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Overview

PowerSync supports defining Sync Bucket Priorities, which allows you to control the sync order for different data sets. This is particularly useful when certain data should be available sooner than others.

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Why Use Sync Bucket Priorities?

PowerSync’s standard sync protocol ensures that:

• The local data view is only updated when a fully consistent checkpoint is available.
• All pending local changes must be uploaded, acknowledged, and synced back before new data is applied.

While this guarantees consistency, it can lead to delays, especially for large datasets or continuous client-side updates. Sync Bucket Priorities provide a way to speed up syncing of high-priority data while still maintaining overall integrity.

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How It Works

Each sync bucket is assigned a priority value between 0 and 3, where:

• 0 is the highest priority and has special behavior (detailed below).
• 3 is the default and lowest priority.
• Lower numbers indicate higher priority.

Buckets with higher priorities sync first, and lower-priority buckets sync later. It’s worth noting that if you only use a single priority, there is no difference between priorities 1-3. The difference only comes in if you use multiple different priorities.

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Syntax and Configuration

Priorities can be defined for a bucket using the priority YAML key, or with the _priority attribute inside parameter queries:

bucket_definitions:
  # Using the `priority` YAML key
  user_data:
    priority: 1
    parameters: SELECT request.user_id() as id where...;
    data: 
      # ...
  
  # Using the `_priority` attribute
  project_data:
    parameters: select id as project_id, 2 as _priority from projects where ...; # This approach is useful when you have multiple parameter queries with different priorities.
    data: 
      # ...

Note:

• Priorities must be static and cannot depend on row values within a parameter query.

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Example: Syncing Lists Before Todos

Consider a scenario where you want to display lists immediately while loading todos in the background. This approach allows users to view and interact with lists right away without waiting for todos to sync. Here’s how to configure sync priorities in your Sync Rules to achieve this:

bucket_definitions:
  user_lists:
    # Sync the user's lists with a higher priority
    priority: 1
    parameters: select id as list_id from lists where user_id = request.user_id()
    data:
      - select * from lists where id = bucket.list_id

  user_todos:
    # Sync the user's todos with a lower priority
    priority: 3
    parameters: select id as todo_id from todos where list_id in (select id from lists where user_id = request.user_id())
    data:
      - select * from todos where list_id = bucket.todo_id

In this configuration:

The lists bucket has the default priority of 1, meaning it syncs first.

The todos bucket is assigned a priority of 2, meaning it may sync only after the lists have been synced.

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Behavioral Considerations

• Interruption for Higher Priority Data: Syncing lower-priority buckets may be interrupted if new data for higher-priority buckets arrives.
• Local Changes & Consistency: If local writes fail due to validation or permission issues, they are only reverted after all buckets sync.
• Deleted Data: Deleted data may only be removed after all buckets have synced. Future updates may improve this behavior.
• Data Ordering: Data in lower-priority buckets will never appear before higher-priority data.

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Special Case: Priority 0

Priority 0 buckets sync regardless of pending uploads.

For example, in a collaborative document editing app (e.g., using Yjs), each change is stored as a separate row. Since out-of-order updates don’t affect document integrity, Priority 0 can ensure immediate availability of updates.

Caution: If misused, Priority 0 may cause flickering or inconsistencies, as updates could arrive out of order.

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Consistency Considerations

PowerSync’s full consistency guarantees only apply once all buckets have completed syncing.

When higher-priority buckets are synced, all inserts and updates within the buckets for the specific priority will be consistent. However, deletes are only applied when the full sync completes, so you may still have some stale data within those buckets.

Consider the following example:

Imagine a task management app where users create lists and todos. Some users have millions of todos. To improve first-load speed:

• Lists are assigned Priority 1, syncing first to allow UI rendering.
• Todos are assigned Priority 2, loading in the background.

Now, if another user adds new todos, it’s possible for the list count (synced at Priority 1) to temporarily not match the actual todos (synced at Priority 2). If real-time accuracy is required, both lists and todos should use the same priority.

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Client-Side Considerations

PowerSync’s client SDKs provide APIs to allow applications to track sync status at different priority levels. Developers can leverage these to ensure critical data is available before proceeding with UI updates or background processing. This includes:

1. waitForFirstSync(priority: int). When passing the optional priority parameter to this method, it will wait for specific priority level to complete syncing.
2. SyncStatus.priorityStatusEntries() A list containing sync information for each priority that was seen by the PowerSync Service.
3. SyncStatus.statusForPriority(priority: int) This method takes a fixed priority and returns the sync state for that priority by looking it up in priorityStatusEntries.

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Example

Using the above we can render a lists component only once the user’s lists (with priority 1) have completed syncing, else display a message indicating that the sync is still in progress:

  // Define the priority level of the lists bucket
  static final _listsPriority = BucketPriority(1);

  @override
  Widget build(BuildContext context) {
    // Use FutureBuilder to wait for the first sync of the specified priority to complete
    return FutureBuilder(
      future: db.waitForFirstSync(priority: _listsPriority),
      builder: (context, snapshot) {
        if (snapshot.connectionState == ConnectionState.done) {
          // Use StreamBuilder to render the lists once the sync completes
          return StreamBuilder(
            stream: TodoList.watchListsWithStats(),
            builder: (context, snapshot) {
              if (snapshot.data case final todoLists?) {
                return ListView(
                  padding: const EdgeInsets.symmetric(vertical: 8.0),
                  children: todoLists.map((list) {
                    return ListItemWidget(list: list);
                  }).toList(),
                );
              } else {
                return const CircularProgressIndicator();
              }
            },
          );
        } else {
          return const Text('Busy with sync...');
        }
      },
    );
  }

Example implementations of prioritized sync are also available in the following apps:

• Flutter: Supabase To-Do List
• React Native: Coming soon.
• JavaScript/Web: Coming soon.
• Kotlin Multiplatform:
  • Supabase To-Do List (KMP)
  • Supabase To-Do List (Android)
• Swift: Coming soon.