docs: add backend sync engine plan

Specifies requirements and architecture for syncing two independent
Ontime backends. Validates the CRDT/Automerge suggestion against the
codebase and proposes a hybrid: Automerge document sync for project
data, replicated playback-intent (recomputed locally) for runtime.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1wpN4k8YeCoZcQhVKXafU

apps/client/src/common/hooks/useEntryAction.ts

@@ -16,6 +16,7 @@ import {
   TimeStrategy,
   isOntimeEvent,
   isOntimeGroup,
+  isOntimeMilestone,
 } from 'ontime-types';
 import {
   MILLIS_PER_SECOND,
@@ -82,6 +83,7 @@ function useEntryActionsForRundown(scopedRundownId: string | undefined) {
     defaultDangerTime,
     defaultTimerType,
     defaultEndAction,
+    inheritGroupColour,
   } = useEditorSettings();
 
   const resolveCurrentRundownQueryKey = useCallback(() => {
@@ -239,6 +241,14 @@ function useEntryActionsForRundown(scopedRundownId: string | undefined) {
         }
       }
 
+      if (inheritGroupColour && (isOntimeEvent(newEntry) || isOntimeMilestone(newEntry)) && !newEntry.colour) {
+        const parentId = resolveInsertParent(rundownData, newEntry);
+        const maybeParent = parentId ? rundownData.entries[parentId] : null;
+        if (maybeParent && isOntimeGroup(maybeParent)) {
+          newEntry.colour = maybeParent.colour;
+        }
+      }
+
       try {
         await addEntryMutation([rundownId, newEntry]);
       } catch (error) {
@@ -254,6 +264,7 @@ function useEntryActionsForRundown(scopedRundownId: string | undefined) {
       defaultTimerType,
       defaultEndAction,
       defaultTimeStrategy,
+      inheritGroupColour,
       addEntryMutation,
     ],
   );

apps/client/src/common/stores/editorSettings.ts

@@ -12,8 +12,10 @@ type EditorSettingsStore = {
   defaultDangerTime: string;
   defaultTimerType: TimerType;
   defaultEndAction: EndAction;
+  inheritGroupColour: boolean;
   setDefaultDuration: (defaultDuration: string) => void;
   setLinkPrevious: (linkPrevious: boolean) => void;
+  setInheritGroupColour: (inheritGroupColour: boolean) => void;
   setTimeStrategy: (timeStrategy: TimeStrategy) => void;
   setWarnTime: (warnTime: string) => void;
   setDangerTime: (dangerTime: string) => void;
@@ -29,6 +31,7 @@ export const editorSettingsDefaults = {
   dangerTime: '00:01:00', // 60000 same as backend
   timerType: TimerType.CountDown,
   endAction: EndAction.None,
+  inheritGroupColour: false,
 };
 
 enum EditorSettingsKeys {
@@ -39,6 +42,7 @@ enum EditorSettingsKeys {
   DefaultDangerTime = 'ontime-default-danger-time',
   DefaultTimerType = 'ontime-default-timer-type',
   DefaultEndAction = 'ontime-default-end-action',
+  InheritGroupColour = 'ontime-inherit-group-colour',
 }
 
 export const useEditorSettings = create<EditorSettingsStore>((set) => {
@@ -59,6 +63,10 @@ export const useEditorSettings = create<EditorSettingsStore>((set) => {
       localStorage.getItem(EditorSettingsKeys.DefaultEndAction),
       editorSettingsDefaults.endAction,
     ),
+    inheritGroupColour: booleanFromLocalStorage(
+      EditorSettingsKeys.InheritGroupColour,
+      editorSettingsDefaults.inheritGroupColour,
+    ),
 
     setDefaultDuration: (defaultDuration) =>
       set(() => {
@@ -97,5 +105,10 @@ export const useEditorSettings = create<EditorSettingsStore>((set) => {
         localStorage.setItem(EditorSettingsKeys.DefaultEndAction, String(defaultEndAction));
         return { defaultEndAction };
       }),
+    setInheritGroupColour: (inheritGroupColour) =>
+      set(() => {
+        localStorage.setItem(EditorSettingsKeys.InheritGroupColour, String(inheritGroupColour));
+        return { inheritGroupColour };
+      }),
   };
 });

apps/client/src/features/app-settings/panel/manage-panel/RundownDefaultSettings.tsx

