# Review: M6 Full-Stack Focus Session Implementation

**Target files:**
- frontend/src/App.tsx
- frontend/src/api.ts  
- src/backend/api/sessions.py
- src/backend/api/settings.py
- src/backend/models.py

**Verdict:** request-changes

**Summary:** Code review found two blocking bugs and one performance issue. The implementation delivers the core features (session tracking, history, settings) with sound backend architecture and idempotency design. Two path/dependency issues block test coverage and initial usage.

---

## Finding 1: History Fetch Effect Dependency Loop

**Severity:** change-required  
**Location:** frontend/src/App.tsx:65-88  
**Category:** State management / React patterns

**Issue:**
```typescript
useEffect(() => {
  const loadHistory = async () => {
    const now = Date.now();
    if (history && now - historyCacheTime < 5 * 60 * 1000) {
      return;
    }
    setHistory(data);
    setHistoryCacheTime(now);
  };
  loadHistory();
}, [historyWindow, history, historyCacheTime]);
```

The effect's dependency array includes `history` and `historyCacheTime`, which are set *inside* the effect via `setHistory` and `setHistoryCacheTime`. Every time these states are updated, the dependency array changes, triggering the effect to re-run. While the 5-minute cache check prevents repeated API calls, the effect function still re-executes and the component still re-renders on every state update.

**Concern:**
This pattern is wasteful and represents a footgun. If the cache logic breaks or a race condition occurs, a tight re-fetch loop could result. More immediately, it wastes CPU cycles on unnecessary effect executions and component renders.

**Request:**
Remove `history` and `historyCacheTime` from the dependency array. Keep only `historyWindow` as the input. Implement caching using `useRef` to track the last fetch time without triggering re-renders:

```typescript
const cacheRef = useRef({ lastWindow: null, lastFetchTime: 0 });

useEffect(() => {
  const loadHistory = async () => {
    const now = Date.now();
    if (cacheRef.current.lastWindow === historyWindow && 
        now - cacheRef.current.lastFetchTime < 5 * 60 * 1000) {
      return;
    }
    
    const data = await api.getSessionHistory(historyWindow);
    setHistory(data);
    cacheRef.current = { lastWindow: historyWindow, lastFetchTime: now };
  };
  loadHistory();
}, [historyWindow]);
```

---

## Finding 2: Health Endpoint Path Mismatch

**Severity:** block  
**Location:** frontend/src/api.ts:170-174 and src/backend/api/__init__.py  
**Category:** API contract / routing

**Issue:**
```typescript
// frontend/src/api.ts
export async function checkHealth(): Promise<{ status: string }> {
  const res = await fetch('/api/health');
  // ...
}
```

```python
# src/backend/api/__init__.py
from src.backend.api.health import router as health_router
api_router.include_router(health_router)  # No prefix!
api_router.include_router(sessions_router, prefix="/api")  # Sessions uses /api
```

The frontend calls `/api/health`, but the health router is mounted without a prefix, so it serves at `/health`. Meanwhile, all other routers (sessions, settings, export) use `prefix="/api"`, creating an inconsistency.

**Concern:**
The fetch to `/api/health` will return 404. Any code calling `checkHealth()` will fail. If tests rely on this endpoint, they will fail. The inconsistency is confusing.

**Request:**
Add `prefix="/api"` to the health router include in src/backend/api/__init__.py:

```python
api_router.include_router(health_router, prefix="/api")
api_router.include_router(sessions_router, prefix="/api")
api_router.include_router(settings_router, prefix="/api")
api_router.include_router(export_router, prefix="/api")
```

This aligns all API routes under `/api/`, matching the frontend's expectations and standard API convention.

---

## Finding 3: Settings Sliders Post API on Every onChange

**Severity:** suggestion  
**Location:** frontend/src/App.tsx:354-361, 369-376  
**Category:** Performance / UX

**Issue:**
```typescript
<input
  type="range"
  value={focusDurationSeconds}
  onChange={(e) => handleSettingsUpdate(parseInt(e.target.value), breakDurationSeconds)}
/>
```

The range input's `onChange` event fires on every slider movement. Each invocation calls `handleSettingsUpdate`, which is async and POSTs to `/api/settings`. A user dragging a slider could generate 50+ API calls in a few seconds.

**Concern:**
While the backend's PATCH endpoint is idempotent (won't corrupt data), this pattern wastes resources: network bandwidth, backend processing, battery/data on mobile, verbose server logs. Given that settings "apply to your next session only," there's no time-critical reason to POST immediately on every slider movement.

**Request:**
Implement debouncing. Wait 300-500ms after the last slider movement before calling `handleSettingsUpdate`. This is a performance optimization and not a blocker for M6 closure, but should be fixed before shipping to production:

```typescript
const debounceTimerRef = useRef<number | null>(null);

const handleSettingsDebounced = (newFocus: number, newBreak: number) => {
  if (debounceTimerRef.current) {
    clearTimeout(debounceTimerRef.current);
  }
  debounceTimerRef.current = window.setTimeout(() => {
    handleSettingsUpdate(newFocus, newBreak);
  }, 300);
};
```

---

## Observations

### Backend Architecture (Positive)

- **Idempotency design:** The `UniqueConstraint('user_id', 'session_id', 'completed_at')` correctly prevents duplicate session recording from retried POSTs. The IntegrityError handling is well-implemented.
- **Session immutability:** Once recorded, sessions cannot be edited. This is a sound design for audit trails and history consistency.
- **Error handling:** Specific, helpful error messages ("Invalid timestamp format", "Failed to record session").
- **Aggregation logic:** History totals correctly sum only focus sessions, excluding breaks.

### Frontend State Management

- **useSession hook:** Overall structure is sound. Session state is correctly persisted to localStorage and recovered on app restart. Timer logic is wall-clock-based (not interval-tick-based), so it tolerates effect re-initialization.
- **View routing:** Clean separation between home, timer, history, and settings views with appropriate conditional rendering.
- **Error boundaries:** Settings errors, session errors, and history errors are all captured and displayed to the user.

---

## Cross-Domain References

- **Hatter (QA):** The history fetch dependency loop could create tight re-fetch loops under race conditions. Verify test scenarios cover settings changes during active sessions and verify auto-completion works correctly.
- **Tweedles:** The health endpoint path bug will cause test_health.py failures. Once fixed, verify the endpoint returns 200 with `{"status": "ok"}`.

---

## Summary for M6 Closure

**Blockers to fix before merge:**
1. Add `prefix="/api"` to health_router include
2. Remove history and historyCacheTime from useEffect dependency array

**Nice-to-have (can defer):**
1. Debounce settings slider API calls

Once blockers are fixed, the implementation should pass all feature tests and deliver the four user-facing stories (focus sessions, history, settings, export).
