# Phase 9: Faster Write-Back — Pattern Map

**Mapped:** 2026-06-12
**Files analyzed:** 5 (1 new, 4 modified)
**Analogs found:** 5 / 5

---

## File Classification

| New/Modified File | Role | Data Flow | Closest Analog | Match Quality |
|---|---|---|---|---|
| `apps/api/src/lib/outboxTrigger.ts` | utility / singleton | event-driven | `apps/api/src/lib/listEmitter.ts` | exact |
| `apps/api/src/broker/outboxWorker.ts` | service / broker | event-driven + batch | `apps/api/src/broker/outboxWorker.ts` (self — extend) | exact |
| `apps/api/src/routes/events.ts` | route / controller | request-response | `apps/api/src/routes/events.ts` (self — extend) | exact |
| `apps/api/src/index.ts` | config / startup | request-response | `apps/api/src/index.ts` (self — extend) | exact |
| `apps/api/tests/broker/outboxWorker.test.ts` | test | batch | `apps/api/tests/broker/outboxWorker.test.ts` (self — extend) | exact |

---

## Pattern Assignments

### `apps/api/src/lib/outboxTrigger.ts` (NEW — utility, event-driven)

**Analog:** `apps/api/src/lib/listEmitter.ts`

**Imports pattern** (lines 18):
```typescript
import { EventEmitter } from 'node:events';
```

**Module-level singleton pattern** (lines 22–23):
```typescript
const emitter = new EventEmitter();
emitter.setMaxListeners(200); // listEmitter sets 200 for fan-out; outboxTrigger needs only 1
// NOTE: for outboxTrigger, setMaxListeners is NOT needed — single subscriber, default 10 is fine
```

**Publish function pattern** (lines 39–41):
```typescript
export function publishListEvent(listId: number, event: ListEvent): void {
  emitter.emit(`list:${listId}`, event);
}
```
For outboxTrigger, adapt as:
```typescript
export function signalOutboxDrain(): void {
  emitter.emit('drain');
}
```

**Subscribe + unsubscribe pattern** (lines 47–54):
```typescript
export function subscribeListEvents(
  listId: number,
  handler: (event: ListEvent) => void,
): () => void {
  const channel = `list:${listId}`;
  emitter.on(channel, handler);
  return () => emitter.off(channel, handler);
}
```
For outboxTrigger, adapt as:
```typescript
export function onOutboxDrain(handler: () => void): () => void {
  emitter.on('drain', handler);
  return () => emitter.off('drain', handler);
}
```

**No imports from broker/, routes/, db/, or index.ts** — `listEmitter.ts` also imports nothing from the project. `outboxTrigger.ts` must follow the same zero-internal-dependency rule to avoid circular imports.

---

### `apps/api/src/broker/outboxWorker.ts` (MODIFIED — service/broker, event-driven + batch)

**Analog:** self (extend existing file)

**Existing `isDraining` flag declaration** (line 159):
```typescript
let isDraining = false;
```
New `drainRequested` flag goes immediately after, at the same module scope:
```typescript
let drainRequested = false;
```

**Existing `runOutboxDrain` guard + finally** (lines 602–605, 783–785):
```typescript
export async function runOutboxDrain(): Promise<void> {
  // CR-05: single-process concurrency guard
  if (isDraining) return;
  isDraining = true;
  // ... (do not touch the body)
  } finally {
    isDraining = false;
  }
}
```
`runOutboxDrain` is NOT modified — `scheduleOutboxDrain` wraps it externally.

**New `scheduleOutboxDrain` wrapper** — add between `isDraining` declaration and `runOutboxDrain`:
```typescript
/**
 * Scheduler entrypoint called by both the EventEmitter listener (initOutboxTrigger)
 * and the 15s setInterval. Implements the drain-again-if-requested loop (D-05) so
 * the last edit in a burst is drained promptly without double-draining (D-02 / D-07).
 * Fire-and-forget — callers do not await this.
 */
export function scheduleOutboxDrain(): void {
  if (isDraining) {
    drainRequested = true;
    return;
  }
  runOutboxDrain()
    .catch((err: unknown) => {
      console.error('[outboxWorker] Unhandled runOutboxDrain error:', err);
    })
    .finally(() => {
      if (drainRequested) {
        drainRequested = false;   // reset FIRST, then recurse (Pitfall 3)
        scheduleOutboxDrain();    // exactly one trailing re-drain (D-05)
      }
    });
}
```