@@ -16,6 +16,7 @@ export default function RundownDefaultSettings() {
     defaultDangerTime,
     defaultTimerType,
     defaultEndAction,
+    inheritGroupColour,
     setDefaultDuration,
     setLinkPrevious,
     setTimeStrategy,
@@ -23,6 +24,7 @@ export default function RundownDefaultSettings() {
     setDangerTime,
     setDefaultTimerType,
     setDefaultEndAction,
+    setInheritGroupColour,
   } = useEditorSettings((state) => state);
 
   const durationInMs = parseUserTime(defaultDuration);
@@ -44,6 +46,13 @@ export default function RundownDefaultSettings() {
               />
               <Switch size='large' checked={linkPrevious} onCheckedChange={setLinkPrevious} />
             </Panel.ListItem>
+            <Panel.ListItem>
+              <Panel.Field
+                title='Inherit group colour'
+                description='Whether new events and milestones inherit the colour of their parent group'
+              />
+              <Switch size='large' checked={inheritGroupColour} onCheckedChange={setInheritGroupColour} />
+            </Panel.ListItem>
             <Panel.ListItem>
               <Panel.Field
                 title='Timer strategy'

apps/server/src/api-data/rundown/__tests__/rundown.utils.test.ts

@@ -2,13 +2,11 @@ import { EndAction, OntimeEvent, TimeStrategy, TimerType } from 'ontime-types';
 import { MILLIS_PER_HOUR, createEvent } from 'ontime-utils';
 import { assertType } from 'vitest';
 
-import { demoDb } from '../../../models/demoProject.js';
 import { makeOntimeEvent, makeOntimeGroup, makeRundown } from '../__mocks__/rundown.mocks.js';
 import {
   calculateDayOffset,
   deleteById,
   doesInvalidateMetadata,
-  duplicateRundown,
   getIntegerAndFraction,
   hasChanges,
   makeDeepClone,
@@ -223,25 +221,6 @@ describe('calculateDayOffset()', () => {
   });
 });
 
-describe('duplicateRundown', () => {
-  it('duplicates a given rundown', () => {
-    const demoRundown = demoDb.rundowns['default'];
-    const title = 'Duplicated Rundown';
-    const duplicatedRundown = duplicateRundown(demoRundown, title);
-
-    expect(duplicatedRundown).toMatchObject({
-      title: title,
-      entries: expect.any(Object),
-      order: expect.any(Array),
-      flatOrder: expect.any(Array),
-    });
-    expect(demoRundown.id).not.toEqual(duplicatedRundown.id);
-    expect(duplicatedRundown.order.length).toEqual(demoRundown.order.length);
-    expect(duplicatedRundown.flatOrder.length).toEqual(demoRundown.flatOrder.length);
-    expect(Object.keys(duplicatedRundown.entries).length).toEqual(Object.keys(demoRundown.entries).length);
-  });
-});
-
 describe('makeDeepClone()', () => {
   it('deep clones a group along with its nested entries', () => {
     const group1 = makeOntimeGroup({ id: 'group1', title: 'Group 1', entries: ['event1', 'event2'] });

apps/server/src/api-data/rundown/rundown.router.ts

@@ -15,16 +15,18 @@ import {
   createNewRundown,
   deleteAllEntries,
   deleteEntries,
+  deleteRundown,
+  duplicateRundown,
   editEntry,
   groupEntries,
-  initRundown,
   loadRundown,
+  renameRundown,
   renumberEntries,
   reorderEntry,
   swapEvents,
   ungroupEntries,
 } from './rundown.service.js';
-import { duplicateRundown, normalisedToRundownArray } from './rundown.utils.js';
+import { normalisedToRundownArray } from './rundown.utils.js';
 import {
   clonePostValidator,
   entryBatchPutValidator,
@@ -34,6 +36,7 @@ import {
   entryReorderValidator,
   entrySwapValidator,
   rundownArrayOfIds,
+  rundownPatchValidator,
   rundownPostValidator,
 } from './rundown.validation.js';
 
