thoughtsync/docs/sync.md

# ThoughtSync sync protocol

The contract the local-first native clients (Tauri desktop, Android) implement
against. The server is the **sync hub**: each client keeps a full local store
(SQLite), works fully offline, and reconciles with the server when linked. The
web app does **not** use this API — it stays on the live REST API (`/api/notes`,
…) as an online-only stopgap.

All sync endpoints live under `/api/sync`. Everything is **owner-scoped** and
**deterministic** (no AI).

> **No Postgres CI lane.** Trigger/migration/sync behavior is verified by the
> operator on deploy, not in CI. Pure logic (LWW comparator, paging cursor,
> token hashing) is unit-tested.

## Authentication — device bearer tokens

Native clients authenticate with a long-lived **device token**, not a session
cookie. Only the token's SHA-256 hash is stored server-side; the plaintext is
shown once at creation.

- **First link (no session):** `POST /api/auth/device-login` with
  `{email, password, name}` → `{token, device, user}`. Store `token`; send it as
  `Authorization: Bearer <token>` on every subsequent call.
- **From the web app (already signed in):** the user creates a token at
  `/account` ("Linked devices"); `POST /api/auth/devices` `{name}` → `{token,
  device}`. They paste it into the native app.
- **Manage:** `GET /api/auth/devices` (list), `DELETE /api/auth/devices/<id>`
  (revoke). A revoked token stops authenticating immediately.

Every authenticated request (sync or otherwise) accepts the bearer token in
place of the session cookie.

## The revision cursor

Every syncable row carries a monotonic **`sync_revision`** (bigint), assigned by
a database trigger from a single shared sequence (`sync_revision_seq`) on every
insert/update. Because it's a shared sequence, one integer is a total-order
watermark across **all** of a user's notes and labels. It is not a timestamp —
it is immune to clock skew and is only ever compared with `>`.

The client persists the highest cursor it has fully consumed and passes it back
as `?since=`. `since=0` (or absent) is a **full initial sync**.

## Entities and what syncs

- **Note** — the primary sync unit. It travels with its **checklist items,
  label memberships, and attachment metadata inline** (same shape as the REST
  serialization). A change to any child bumps the parent note's `sync_revision`,
  so pulling the note re-syncs the whole thing.
- **Label** — the label *catalog* (name + color) syncs as its own entity so a
  rename/recolor/delete propagates independently of notes.
- **Derived, NOT synced:** `[[wiki-links]]` and `#tags` are parsed from the note
  body. Clients recompute them locally; the server recomputes them on push. They
  never travel over the wire.
- **Attachment blobs** sync by id over the existing upload/download routes (see
  Attachments below); only their metadata rides the delta feed.

## Tombstones (deletes)

Two levels, both propagate:

- **Trash** — `deleted_at` is a normal field. A trashed note still syncs with its
  content; the client shows it in its Trash. Restoring clears `deleted_at`.
- **Purge (permanent delete)** — becomes a **content-less tombstone**: `purged_at`
  is set, title/body/items/labels/attachments are cleared/removed, and the row is
  kept. A client seeing `purged_at != null` deletes the row from its local store.
  Tombstones are retained indefinitely (cheap for a personal store); revisit if
  they ever grow large.

## Pull — `GET /api/sync/changes`

Query: `?since=<cursor>&limit=<n>` (limit default 500, max 1000).

Response:

```json
{
  "notes":  [ { "...full note...", "sync_revision": 42, "purged_at": null } ],
  "labels": [ { "id": "...", "name": "...", "color": "...",
               "sync_revision": 43, "purged_at": null, "created_at": "..." } ],
  "cursor": 43,
  "has_more": false
}
```

Returns **all** of the caller's notes + labels (any state — active, archived,
trash, purged) whose `sync_revision > since`, ascending by revision. Notes and
labels share the sequence, so paging merges the two streams: when either stream
fills a page, `cursor` advances only to the **smaller** of the two page
boundaries, so nothing between `cursor` and the next pull is skipped. Loop while
`has_more` is true, advancing `since = cursor` each time.

Note attachment metadata carries `{id, url, mime, size, sha256}`.

## Push — `POST /api/sync/push`

Body: `{ "changes": [ ... ] }` (max 1000 per batch). Each change:

```json
{ "entity": "note", "id": "<uuid>", "op": "upsert", "edited_at": "<iso8601>",
  "title": "...", "body": "...", "color": "blue", "kind": "text",
  "pinned": false, "archived": false, "trashed": false, "remind_at": null,
  "position": 0, "items": [ {"text": "...", "checked": false} ],
  "label_ids": ["<uuid>", ...], "created_at": "<iso8601, on create>" }
```

- **Client-generated ids.** Notes/labels are UUIDs; the client mints the id when
  it creates the row offline and sends it here. Create-if-absent, else update.
- **Whole-note semantics.** A note upsert carries the client's *full* current
  state (not a partial patch) — the server overwrites all scalar fields, replaces
  items, and sets manual label memberships from `label_ids` (tag-sourced labels
  are re-derived from the body). `[[links]]`/`#tags` are recomputed server-side.
- **`op: "delete"`** purges (tombstones) the row. Trashing is just an upsert with
  `trashed: true`.
- **Labels:** `{entity: "label", op: "upsert"|"delete", id, edited_at, name,
  color}`. A per-owner name clash on a *different* id is rejected (fix locally
  and retry).

### Conflict resolution — last-write-wins + history

On a clash the **most-recently-edited version wins**, by `edited_at`
(client wall-clock) compared against the server row's last-edit time
(`updated_at`). The client applies iff `client.edited_at >= server.updated_at`.
When a winning upsert overwrites an existing title/body, the server first
**snapshots the overwritten version into the note's version history**
(`note_revisions`) — so a "lost" edit is never truly lost; it's one Restore away.
A stale delete loses to a newer server edit.

Response — per item, so the client can mark its local rows synced:

```json
{ "results": [
  { "id": "...", "entity": "note", "status": "created", "sync_revision": 44 },
  { "id": "...", "entity": "note", "status": "kept",    "sync_revision": 40 },
  { "id": "...", "entity": "label","status": "rejected","error": "name in use" }
] }
```

`status` ∈ `created | applied | kept | noop | rejected`. `kept` means the server
had a newer edit and the client should adopt the server version on its next pull.

## Attachments (blobs)

Metadata rides the delta feed (`id, url, mime, size, sha256`); the bytes move
over the existing routes:

- **Upload:** `POST /api/notes/<note_id>/attachments` (multipart, field `file`;
  optional field `id` to keep a client-minted attachment id). Re-uploading an id
  the note already has is an idempotent no-op. Server stores + hashes the bytes.
- **Download:** `GET /api/notes/<note_id>/attachments/<id>` (owner/shared scoped).
- The client uses `sha256` to skip blobs it already holds and to verify
  integrity after download. (Currently image mimes only; broadening to any file
  is tracked separately.)

## A sync cycle

1. **Push** local changes since the last sync (batched). Apply the per-item
   results (mark synced, adopt server version where `kept`).
2. **Pull** from the stored `since` cursor until `has_more` is false. Upsert
   notes/labels into the local store; delete rows whose `purged_at` is set;
   download any attachment blobs referenced by a new/changed `sha256`.
3. Persist the new `cursor`.

Initial sync is the same with `since=0`. Because everything is keyed by stable
ids and a monotonic cursor, the cycle is **idempotent and resumable** — a client
can crash mid-sync and simply resume from its last persisted cursor.