# Sync Feature — Implementation Notes

## DB: better-sqlite3 (migrated from @libsql/client)

**Why migrated:** `@libsql/client` caused SIGSEGV (exit code 139) on Azure App Service due to Rust native module incompatibility with seccomp profile.

**Key constraints of better-sqlite3:**
- All operations are **synchronous** — no `await` on db calls
- `db.transaction()` callback **must be synchronous** — `async (tx) => {}` throws `TypeError: Transaction function cannot return a promise`
- Drizzle ORM for better-sqlite3: `drizzle-orm/better-sqlite3` + `drizzle-orm/better-sqlite3/migrator`
- Result object uses `.changes` (not `.rowsAffected`) → use `(result as any).changes ?? (result as any).rowsAffected ?? 0`

**Config:**
- `drizzle.config.ts`: `url` without `file:` prefix
- Docker base: `node:22-slim` (prebuilt binaries available)
- No build tools needed in Dockerfile (prebuilts used on Linux)

**Migration:** `migrate(db, { migrationsFolder })` called synchronously at startup in `connection.ts`.

## sync-service.ts

`pushChanges` processes entities in FK order: contexts → topics → historyEntries → ratings → imageBlobs → pages → notebooks → pageNotebooks.

`upsertEntity` logic: insert if not exists, update if `clientVersion > serverVersion`, else record conflict.

**No transaction wrapper** — the old `db.transaction(async ...)` was dead code and was removed. Each entity is upserted individually.

## API Keys (M2M Auth)

Added `api_keys` table in schema.ts. Format: `ka_<base64url-32bytes>`. Only SHA-256 hash stored in DB.

Auth middleware (`auth.ts`) detects `ka_...` Bearer tokens **before** JWT path, looks up by hash, fires `lastUsedAt` update async.

Routes: `GET /api/api-keys`, `POST /api/api-keys`, `DELETE /api/api-keys/:id` (soft-delete).

Raw key returned **once** on creation — not recoverable after that.

## Browser API Key (client-side)

`browserApiKey` store in `authStore.ts` backed by `localStorage['ka-note-browser-key']`.

`getAccessToken()` returns browser key first, then falls back to MSAL.

`isAuthenticated` derived from `account OR browserApiKey`.

UI in Settings → "Browser-Authentifizierung" section.

Use case: avoids MSAL silent refresh failures (Azure AD returning HTTP 400 on refresh token).

**401 auto-invalidation:** `authFetch` in `apiClient.ts` detects 401 responses when a browser key was used and calls `setBrowserApiKey(null)` — clears `localStorage` and the store. `isAuthenticated` becomes `false`, restoring the MSAL login UI. Prevents the stuck state where a revoked/expired key blocks both sync and manual login.

## Deploy

`deploy.ps1` uses normal `docker build` with layer caching. Only changed layers are pushed to ACR.

`COPY server/ server/` triggers a cache miss whenever server source changes → tsc reruns automatically. `--no-cache` is not needed and was removed (it caused all layers to be rebuilt and pushed on every deploy, >40 MB).

## Delta Push (client-side filtering)

`pushAll(since)` only sends entities with `updatedAt > since.toISOString()`. A read-only client sends 0 entities and skips the HTTP request entirely.

`fullSync()` passes `since=null` → pushes everything (used after DB reset or first sync).

**Root cause for all-conflicts:** `upsertEntity` accepts only `clientVersion > serverVersion`. After a pull, client has `version == serverVersion` → equal-version entities are never pushed (filtered out by delta push). Without delta push, a read-only client would produce 700+ conflicts on every sync cycle.

## Client identification (`X-Client-Id` header)

Each browser instance generates a stable random ID in `localStorage['ka-clientId']`. The header value is `Browser·OS·id` (e.g. `Edge·Win·a3f2`, `Safari·iOS·x9k2`).

Server logs `client:` field in `[sync/push]` and `[sync/pull]` entries.

## ECONNRESET on POST /api/sync/push

`Error: aborted / ECONNRESET` logged as 500 is a **client-side disconnect**, not a server error. Hono logs 500 because response couldn't be sent. Causes: tab backgrounded on iOS, Azure idle timeout, browser request timeout.

Not actionable on server side unless response time is consistently >10s (indicates oversized payload, e.g. first full sync after DB reset).