Manage Sync Subscriptions - Node.js SDK

Subscribe to a Query

We recommend that you name your subscriptions. This makes finding and managing your subscriptions easier. Subscription names must be unique. Trying to add a subscription with the same name as an existing subscription throws an error.

To subscribe to a query:

1. Query for the objects that you want to read and write.

2. Call subscribe() on the query results to create a sync subscription for objects matching the query.

3. Pass a SubscriptionOptions object that contains the name property to subscribe().

Subscribe to a Query without a Subscription Name

Most of the time, you should give your subscriptions a name. If you don't, the name is set to null.

If you use filtered() on an unnamed query subscription, the subscription identifier is based on the filtered query. This means that every time your query string changes, subscribe() will create a new subscription.

Wait for a Query Subscription to Sync

When you subscribe to a query's results, the results do not contain objects until synced data is downloaded. When you do need to wait for syned objects to finish downloading, use waitForSync. You can specify different behavior for your subscriptions and how they handle witing for downloads.

This example uses the FirstTime option, which is the default behavior. A subscription with FirstTime behavior only waits for sync to finish when a subscription is first created.

import { WaitForSync } from "realm";
// Get tasks that have a status of "in progress".
const completedTasks = realm
  .objects(Task)
  .filtered("status == 'completed'");
// Only waits for sync to finish on the initial sync.
await completedTasks.subscribe({
  behavior: WaitForSync.FirstTime,
  name: "First time sync only",
});

The other supported WaitForSync options are:

• Always: Wait to download matching objects every time your app launches. The app must have an internet connection at every launch.

• Never: Never wait to download matching objects. The app needs an internet connection for the user to authenticate the first time the app launches, but can open offline on subsequent launches using cached credentials.

You can optionally specify a timeout value to limit how long the sync download runs:

import { WaitForSync } from "realm";
// Get tasks that have a status of "in progress".
const completedTasks = realm
  .objects(Task)
  .filtered("status == 'completed'");
// Add subscription with timeout
// If timeout expires before sync is completed, currently-downloaded
// objects are returned and sync download continues in the background.
const taskSubscription = await completedTasks.subscribe({
  behavior: WaitForSync.Always,
  timeout: 500,
});

Unsubscribe from a Query

You can unsubscribe from a query's results using unsubscribe():

import { WaitForSync } from "realm";
// Get tasks that have a status of "in progress".
const completedTasks = realm
  .objects(Task)
  .filtered("status == 'completed'");
// Only waits for sync to finish on the initial sync.
await completedTasks.subscribe({
  behavior: WaitForSync.FirstTime,
  name: "First time sync only",
});
// Unsubscribe
completedTasks.unsubscribe();

This removes the subscription from the list of active subscriptions, similar to manually removing a subscription.

A results list may still contain objects after calling unsubscribe() if another subscription exists that contains overlapping objects.

When you call unsubscribe(), the associated subscription is removed. Subscriptions are removed by name. If they don't have a name, unsubscribe() removes any queries that exactly match the one you call unsubscribe() on.

The unsubscribe() method returns before objects matching the removed subscription are deleted from the realm. Sync continues in the background based on the new set of subscriptions.

Get All Subscriptions

When using a flexible synced realm, you can access a SubscriptionSet, a collection of subscriptions, through the realm.subscriptions property.

// get the SubscriptionSet for the realm
const subscriptions = realm.subscriptions;

Add a Subscription

Subscriptions are based on the results of realm queries.

In the following example, completed and progressMinutes have been set as queryable fields in an App Services App. In the client code, we create filtered queries and then subscribe to their results:

• Completed tasks

• Completed tasks that have taken over 120 progressMinutes

const tasks = realm.objects("Task");
const longRunningTasks = tasks.filtered(
  'status == "completed" && progressMinutes > 120'
);
await realm.subscriptions.update((mutableSubs) => {
  mutableSubs.add(longRunningTasks, {
    name: "longRunningTasksSubscription",
  });
  mutableSubs.add(realm.objects("Team"), {
    name: "teamsSubscription",
  });
});

Set Initial Subscriptions

You must have at least one subscription before you can read from or write to a Flexible Sync realm. You can add an initial subscription when opening a realm.

To set initial subscriptions, include the initialSubscriptions field in your realm's SyncConfiguration. Within the initialSubscriptions object, add an update field set to a callback that subscribes to queries:

By default, initial subscriptions are only created the first time a realm is opened. If your app needs to rerun this initial subscription every time the app starts, you can set rerunOnOpen to true. You might need to do this to re-run dynamic time ranges or other queries that require a re-computation of static variables for the subscription.

Check the Status of Subscriptions

You can check the subscription state to see if the server has acknowledged the subscription and the device has downloaded the data locally.

You can use subscription state to:

• Trigger error handling

• Show if the transaction is pending or has completed

• Find out when a subscription set is superseded, and you should obtain a new instance of the subscription set to write a subscription change

Node.js v12.0.0 added the SubscriptionSetState enum that you can use to get the status of a subscription.