**`initOutboxTrigger` export** — add in the Scheduler section alongside `startOutboxWorker`:
```typescript
import { onOutboxDrain } from '../lib/outboxTrigger.js';

export function initOutboxTrigger(): void {
  onOutboxDrain(() => scheduleOutboxDrain());
}
```

**Existing `startOutboxWorker`** (lines 796–802) — update to call `scheduleOutboxDrain` instead of `runOutboxDrain` directly:
```typescript
export function startOutboxWorker(): void {
  setInterval(() => {
    scheduleOutboxDrain();
  }, 15 * 1000);
}
```
(Existing `.catch` wrapper is absorbed into `scheduleOutboxDrain` — remove the per-call `.catch` in the interval body.)

**Export rationale comment** (line 19 in file header):
```
* runOutboxDrain is exported for unit testing.
* startOutboxWorker wraps it in a 15-second setInterval.
```
Add: `scheduleOutboxDrain is exported for unit testing.`

---

### `apps/api/src/routes/events.ts` (MODIFIED — route/controller, request-response)

**Analog:** self (extend existing file)

**Import to add** (after existing imports, lines 24–35):
```typescript
import { signalOutboxDrain } from '../lib/outboxTrigger.js';
```

**Site 1 — POST /create, single insert** (lines 300–309). Insert is a bare `await db.insert(...)` (no transaction). Signal fires after the awaited insert, before the `return`:
```typescript
await db.insert(calendarOutbox).values({
  userId: currentUserId,
  operation: 'create',
  status: 'pending',
  uid,
  calendarUrl: targetCalendarUrl,
  payload: JSON.stringify(payload),
});
signalOutboxDrain(); // fire-and-forget (D-04)
return c.json({ uid }, 202);
```

**Site 2 — PATCH /:uid/edit, edit-as-move transaction** (lines 408–432). Signal fires AFTER `await db.transaction(...)` resolves — NOT inside the callback:
```typescript
await db.transaction(async (tx) => {
  await tx.insert(calendarOutbox).values({ operation: 'delete', ... });
  await tx.insert(calendarOutbox).values({ operation: 'create', ... });
});
signalOutboxDrain(); // after transaction commits — both rows durable (D-03)
return c.json({ uid: newUid }, 202);
```

**Site 3 — PATCH /:uid/edit, same-calendar update** (lines 436–447):
```typescript
await db.insert(calendarOutbox).values({ operation: 'update', ... });
signalOutboxDrain(); // fire-and-forget (D-04)
return c.json({ uid }, 202);
```

**Site 4 — DELETE /:uid** (lines 511–521):
```typescript
await db.insert(calendarOutbox).values({ operation: 'delete', ... });
signalOutboxDrain(); // fire-and-forget (D-04)
return c.json({ uid }, 202);
```

All four sites are inside existing `try/catch` blocks — the signal call slots between the `await` and the `return`, matching the same pattern at each site.

---

### `apps/api/src/index.ts` (MODIFIED — config/startup, request-response)

**Analog:** self (extend existing file)

**Existing import** (line 16):
```typescript
import { startOutboxWorker } from './broker/outboxWorker.js';
```
Update to:
```typescript
import { startOutboxWorker, initOutboxTrigger } from './broker/outboxWorker.js';
```

**Existing startup block** (lines 112, 136–141):
```typescript
if (isMainModule()) {
  // ... VAPID setup ...
  startBrokerPoller();
  startOutboxWorker();
  startReminderScheduler();
  serve({ fetch: app.fetch, port: 3000 }, ...);
}
```
Add `initOutboxTrigger()` immediately after `startOutboxWorker()`:
```typescript
  startOutboxWorker();
  initOutboxTrigger(); // subscribe drain signal listener (D-01)
  startReminderScheduler();
```

**`isMainModule()` function** (lines 100–107) — do not modify. The trigger listener must remain gated by this guard (same as all other background workers) to prevent listener registration when tests import `app`.