@@ -44,7 +47,7 @@ export const router: Router = express.Router();
 /**
  * Returns all rundowns in the project
  */
-router.get('/', async (_req: Request, res: Response<ProjectRundownsList>) => {
+router.get('/', (_req: Request, res: Response<ProjectRundownsList>) => {
   const projectRundowns = getDataProvider().getProjectRundowns();
   res.json({ loaded: getCurrentRundown().id, rundowns: normalisedToRundownArray(projectRundowns) });
 });
@@ -52,7 +55,7 @@ router.get('/', async (_req: Request, res: Response<ProjectRundownsList>) => {
 /**
  * Returns the current rundown
  */
-router.get('/current', async (_req: Request, res: Response<Rundown>) => {
+router.get('/current', (_req: Request, res: Response<Rundown>) => {
   const rundown = getCurrentRundown();
   res.json(rundown);
 });
@@ -60,7 +63,7 @@ router.get('/current', async (_req: Request, res: Response<Rundown>) => {
 /**
  * Returns a given rundown in its normalised client shape
  */
-router.get('/:id', paramsWithId, async (req: Request, res: Response<Rundown | ErrorResponse>) => {
+router.get('/:id', paramsWithId, (req: Request, res: Response<Rundown | ErrorResponse>) => {
   try {
     const rundown = getProcessedRundown(req.params.id);
     res.json(rundown);
@@ -104,13 +107,7 @@ router.post(
   paramsWithId,
   async (req: Request, res: Response<ProjectRundownsList | ErrorResponse>) => {
     try {
-      const dataProvider = getDataProvider();
-      const rundown = dataProvider.getRundown(req.params.id);
-
-      const duplicatedRundown: Rundown = duplicateRundown(rundown, `Copy of ${rundown.title}`);
-      await dataProvider.setRundown(duplicatedRundown.id, duplicatedRundown);
-
-      const projectRundowns = getDataProvider().getProjectRundowns();
+      const projectRundowns = await duplicateRundown(req.params.id);
       res.status(201).json({ loaded: getCurrentRundown().id, rundowns: normalisedToRundownArray(projectRundowns) });
     } catch (error) {
       const message = getErrorMessage(error);
@@ -123,54 +120,26 @@ router.post(
  * Patches the data of an existing rundown
  * Currently only the title can be changed
  */
-router.patch('/:id', paramsWithId, async (req: Request, res: Response<ProjectRundownsList | ErrorResponse>) => {
-  try {
-    const dataProvider = getDataProvider();
-    const rundown = dataProvider.getRundown(req.params.id);
-    if (!rundown) throw new Error(`Rundown with ID ${req.params.id} not found`);
-    if (!req.body.title) throw new Error('No title provided');
-
-    await dataProvider.setRundown(rundown.id, { ...rundown, title: req.body.title });
-
-    /**
-     * If loaded we re-init the rundown
-     * This is likely over-kill but the simplest way to ensure state consistency
-     */
-    if (req.params.id === getCurrentRundown().id) {
-      const rundown = dataProvider.getRundown(req.params.id);
-      const customField = dataProvider.getCustomFields();
-      await initRundown(rundown, customField);
+router.patch(
+  '/:id',
+  rundownPatchValidator,
+  async (req: Request, res: Response<ProjectRundownsList | ErrorResponse>) => {
+    try {
+      const projectRundowns = await renameRundown(req.params.id, req.body.title);
+      res.status(200).json({ loaded: getCurrentRundown().id, rundowns: normalisedToRundownArray(projectRundowns) });
+    } catch (error) {
+      const message = getErrorMessage(error);
+      res.status(400).send({ message });
     }
-
-    const projectRundowns = getDataProvider().getProjectRundowns();
-    res.status(201).json({ loaded: getCurrentRundown().id, rundowns: normalisedToRundownArray(projectRundowns) });
-  } catch (error) {
-    const message = getErrorMessage(error);
-    res.status(400).send({ message });
-  }
-});
+  },
+);
 
 /**
  * Deletes a rundown if not loaded
  */
 router.delete('/:id', paramsWithId, async (req: Request, res: Response<ProjectRundownsList | ErrorResponse>) => {
   try {
-    if (req.params.id === getCurrentRundown().id) {
-      res.status(400).send({ message: 'Cannot delete loaded rundown' });
-      return;
-    }
-
-    const dataProvider = getDataProvider();
-    const projectRundowns = dataProvider.getProjectRundowns();
-
-    if (Object.keys(projectRundowns).length <= 1) {
-      // might never hit this as it is likely covered by the case of trying to delete the loaded rundown
-      res.status(400).send({ message: 'Cannot delete the last rundown' });
-      return;
-    }
-
-    await dataProvider.deleteRundown(req.params.id);
-    const newProjectRundowns = getDataProvider().getProjectRundowns();
+    const newProjectRundowns = await deleteRundown(req.params.id);
     res.status(200).json({ loaded: getCurrentRundown().id, rundowns: normalisedToRundownArray(newProjectRundowns) });
   } catch (error) {
     const message = getErrorMessage(error);

