attachments topic

Attachments

In many cases, you might want to sync large binary data (like images) along with the data synced by PowerSync. Embedding this data directly in your source databases is inefficient and not recommended.

Instead, the PowerSync SDK for Dart and Flutter provides utilities you can use to reference this binary data in your primary data model, and then download it from a secondary data store such as S3. Because binary data is not directly stored in the source database in this model, we call these files attachments.

Alpha release

The attachment helpers described in this document are currently in an alpha state, intended for testing. Expect breaking changes and instability as development continues. The attachments API is marked as @experimental for this reason.

Do not rely on these libraries for production use.

Usage

An AttachmentQueue instance is used to manage and sync attachments in your app. The attachments' state is stored in a local-only attachments table.

Key assumptions

• Each attachment is identified by a unique id.
• Attachments are immutable once created.
• Relational data should reference attachments using a foreign key column.
• Relational data should reflect the holistic state of attachments at any given time. Any existing local attachment will be deleted locally if no relational data references it.

Example implementation

See the supabase todolist demo for a basic example of attachment syncing.

Setup

First, add a table storing local attachment state to your database schema.

final schema = Schema([
  AttachmentsQueueTable(),
  // In this document, we assume the photo_id column of the todos table references an optional photo
  // stored as an attachment.
  Table('todos', [
    Column.text('list_id'),
    Column.text('photo_id'),
    Column.text('description'),
    Column.integer('completed'),
  ]),
]);

Next, create an AttachmentQueue instance. This class provides default syncing utilities and implements a default sync strategy. This class can be extended for custom functionality, if needed.

final directory = await getApplicationDocumentsDirectory();

final attachmentQueue = AttachmentQueue(
  db: db,
  remoteStorage: SupabaseStorageAdapter(), // instance responsible for uploads and downloads
  logger: logger,
  localStorage: IOLocalStorage(appDocDir), // IOLocalStorage requires `dart:io` and is not available on the web
   watchAttachments: () => db.watch('''
      SELECT photo_id as id FROM todos WHERE photo_id IS NOT NULL
   ''').map((results) => [
      for (final row in results)
        WatchedAttachmentItem(
          id: row['id'] as String,
          fileExtension: 'jpg',
        )
      ],
    ),
);

Here,

• An instance of LocalStorageAdapter, such as the IOLocalStorage provided by the SDK, is responsible for storing attachment contents locally.
• An instance of RemoteStorageAdapter is responsible for downloading and uploading attachment contents to the secondary service, such as S3, Firebase cloud storage or Supabase storage.
• watchAttachments is a function emitting a stream of attachment items that are considered to be referenced from the current database state. In this example, todos.photo_id is the only column referencing attachments.

Next, start the sync process by calling attachmentQueue.startSync().

Storing attachments

To create a new attachment locally, call AttachmentQueue.saveFile. To represent the attachment, this method takes the contents to store, the media type, an optional file extension and id. The queue will store the contents in a local file and mark it as queued for upload. It also invokes a callback responsible for referencing the id of the generated attachment in the primary data model:

Future<Attachment> savePhotoAttachment(
    Stream<List<int>> photoData, String todoId,
    {String mediaType = 'image/jpeg'}) async {
  // Save the file using the AttachmentQueue API
  return await attachmentQueue.saveFile(
    data: photoData,
    mediaType: mediaType,
    fileExtension: 'jpg',
    metaData: 'Photo attachment for todo: $todoId',
    updateHook: (context, attachment) async {
      // Update the todo item to reference this attachment
      await context.execute(
        'UPDATE todos SET photo_id = ? WHERE id = ?',
        [attachment.id, todoId],
      );
    },
  );
}

Deleting attachments

To delete attachments, it is sufficient to stop referencing them in the data model, e.g. via UPDATE todos SET photo_id = NULL in this example. The attachment sync implementation will eventually delete orphaned attachments from the local storage.