Update Subscriptions with a New Query

You can update a named subscription with a new query. To update a subscription's query, pass the new query and a subscription option with the name of the subscription that you want to update to the MutableSubscriptionSet.add() method. Like adding a new subscription, you must update a subscription within a transaction by calling subscriptions.update().

In the following example, long-running tasks are re-defined to be any tasks that have taken more than 180 minutes.

realm.subscriptions.update((mutableSubs) => {
  mutableSubs.add(
    tasks.filtered('status == "completed" && progressMinutes > 180'),
    {
      name: "longRunningTasksSubscription",
    }
  );
});

Remove Subscriptions

You can remove subscriptions in several ways:

• Remove a single subscription with a specific query

• Remove a single subscription with a specific name

• Remove all subscriptions to a specific object model

• Remove all unnamed subscriptions

• Remove all subscriptions

When you remove a subscription query, the server also removes synced data from the client device.

realm.subscriptions.update((mutableSubs) => {
  // remove a subscription with a specific query
  mutableSubs.remove(tasks.filtered('owner == "Ben"'));
});

realm.subscriptions.update((mutableSubs) => {
  // remove a subscription with a specific name
  mutableSubs.removeByName("longRunningTasksSubscription");
});

let subscriptionReference;
realm.subscriptions.update((mutableSubs) => {
  subscriptionReference = mutableSubs.add(realm.objects("Task"));
});
// later..
realm.subscriptions.removeSubscription(subscriptionReference);

realm.subscriptions.update((mutableSubs) => {
  mutableSubs.removeByObjectType("Team");
});

// Remove unnamed subscriptions.
let numberRemovedSubscriptions = 0;
await realm.subscriptions.update((mutableSubs) => {
  numberRemovedSubscriptions = mutableSubs.removeUnnamed();
});

realm.subscriptions.update((mutableSubs) => {
  mutableSubs.removeAll();
});

API Efficiency

Managing multiple subscriptions with the subscribe() and unsubscribe() APIs described in the Subscribe to Queries section is less efficient than performing batch updates when you manually manage subscriptions.

For better performance when making multiple subscription changes, use the subscriptions API to update all the subscriptions in a single transaction. To learn how, see Manually Manage Subscriptions.

Group Updates for Improved Performance

Every write transaction for a subscription set has a performance cost. If you need to make multiple updates to a Realm object during a session, consider keeping edited objects in memory until all changes are complete. This improves sync performance by only writing the complete and updated object to your realm instead of every change.

Indexed Queryable Fields Subscription Requirements

Adding an indexed queryable field to your App can improve performance for simple queries on data that is strongly partitioned. For example, an app where queries strongly map data to a device, store, or user, such as user_id == $0, “641374b03725038381d2e1fb”, is a good candidate for an indexed queryable field. However, an indexed queryable field has specific requirements for use in a query subscription:

• The indexed queryable field must be used in every subscription query. It cannot be missing from the query.

• The indexed queryable field must use an == or IN comparison against a constant at least once in the subscription query. For example, user_id == $0, "641374b03725038381d2e1fb" or store_id IN $0, {1,2,3}.

You can optionally include an AND comparison as long as the indexed queryable field is directly compared against a constant using == or IN at least once. For example, store_id IN {1,2,3} AND region=="Northeast" or store_id == 1 AND (active_promotions < 5 OR num_employees < 10).

Invalid Flexible Sync queries on an indexed queryable field include queries where:

• The indexed queryable field does not use AND with the rest of the query. For example store_id IN {1,2,3} OR region=="Northeast" is invalid because it uses OR instead of AND. Similarly, store_id == 1 AND active_promotions < 5 OR num_employees < 10 is invalid because the AND only applies to the term next to it, not the entire query.

• The indexed queryable field is not used in an equality operator. For example store_id > 2 AND region=="Northeast" is invalid because it uses only the > operator with the indexed queryable field and does not have an equality comparison.

• The query is missing the indexed queryable field entirely. For example, region=="Northeast or truepredicate are invalid because they do not contain the indexed queryable field.

Unsupported Query Operators in Flexible Sync

Flexible Sync has some limitations when using RQL operators. When you write the query subscription that determines which data to sync, the server does not support these query operators. However, you can still use the full range of RQL features to query the synced data set in the client application.

Case insensitive queries ([c]) cannot use indexes effectively. As a result, case insensitive queries are not recommended, since they could lead to performance problems.

Flexible Sync only supports @count for array fields.

List Queries

Flexible Sync supports querying lists using the IN operator.

You can query a list of constants to see if it contains the value of a queryable field:

// Query a constant list for a queryable field value
"priority IN { 1, 2, 3 }"

If a queryable field has an array value, you can query to see if it contains a constant value:

// Query an array-valued queryable field for a constant value
"'comedy' IN genres"

// Invalid Flexible Sync query. Do not do this!
"{'comedy', 'horror', 'suspense'} IN genres"
// Another invalid Flexible Sync query. Do not do this!
"ANY {'comedy', 'horror', 'suspense'} != ANY genres"

Embedded or Linked Objects

Flexible Sync does not support querying on properties in Embedded Objects or links. For example, obj1.field == "foo".