apps/server/src/api-data/rundown/rundown.service.ts

@@ -16,7 +16,7 @@ import {
   isOntimeEvent,
   isOntimeGroup,
 } from 'ontime-types';
-import { customFieldLabelToKey, getInsertAfterId, resolveInsertParent } from 'ontime-utils';
+import { customFieldLabelToKey, generateId, getInsertAfterId, resolveInsertParent } from 'ontime-utils';
 
 import { sendRefetch } from '../../adapters/WebsocketAdapter.js';
 import { getDataProvider } from '../../classes/data-provider/DataProvider.js';
@@ -643,7 +643,7 @@ export function isCurrentRundown(id: string) {
 /**
  * @throws if the provided id does not exist
  */
-export async function loadRundown(id: string) {
+export function loadRundown(id: string) {
   const dataProvider = getDataProvider();
   if (isCurrentRundown(id)) {
     return dataProvider.getProjectRundowns();
@@ -651,7 +651,7 @@ export async function loadRundown(id: string) {
 
   const rundown = dataProvider.getRundown(id);
   const customField = dataProvider.getCustomFields();
-  await initRundown(rundown, customField);
+  initRundown(rundown, customField);
   return dataProvider.getProjectRundowns();
 }
 
@@ -659,11 +659,7 @@ export async function loadRundown(id: string) {
  * Sets a new rundown in the cache
  * and marks it as the currently loaded one
  */
-export async function initRundown(
-  rundown: Readonly<Rundown>,
-  customFields: Readonly<CustomFields>,
-  reload: boolean = false,
-) {
+export function initRundown(rundown: Readonly<Rundown>, customFields: Readonly<CustomFields>, reload: boolean = false) {
   runtimeService.stop();
   const { rundownMetadata, revision } = rundownCache.init(rundown, customFields);
   logger.info(LogOrigin.Server, `Switch to rundown: ${rundown.id}`);
@@ -692,3 +688,73 @@ export async function createNewRundown(title: string) {
 
   return projectRundowns;
 }
+
+/**
+ * duplicate a rundown
+ * @throws
+ */
+export async function duplicateRundown(id: string) {
+  const dataProvider = getDataProvider();
+  const rundown = dataProvider.getRundown(id);
+
+  const newRundownId = generateId();
+  const newRundown: Rundown = structuredClone(rundown);
+  newRundown.id = newRundownId;
+  newRundown.title = `Copy of ${rundown.title}`;
+  newRundown.revision = 0;
+
+  const newProjectRundowns = await dataProvider.setRundown(newRundownId, newRundown);
+
+  setImmediate(() => {
+    sendRefetch(RefetchKey.ProjectRundowns);
+  });
+
+  return newProjectRundowns;
+}
+
+/**
+ * rename a rundown
+ * @throws
+ */
+export async function renameRundown(id: string, title: string) {
+  const dataProvider = getDataProvider();
+  const rundown = dataProvider.getRundown(id);
+  const newProjectRundowns = await dataProvider.setRundown(rundown.id, { ...rundown, title });
+
+  /**
+   * If we are modifying the loaded rundown we re-init it
+   * This is likely over-kill but the simplest way to ensure state consistency
+   */
+  if (isCurrentRundown(id)) {
+    const rundown = dataProvider.getRundown(id);
+    const customField = dataProvider.getCustomFields();
+    initRundown(rundown, customField);
+  } else {
+    setImmediate(() => {
+      sendRefetch(RefetchKey.ProjectRundowns);
+    });
+  }
+
+  return newProjectRundowns;
+}
+
+/**
+ * delete a rundown
+ * @throws
+ */
+export async function deleteRundown(id: string) {
+  if (isCurrentRundown(id)) throw new Error('Cannot delete loaded rundown');
+
+  const dataProvider = getDataProvider();
+  const projectRundowns = dataProvider.getProjectRundowns();
+
+  // might never hit this as it is likely covered by the case of trying to delete the loaded rundown
+  if (Object.keys(projectRundowns).length <= 1) throw new Error('Cannot delete the last rundown');
+  const newProjectRundowns = await dataProvider.deleteRundown(id);
+
+  setImmediate(() => {
+    sendRefetch(RefetchKey.ProjectRundowns);
+  });
+
+  return newProjectRundowns;
+}

apps/server/src/api-data/rundown/rundown.utils.ts

@@ -460,20 +460,6 @@ export function normalisedToRundownArray(rundowns: ProjectRundowns): ProjectRund
   });
 }
 