---

### `apps/api/tests/broker/outboxWorker.test.ts` (MODIFIED — test, batch)

**Analog:** self (extend existing file)

**Existing import line** (line 21):
```typescript
import { runOutboxDrain, assembleRruleString } from '../../src/broker/outboxWorker.js';
```
Extend to include new exports:
```typescript
import { runOutboxDrain, assembleRruleString, scheduleOutboxDrain } from '../../src/broker/outboxWorker.js';
```
Also import the signal function for trigger wiring tests:
```typescript
import { signalOutboxDrain } from '../../src/lib/outboxTrigger.js';
```

**`vi.hoisted` + `vi.mock` scaffold** (lines 31–101) — copy verbatim into new describe block. The `mockPendingRows`, `makeRow`, `makeResponse`, and `wireMockChain` helpers are already at module scope and are reusable from the new tests without re-declaration.

**`beforeEach` pattern** (lines 170–174):
```typescript
beforeEach(() => {
  vi.resetAllMocks();
  mockPendingRows = [];
  wireMockChain();
});
```
New describe block uses the same `beforeEach`. Note: `vi.resetAllMocks()` clears all mock state between tests — adequate for trigger wiring tests since `isDraining` and `drainRequested` are module-level state that resets between test runs only if the module is re-imported. Use `vi.isolateModules` or reset state explicitly if module-level flags need isolation between tests.

**`makeResponse` helper** (line 131–132):
```typescript
const makeResponse = (status: number): Response =>
  ({ status, ok: status >= 200 && status < 300, headers: new Headers() }) as unknown as Response;
```
Reuse in new tests — already in scope.

**Async flush pattern for signal→drain tests** (no fake timers needed):
```typescript
// Flush microtask queue so the promise chain from scheduleOutboxDrain completes
await new Promise<void>(resolve => setImmediate(resolve));
```

**`vi.useFakeTimers()` pattern** — needed only for tests that advance the 15s interval. Not needed for pure signal→drain tests. See existing `startOutboxWorker` interval tests for the pattern if needed.

**New describe block placement** — add after the final existing `describe` block, before EOF:
```typescript
describe('scheduleOutboxDrain — trigger wiring (D-09)', () => {
  beforeEach(() => {
    vi.resetAllMocks();
    mockPendingRows = [];
    wireMockChain();
  });

  // Test A: signal → drain without timer wait (SC-1)
  // Test B: signal mid-drain → exactly one trailing re-drain (D-05)
  // Test C: concurrent scheduleOutboxDrain calls → each row dispatched once (D-07)
});
```

---

## Shared Patterns

### `isMainModule()` startup gate
**Source:** `apps/api/src/index.ts` lines 100–107 + 112
**Apply to:** `initOutboxTrigger()` call in index.ts
```typescript
function isMainModule(): boolean {
  if (!process.argv[1]) return false;
  try {
    return fileURLToPath(import.meta.url) === realpathSync(process.argv[1]);
  } catch {
    return false;
  }
}

if (isMainModule()) {
  // all background worker starts here
}
```
All listener registrations and `setInterval` calls must be inside this guard. `initOutboxTrigger` follows the same gating as `startBrokerPoller`, `startOutboxWorker`, `startReminderScheduler`.

### Error swallowing in setInterval workers
**Source:** `apps/api/src/broker/outboxWorker.ts` lines 797–801
**Apply to:** `scheduleOutboxDrain` wrapper (the `.catch` on the `runOutboxDrain()` call)
```typescript
runOutboxDrain().catch((err: unknown) => {
  console.error('[outboxWorker] Unhandled runOutboxDrain error:', err);
});
```
`scheduleOutboxDrain` absorbs this pattern into its own `.catch` — the interval body becomes a bare `scheduleOutboxDrain()` call with no inline error handling.

### Module-level singleton + typed publish/subscribe API
**Source:** `apps/api/src/lib/listEmitter.ts` lines 18–54
**Apply to:** `apps/api/src/lib/outboxTrigger.ts`
Pattern: `const emitter = new EventEmitter()` at module scope; named export functions wrap all emitter access; no direct EventEmitter reference leaks to callers. `outboxTrigger.ts` follows this exactly.

---

## No Analog Found

None. All five files have direct analogs (three are self-extensions of existing files).

---

## Metadata

**Analog search scope:** `apps/api/src/lib/`, `apps/api/src/broker/`, `apps/api/src/routes/`, `apps/api/src/`, `apps/api/tests/broker/`
**Files read:** `listEmitter.ts`, `outboxWorker.ts`, `events.ts`, `index.ts`, `tests/broker/outboxWorker.test.ts`
**Pattern extraction date:** 2026-06-12