 매 partial function.
- **Initial state (q₀)**: `idle`.
- **Final states (F)**: optional — terminal nodes.

### 매 Redux 와 mapping
- **Reducer = δ**: `(state, action) => newState` 의 pure transition.
- **Action = Σ event**: typed discriminated union.
- **Store state = Q**: 매 status field 의 enum.
- **Selector**: 매 derived view — `isLoading = state.status === 'loading'`.

### 매 응용
1. Form wizard — multi-step flow 의 state 의 explicit.
2. Data fetching — idle/loading/success/error 의 4-state.
3. Auth flow — anonymous/authenticating/authenticated/expired.

## 💻 패턴

### Discriminated union state
```typescript
type FetchState<T> =
  | { status: 'idle' }
  | { status: 'loading' }
  | { status: 'success'; data: T }
  | { status: 'error'; error: Error };

// 매 impossible state ('loading' + data) 의 unrepresentable.
```

### RTK slice = FSM reducer
```typescript
import { createSlice, PayloadAction } from '@reduxjs/toolkit';

const userSlice = createSlice({
  name: 'user',
  initialState: { status: 'idle' } as FetchState<User>,
  reducers: {
    fetch: (state) => {
      if (state.status === 'idle' || state.status === 'error') {
        return { status: 'loading' };
      }
      return state; // ignore invalid transition
    },
    resolve: (_, action: PayloadAction<User>) => ({
      status: 'success',
      data: action.payload,
    }),
    reject: (_, action: PayloadAction<Error>) => ({
      status: 'error',
      error: action.payload,
    }),
  },
});
```

### Transition guard table
```typescript
const TRANSITIONS = {
  idle:    { FETCH: 'loading' },
  loading: { RESOLVE: 'success', REJECT: 'error' },
  success: { FETCH: 'loading' },
  error:   { FETCH: 'loading' },
} as const;

function transition<S extends keyof typeof TRANSITIONS>(
  state: S,
  event: string
): string {
  return (TRANSITIONS[state] as any)[event] ?? state;
}
```

### XState 5 + Redux integration
```typescript
import { createMachine, createActor } from 'xstate';

const fetchMachine = createMachine({
  id: 'fetch',
  initial: 'idle',
  states: {
    idle: { on: { FETCH: 'loading' } },
    loading: {
      invoke: {
        src: 'fetchUser',
        onDone: { target: 'success', actions: 'storeData' },
        onError: 'error',
      },
    },
    success: { on: { REFRESH: 'loading' } },
    error: { on: { RETRY: 'loading' } },
  },
});

const actor = createActor(fetchMachine).start();
actor.subscribe((snapshot) => store.dispatch({ type: 'fsm/sync', payload: snapshot.value }));
```

### Hierarchical (nested) state
```typescript
// 매 auth flow — 매 sub-state machine.
const authMachine = createMachine({
  initial: 'unauthenticated',
  states: {
    unauthenticated: {
      initial: 'idle',
      states: {
        idle: { on: { LOGIN: 'submitting' } },
        submitting: { on: { SUCCESS: '#auth.authenticated', FAIL: 'idle' } },
      },
    },
    authenticated: {
      on: { LOGOUT: 'unauthenticated' },
    },
  },
}, { id: 'auth' });
```

### Parallel state regions
```typescript
const editorMachine = createMachine({
  type: 'parallel',
  states: {
    document: { initial: 'clean', states: { clean: {}, dirty: {} } },
    network:  { initial: 'online', states: { online: {}, offline: {} } },
  },
});
// 매 state = (document, network) 의 cross product — explicit.
```

### Test as transition table
```typescript
test.each([
  ['idle',    'FETCH',   'loading'],
  ['loading', 'RESOLVE', 'success'],
  ['loading', 'REJECT',  'error'],
  ['success', 'FETCH',   'loading'],
])('transition %s --%s--> %s', (from, event, to) => {
  expect(transition(from, event)).toBe(to);
});
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| Simple async (fetch) | RTK `createAsyncThunk` + discriminated union |
| Multi-step wizard | XState machine |
| Complex auth | XState hierarchical machine |
| Parallel concerns | XState parallel states |
| Pure UI toggle | useState (not Redux) |

**기본값**: RTK + discriminated union state. 복잡 시 XState 5.

## 🔗 Graph
- 부모: [[Finite State Machine]] · [[Redux]]
- 변형: [[XState]] · [[Statecharts]]
- 응용: [[Form Wizard]] · [[Data Fetching]] · [[Auth Flow]]
- Adjacent: [[Discriminated Union]] · [[Pure Function]] · [[Event Sourcing]]

## 🤖 LLM 활용
**언제**: state 가 3+ 개 + transition 의 explicit rule 의 필요 시.
**언제 X**: simple boolean toggle 또는 single-shot async — 매 overkill.

## ❌ 안티패턴
- **Boolean explosion**: `isLoading + isError + isSuccess` — impossible state (true+true) 의 representable.
- **Implicit transition**: reducer 안 valid state check 의 X — 매 invalid transition 의 silently 발생.
- **God reducer**: 매 모든 logic 의 한 reducer — 매 split.
- **Side effect in reducer**: `fetch()` 의 reducer 내부 — 매 thunk/saga/middleware 사용.

## 🧪 검증 / 중복
- Verified (Hopcroft & Ullman *Automata Theory*; Redux docs; XState v5 docs 2026).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — FSM↔Redux mapping + XState 5 patterns |