-/**
- * Duplicates an existing rundown ensuring all IDs are unique
- */
-export function duplicateRundown(rundown: Rundown, newTitle: string): Rundown {
-  const newRundownId = generateId();
-
-  const newRundown = structuredClone(rundown);
-  newRundown.id = newRundownId;
-  newRundown.title = newTitle;
-  newRundown.revision = 0;
-
-  return newRundown;
-}
-
 export type IncrementNumber = {
   integer: number;
   faction: number;

apps/server/src/api-data/rundown/rundown.validation.ts

@@ -5,6 +5,11 @@ import { requestValidationFunction } from '../validation-utils/validationFunctio
 // #region operations on project rundowns =========================
 
 export const rundownPostValidator = [body('title').isString().trim().notEmpty(), requestValidationFunction];
+export const rundownPatchValidator = [
+  param('id').isString().trim().notEmpty(),
+  body('title').isString().trim().notEmpty().withMessage('No title provided'),
+  requestValidationFunction,
+];
 
 // #endregion operations on project rundowns ======================
 // #region operations on rundown entries ==========================

apps/server/src/classes/data-provider/DataProvider.ts

@@ -101,9 +101,10 @@ function getCustomFields(): Readonly<CustomFields> {
   return db.data.customFields;
 }
 
-async function setRundown(rundownKey: string, newData: Rundown): Promise<void> {
+async function setRundown(rundownKey: string, newData: Rundown): ReadonlyPromise<ProjectRundowns> {
   db.data.rundowns[rundownKey] = structuredClone(newData);
   await persist();
+  return db.data.rundowns;
 }
 
