# Phase 4 Step 2: Efficient Sync Research & Implementation

## Status: COMPLETE

## Goal
Improve the efficiency of data synchronization to reduce API calls and latency. Previously, the dashboard fetched all data on every refresh.

## Implementation

### Todoist Sync API v9
- **Endpoint**: `https://api.todoist.com/sync/v9/sync`
- **Method**: POST with `sync_token` and `resource_types`
- **Full sync**: When token is `*` or empty, returns all items
- **Incremental sync**: With valid token, returns only changes since last sync
- **Response**: Contains `sync_token` (for next request), `full_sync` flag, `items`, `projects`

### Storage
- New `sync_tokens` table in SQLite for persisting tokens across restarts
- Store methods: `GetSyncToken()`, `SetSyncToken()`, `ClearSyncToken()`
- Incremental update methods: `UpsertTask()`, `DeleteTasksByIDs()`

### Handler Logic
- `fetchTasks()` uses Sync API with stored token
- If `FullSync=true`: Replace all cached tasks
- If `FullSync=false`: Merge changes (upsert active, delete completed/deleted)
- Falls back to cached data on API errors

### Trello Optimization
- Added `fields` parameter to API calls to reduce response payload:
  - `GetBoards()`: `fields=id,name`
  - `GetCards()`: `fields=id,name,idList,due,url,idBoard`
  - `getLists()`: `fields=id,name`

## Files Modified
- `migrations/003_add_sync_tokens.sql` (new)
- `internal/store/sqlite.go` - Added sync token and incremental update methods
- `internal/api/todoist.go` - Added Sync API support
- `internal/api/interfaces.go` - Added Sync method to interface
- `internal/api/trello.go` - Added field filtering
- `internal/handlers/handlers.go` - Updated fetchTasks to use Sync API
- `internal/handlers/handlers_test.go` - Updated mock to implement Sync

## Benefits
- **Todoist**: Reduced data transfer on subsequent syncs (only changes returned)
- **Trello**: Smaller API responses (only required fields)
- **Persistence**: Sync token survives restarts, enabling incremental sync on startup