@powersync/attachments

A PowerSync library to manage attachments in React Native and JavaScript/TypeScript apps.

Installation

yarn

yarn add @powersync/attachments

pnpm

pnpm add @powersync/attachments

npm

npm install @powersync/attachments

Usage

The AttachmentQueue class is used to manage and sync attachments in your app.

Example

In this example, the user captures photos when checklist items are completed as part of an inspection workflow.

The schema for the checklist table:

const checklists = new Table(
  {
    photo_id: column.text,
    description: column.text,
    completed: column.integer,
    completed_at: column.text,
    completed_by: column.text
  },
  { indexes: { inspections: ['checklist_id'] } }
);

const AppSchema = new Schema({
  checklists
});

Steps to implement

1. Create a new class AttachmentQueue that extends AbstractAttachmentQueue from @powersync/attachments.

import { AbstractAttachmentQueue } from '@powersync/attachments';

export class AttachmentQueue extends AbstractAttachmentQueue {}

2. Implement onAttachmentIdsChange, which takes in a callback to handle an array of string values of IDs that relate to attachments in your app. We recommend using PowerSync's watch query to return the all IDs of attachments in your app.

   In this example, we query all photos that have been captured as part of an inspection and map these to an array of string values.

import { AbstractAttachmentQueue } from '@powersync/attachments';

export class AttachmentQueue extends AbstractAttachmentQueue {
  onAttachmentIdsChange(onUpdate) {
    this.powersync.watch('SELECT photo_id as id FROM checklists WHERE photo_id IS NOT NULL', [], {
      onResult: (result) => onUpdate(result.rows?._array.map((r) => r.id) ?? [])
    });
  }
}

3. Implement newAttachmentRecord to return an object that represents the attachment record in your app.

   In this example we always work with JPEG images, but you can use any media type that is supported by your app and storage solution. Note: we are set the state to QUEUED_UPLOAD when creating a new photo record which assumes that the photo data is already on the device.

import { AbstractAttachmentQueue } from '@powersync/attachments';

export class AttachmentQueue extends AbstractAttachmentQueue {
  // ...
  async newAttachmentRecord(record) {
    const photoId = record?.id ?? uuid();
    const filename = record?.filename ?? `${photoId}.jpg`;
    return {
      id: photoId,
      filename,
      media_type: 'image/jpeg',
      state: AttachmentState.QUEUED_UPLOAD,
      ...record
    };
  }
}

4. Add an AttachmentTable to your app's PowerSync Schema:

import { AttachmentTable } from '@powersync/attachments';

const AppSchema = new Schema({
  // ... other tables
  attachments: new AttachmentTable({
    name: 'attachments',
  }),
});

In addition to Table options, the AttachmentTable can optionally be configured with the following options:

Option	Description	Default
name	The name of the table	attachments
additionalColumns	An array of addition Column objects added to the default columns in the table	See below for default columns

The default columns in AttachmentTable:

Column Name	Type	Description
id	TEXT	The ID of the attachment record
filename	TEXT	The filename of the attachment
media_type	TEXT	The media type of the attachment
state	INTEGER	The state of the attachment, one of AttachmentState enum values
timestamp	INTEGER	The timestamp of last update to the attachment record
size	INTEGER	The size of the attachment in bytes

5. To instantiate an AttachmentQueue, one needs to provide an instance of AbstractPowerSyncDatabase from PowerSync and an instance of StorageAdapter. See the StorageAdapter interface definition here.

6. Instantiate a new AttachmentQueue and call init() to start syncing attachments. Our example, uses a StorageAdapter that integrates with Supabase Storage.

this.storage = this.supabaseConnector.storage;
this.powersync = new PowerSyncDatabase({
  schema: AppSchema,
  database: {
    dbFilename: 'sqlite.db'
  }
});

this.attachmentQueue = new AttachmentQueue({
  powersync: this.powersync,
  storage: this.storage
});

// Initialize and connect PowerSync ...
// Then initialize the attachment queue
await this.attachmentQueue.init();

7. Finally, to create an attachment and add it to the queue, call saveToQueue().

   In our example we added a savePhoto() method to our AttachmentQueue class, that does this:

export class AttachmentQueue extends AbstractAttachmentQueue {
  // ...
  async savePhoto(base64Data) {
    const photoAttachment = await this.newAttachmentRecord();
    photoAttachment.local_uri = this.getLocalFilePathSuffix(photoAttachment.filename);

    const localFilePathUri = this.getLocalUri(photoAttachment.local_uri);

    await this.storage.writeFile(localFilePathUri, base64Data, { encoding: 'base64' });

    return this.saveToQueue(photoAttachment);
  }
}

Implementation details

Attachment State

The AttachmentQueue class manages attachments in your app by tracking their state.

The state of an attachment can be one of the following:

State	Description
QUEUED_SYNC	Check if the attachment needs to be uploaded or downloaded
QUEUED_UPLOAD	The attachment has been queued for upload to the cloud storage
QUEUED_DOWNLOAD	The attachment has been queued for download from the cloud storage
SYNCED	The attachment has been synced
ARCHIVED	The attachment has been orphaned, i.e. the associated record has been deleted

Initial sync

Upon initializing the AttachmentQueue, an initial sync of attachments will take place if the performInitialSync is set to true. Any AttachmentRecord with id in first set of IDs retrieved from the watch query will be marked as QUEUED_SYNC, and these records will be rechecked to see if they need to be uploaded or downloaded.

Syncing attachments

The AttachmentQueue sets up two watch queries on the attachments table, one for records in QUEUED_UPLOAD state and one for QUEUED_DOWNLOAD state.

In addition to watching for changes, the AttachmentQueue also triggers a sync every few seconds. This will retry any failed uploads/downloads, in particular after the app was offline.

By default, this is every 30 seconds, but can be configured by setting syncInterval in the AttachmentQueue constructor options, or disabled by setting the interval to 0.

Uploading

• An AttachmentRecord is created or updated with a state of QUEUED_UPLOAD.
• The AttachmentQueue picks this up and upon successful upload to Supabase, sets the state to SYNCED.
• If the upload is not successful, the record remains in QUEUED_UPLOAD state and uploading will be retried when syncing triggers again.

Downloading

• An AttachmentRecord is created or updated with QUEUED_DOWNLOAD state.
• The watch query adds the id into a queue of IDs to download and triggers the download process
• This checks whether the photo is already on the device and if so, skips downloading.
• If the photo is not on the device, it is downloaded from cloud storage.
• Writes file to the user's local storage.
• If this is successful, update the AttachmentRecord state to SYNCED.
• If any of these fail, the download is retried in the next sync trigger.

Deleting attachments

When an attachment is deleted by a user action or cache expiration:

• Related AttachmentRecord is removed from attachments table.
• Local file (if exists) is deleted.
• File on cloud storage is deleted.

Expire Cache

When PowerSync removes a record, as a result of coming back online or conflict resolution for instance:

• Any associated AttachmentRecord is orphaned.
• On the next sync trigger, the AttachmentQueue sets all records that are orphaned to ARCHIVED state.
• By default, the AttachmentQueue only keeps the last 100 attachment records and then expires the rest.
• This can be configured by setting cacheLimit in the AttachmentQueue constructor options.