 function getSettings(): Readonly<Settings> {

docs/sync-engine/backend-plan.md

@@ -0,0 +1,260 @@
+# Ontime Sync Engine — Backend Plan
+
+> Status: **draft / requirements**. Scope: backend only. The client/UI integration is
+> deliberately out of scope and will be specified separately.
+
+## 1. Goal
+
+Run **two independent Ontime backends** (typically one **local** and one in the **cloud**) and
+keep them in sync:
+
+- The user, from the client of *one* backend, initiates a sync between the two.
+- After the initial sync, both backends hold the **same project data**.
+- Ongoing **data changes** (rundown edits, custom fields, settings, …) propagate to both.
+- Ongoing **playback actions** (start / stop / pause / roll / load / add-time, messages,
+  aux timers) propagate to both, so both report the same runtime state.
+
+The brief suggests leaning on a CRDT library (Automerge / `automerge-repo`) to do the heavy
+lifting. This document validates that idea against the codebase and proposes a concrete
+architecture.
+
+## 2. What the code actually looks like (validated)
+
+There are **three distinct kinds of state** in the server, and they have very different sync
+requirements. This distinction drives the whole design.
+
+### 2.1 Persistent project data — *the document*
+
+`DatabaseModel` (`packages/types/src/definitions/DataModel.type.ts`):
+
+```ts
+type DatabaseModel = {
+  rundowns: ProjectRundowns;   // Record<RundownId, Rundown>
+  project: ProjectData;
+  settings: Settings;
+  viewSettings: ViewSettings;
+  urlPresets: URLPreset[];
+  customFields: CustomFields;
+  automation: AutomationSettings;
+};
+```
+
+- Persisted by `DataProvider` (`apps/server/src/classes/data-provider/DataProvider.ts`) via
+  **lowdb** to a JSON file, with a **3 s trailing-edge debounced write** (`persist()`).
+- A `Rundown` carries `{ id, title, order, flatOrder, entries, revision }`. `entries` is a
+  `Record<EntryId, OntimeEntry>` — i.e. a map keyed by stable id. `order`/`flatOrder` are
+  arrays of ids.
+- This is **collaborative-document-shaped data**. It is the natural fit for a CRDT.
+
+### 2.2 Rundown cache + transaction layer
+
+`apps/server/src/api-data/rundown/rundown.dao.ts`:
+
+- The **currently loaded** rundown lives in an in-memory `cachedRundown` plus derived
+  `rundownMetadata` (computed schedule: gaps, delays, group times, ordered lists).
+- All edits go through `createTransaction({ rundownId, mutableRundown })` →
+  `rundownMutation.*` (add/edit/remove/reorder/applyDelay/swap/clone/group/ungroup/renumber)
+  → `commit()`. `commit()`:
+  1. bumps `cachedRundown.revision`,
+  2. re-processes derived metadata (`processRundown`),
+  3. persists through `DataProvider.setRundown`.
+- **Non-loaded ("background") rundowns** bypass the cache: read from disk, mutate, persist.
+- `rundown.service.ts` wraps every mutation and, in a `setImmediate`, fires **side effects**:
+  - `updateRuntimeOnChange()` → pushes derived counts into runtime state,
+  - `notifyChanges()` → `runtimeService.notifyOfChangedEvents()` (timer) and
+    `sendRefetch(RefetchKey.Rundown, revision, rundownId)` (tells clients to re-pull).
+
+> Key takeaway: **mutations are funnelled through a single, well-defined chokepoint** with a
+> post-commit side-effect hook. That hook is exactly where remote (synced) changes must also
+> be injected, so that a change arriving from the peer triggers the same cache rebuild +
+> client refetch as a local edit.
+
+The other persistent slices (`project`, `settings`, `viewSettings`, `urlPresets`,
+`customFields`, `automation`) are written **directly** through `DataProvider` setters and do
+**not** go through the transaction/side-effect layer — they emit their own `Refetch` from
+their routers. Sync must cover these too.
+
+### 2.3 Runtime / playback state — *not document data*
+
+`apps/server/src/stores/runtimeState.ts` + `EventTimer` + `runtime.service.ts`:
+
+- Live timer state is **derived every tick from the local wall clock** (`timeCore.now()` =
+  `Date.now()`), recomputed at 30 fps and broadcast to clients at ~1 fps via `eventStore`
+  over the websocket (`MessageTag.RuntimeData`).
+- The **entire playback state collapses to a tiny serialisable record** — the existing
+  `RestorePoint` (`services/restore-service/restore.type.ts`):
+
+  ```ts
+  type RestorePoint = {
+    playback: Playback;
+    selectedEventId: MaybeString;
+    startedAt: MaybeNumber;     // TimeOfDay (ms since local midnight)
+    addedTime: number;
+    pausedAt: MaybeNumber;      // TimeOfDay
+    firstStart: MaybeNumber;    // TimeOfDay
+    startEpoch: Maybe<Instant>; // absolute epoch ms  ← timezone independent
+    currentDay: MaybeNumber;
+  };
+  ```
+
+- `runtimeState.resume(restorePoint, event, rundown, metadata)` already **reconstructs a live
+  playing timer from this record** — this is the mechanism a follower backend will reuse to
+  adopt remote playback state.
+- **Commands** (`start/startById/stop/pause/roll/load*/addTime/setOffsetMode`) all live on the
+  `runtimeService` singleton, decorated with `@broadcastResult`. External callers reach them
+  through `dispatchFromAdapter()` (`integration.controller.ts`) from WS/OSC/HTTP.
+- **Messages** (`message.service.ts`) and **aux timers** (`AuxTimerService`) are additional
+  ephemeral runtime state held in `eventStore`, not in the DB.
+
+> Critical timing observation: `timeCore.toTimeOfDay()` uses the **machine's local timezone
+> offset** (`getTimezoneOffset`). `startEpoch` is absolute and TZ-independent, but `startedAt`,
+> `pausedAt`, `firstStart`, `clock` are all *TimeOfDay in the originating machine's TZ*. A
+> local box and a cloud box in different timezones will **not** interpret a replicated
+> TimeOfDay the same way. Playback sync must therefore anchor on **absolute epoch + the
+> project's configured timezone**, and the follower must **recompute** TimeOfDay-derived
+> fields locally rather than copying them verbatim.
+
+## 3. Does a CRDT fit? — Verdict
+
+**Yes, but only for §2.1 (the project document).** Automerge is a strong fit there:
+
+- Edits are keyed by stable ids (`entries[id]`, `customFields[key]`, rundowns by id), so
+  concurrent edits to *different* entries merge cleanly (Automerge maps merge per-key).
+- It removes the need to hand-roll conflict resolution, op ordering, and incremental
+  catch-up after disconnection.
+
+**No for §2.3 (playback/runtime).** A CRDT is the wrong tool for real-time control:
+
+- Timer ticks must **not** be streamed over the network — each backend already derives them
+  locally from the wall clock. We only need to replicate **intent transitions**.
+- Playback is a control-plane concern with a "last command wins" nature, not a mergeable
+  document.
+
+So the recommendation is a **hybrid**:
+
+| Domain | Mechanism | Library |
+| --- | --- | --- |
+| Project document (`DatabaseModel`) | CRDT document, incremental sync | `@automerge/automerge-repo` + WS network adapter |
+| Playback / messages / aux timers | Replicated **intent** (LWW register w/ logical clock), recomputed locally | small in-house module over the same socket |
+| Live timer ticks | **Not synced** — derived locally on each node | existing `EventTimer` |
+
+## 4. Topology & connection model
+
+- **Initiation is directional, ongoing sync is bidirectional.** "Sync now" from backend A
+  must choose a **baseline owner** (whose project seeds the shared document). Merging two
+  *unrelated* projects with a CRDT yields a union of both rundowns — almost never what the
+  user wants. So:
+  1. On "sync", A and B establish a connection.
+  2. The chosen baseline (say A) exports its current project as the shared Automerge document;
+     B **adopts** it (forks from A's document so they share lineage/history).
+  3. From then on, both edit the *same* document and `automerge-repo` reconciles incrementally
+     and bidirectionally.
+- **Who dials whom:** the cloud instance (`IS_CLOUD`) has a reachable public endpoint; the
+  local instance is usually behind NAT. The **local node dials out to the cloud node**, and
+  the cloud node acts as the `automerge-repo` sync server / relay. This is exactly the
+  `automerge-repo` WebSocket server/client split.
+- **Sync targets the currently loaded project only** (one Automerge `DocumentId` ↔ one Ontime
+  project). Switching projects detaches/attaches the sync session.
+- **Auth:** the sync socket must authenticate. Reuse the existing auth (`makeAuthenticateMiddleware`
+  / login flow + shared token). The "sync" action carries the peer URL + credentials.
+
+## 5. Integration points (where code hooks in)
+
+1. **DataProvider becomes CRDT-backed (the document).**
+   - The shared Automerge doc holds the persistent `DatabaseModel`.
+   - `DataProvider` read paths return the doc's current value; write paths (`setRundown`,
+     `setCustomFields`, `setSettings`, …) are re-expressed as Automerge `change()` calls.
+   - Granularity: the existing mutations already operate at **per-entry / per-key** level
+     (`rundown.entries[id] = …`, `order.splice(...)`, `customFields[key] = …`). Re-expressing
+     them as Automerge changes at that same granularity gives good merge behaviour without a
+     full rewrite of the mutation algorithms. **Avoid replacing whole `entries`/`order`
+     objects wholesale** — that defeats per-key merge. The current `commit()` reassigns
+     `cachedRundown.entries = entries`; the CRDT adapter needs to apply the *delta* instead.
+   - lowdb persistence stays as a **local durability layer** (or is replaced by Automerge's
+     own storage adapter). Either way the 3 s debounce semantics should be preserved.
+
+2. **Remote-change observer → existing side-effect path.**
+   - Subscribe to Automerge doc changes. When a change arrives **from the peer** (not from a
+     local mutation), run the same post-commit side effects that a local edit would:
+     - rebuild the loaded-rundown cache (`rundownCache.init` / `runtimeState.updateAll`),
+     - `runtimeService.notifyOfChangedEvents(metadata)`,
+     - `sendRefetch(RefetchKey.Rundown | …, revision, rundownId)` to local clients.
+   - This is the single most important hook: it makes remote edits indistinguishable from
+     local edits to everything downstream (clients, timer, integrations).
+
+3. **`revision` semantics.** Today `revision` is a per-rundown monotonic counter used only to
+   tell clients "you're stale, refetch". With two writers it can collide. Options: derive the
+   client-facing revision from the Automerge document heads/hash, or keep the counter as
+   advisory and rely on the refetch always pulling current truth. Recommend deriving a stable
+   version token from Automerge heads.
+
+4. **Playback intent channel.**
+   - Define a replicated `PlaybackIntent` ≈ `RestorePoint` + `offsetMode`, plus `messages` and
+     `auxTimers[1..3]` intent (`{playback, startedAtEpoch, duration, direction}`).
+   - Model as a **LWW register stamped with a logical (Lamport) clock + originating peer id**.
+     Every `runtimeService` command updates the local intent and publishes it; the peer
+     applies it if its stamp is newer.
+   - The follower applies intent via a **resume-style path** (`runtimeState.resume`-like) that
+     **recomputes TimeOfDay fields from `startEpoch` + clock offset + project timezone** — it
+     does *not* copy `startedAt`/`pausedAt` verbatim (see §2.3 timing note).
+   - Live ticks remain local; both nodes converge because they share intent + a common clock.
+
+5. **Clock synchronisation.**
+   - Both nodes must agree on epoch time within tolerance (target sub-100 ms for broadcast use).
+   - Recommend an **application-level offset estimate** over the sync socket (periodic
+     timestamped ping ⇒ Cristian's algorithm / NTP-lite), applied by the follower when
+     interpreting `startEpoch`. Do not assume both machines are NTP-disciplined, but benefit
+     from it when they are.
+
+## 6. Conflict & authority model
+
+- **Document edits:** resolved by Automerge (per-key map merge, RGA for arrays). Define a
+  policy for the rare same-key concurrent edit (Automerge picks a deterministic winner; we
+  may surface a "changed remotely" hint to the editor). Concurrent reorders of the same list
+  are the main thing to test (array CRDT semantics).
+- **Playback:** a human operator drives it; genuinely simultaneous conflicting commands are
+  rare. LWW on the intent register (logical clock + peer id tiebreak) is sufficient for a
+  2-node system and far simpler than a leader-election protocol. Revisit if N>2 is ever needed.
+
+## 7. Phased delivery
+
+1. **Phase 0 — Spec & spike.** Lock requirements (this doc). Spike `automerge-repo` WS
+   client/server between two local server instances; prove a doc round-trips.
+2. **Phase 1 — Document sync (data only).** CRDT-back the `DatabaseModel`; remote-change
+   observer wired into the existing refetch/cache side-effect path. Directional initial seed.
+   No playback sync yet. Deliverable: edits on either node appear on both.
+3. **Phase 2 — Clock sync + playback intent.** Offset estimation; replicate `PlaybackIntent`;
+   follower derives ticks locally. Deliverable: start/stop/pause/roll/load/add-time mirror.
+4. **Phase 3 — Messages & aux timers.** Extend intent channel.
+5. **Phase 4 — Resilience.** Reconnection/catch-up, project-switch handling, auth hardening,
+   conflict UX hints, observability (drift metrics, sync status).
+
+## 8. Open questions / decisions needed
+
+1. **Library:** confirm `@automerge/automerge-repo` (WASM core) vs alternatives (Yjs). Automerge
+   matches the keyed-map data model and brittle-free merges; Yjs is leaner/faster but more
+   text-CRDT oriented. *Recommendation: Automerge.*
+2. **Baseline-owner UX:** when the two projects differ at initiation, is it always
+   "push mine / overwrite theirs", or do we offer "pull theirs"? (Merging unrelated projects is
+   explicitly discouraged.)
+3. **Persistence:** keep lowdb as the local store and treat Automerge as the in-memory
+   sync truth, or move durability to an Automerge storage adapter? Affects crash recovery and
+   the existing `flushPendingWrites`/restore flow.
+4. **Timezone authority:** anchor playback on the **project's configured timezone** (not each
+   machine's local TZ). Confirm where that timezone lives / whether it must be added.
+5. **`report` data** (run history): sync as part of the document, or keep per-instance?
+6. **Scope of N:** is 2 nodes the hard ceiling, or should the intent/authority model leave room
+   for more peers?
+7. **Multiple loaded rundowns / background rundowns:** confirm the whole project document syncs
+   (all rundowns), while only the *loaded* one drives the runtime on each node.
+
+## 9. Risks
+
+- **TimeOfDay vs absolute epoch** across timezones (the single biggest playback-sync trap; §2.3).
+- **Array/order merge** semantics for concurrent reorders — needs explicit test coverage.
+- **`structuredClone`-and-replace** mutation style must be converted to deltas or it will
+  clobber concurrent edits and negate the CRDT.
+- **Document growth / compaction** — Automerge history grows; plan periodic compaction/snapshots.
+- **Bandwidth on the local↔cloud link** — fine for doc deltas + intent; would be a problem if
+  timer ticks were ever streamed (they must not be).
+```

e2e/tests/features/202-cuesheet.spec.ts

@@ -30,6 +30,8 @@ test('cuesheet datagrid does not submit timer cells on tab-out or escape', async
 
   // re-enter edit mode: original value should be unchanged
   await durationCell.click();
+  // tabbing selects the next input field so we have to click twice to first leave input field and then select
+  await durationCell.click();
   await expect(durationCell.locator('input')).toHaveValue(originalDuration);
   await durationCell.locator('input').press('Escape');