Configure & Open a Synced Realm - Node.js SDK

Synced Realms

You can configure a realm to automatically synchronize data between many devices that each have their own local copy of the data. Synced realms use a different configuration than local-only realms and require an Atlas App Services backend to handle the synchronization process.

Applications can always create, modify, and delete synced realm objects locally, even when offline. Whenever a network connection is available, the Realm SDK opens a connection to an application server and syncs changes to and from other clients. The Atlas Device Sync protocol and server-side operational transforms guarantee that all fully synced instances of a realm see exactly the same data, even if some changes occurred offline and/or were received out of order.

Prerequisites

Before you configure a realm with Flexible Sync in a Node.js application:

1. Enable Flexible Sync on the backend. You must configure Flexible Sync in the backend before you can use it with your client application.

2. Initialize the App client.

3. Authenticate a user in your client project.

Open a Synced Realm

The first step in implementing Device Sync is to open the synced Realm. The following information pertains to an app using Flexible Sync. If your existing app uses the older Partition-Based Sync, refer to Open a Partition-Based Synced Realm. If you have not yet decided or are unsure which to use, read the Choose Your Sync Mode page.

To open a realm using Flexible Sync, call Realm.open(). Pass in a ConfigurationWithSync object, which must include the sync property defining a SyncConfiguration object. In the SyncConfiguration, you must include include a user and flexible:true.

const realm = await Realm.open({
  schema: [TaskSchema, TeamSchema],
  sync: {
    user: app.currentUser,
    flexible: true,
  },
});

By default, Realm syncs all data from the server before returning. If you want to sync data in the background, read the Open a Synced Realm While Offline section.

const app = new Realm.App({ id: APP_ID, baseFilePath: customPath });
const user = await app.logIn(Realm.Credentials.anonymous());
const realm = await Realm.open({
  schema: [Car],
  sync: {
    flexible: true,
    user,
  },
});

Open a Synced Realm While Offline

When your Realm application authenticates a user, it caches the user's credentials. You can check for existing user credentials to bypass the login flow and access the cached user. Use this to open a realm offline.

// Log user into your App Services App.
// On first login, the user must have a network connection.
const getUser = async () => {
  // If the device has no cached user credentials, log in.
  if (!app.currentUser) {
    const credentials = Realm.Credentials.anonymous();
    await app.logIn(credentials);
  }
  // If the app is offline, but credentials are
  // cached, return existing user.
  return app.currentUser!;
};

The following subsections show how to use background synchronization to access a realm while offline. To do this, use the cached user and an OpenRealmBehaviorConfiguration object.

Within your Sync Configuration, set the optional newRealmFileBehavior and existingRealmFileBehavior fields to your OpenRealmBehaviorConfiguration object to enable background synchronization.

Sync Changes in the Background

You may want to sync changes in the background to display partial data to the user while the synced realm downloads data from the server, preventing the user experience from being blocked. We recommend syncing changes in the background for applications in which the user's device may go offline. To sync changes in the background, open a synced realm synchronously.

Create a OpenRealmBehaviorConfiguration object and set its type to "openImmediately".

Create a Configuration object, which must include the sync property defining a SyncConfiguration object. Set this OpenRealmBehaviorConfiguration object as the value for the newRealmFileBehavior and existingRealmFileBehavior fields of the SyncConfiguration.

Finally, call Realm.open() to open a synced realm. This will create a sync session and begin downloading any existing data from the server in the background.

const realm = await Realm.open(config);

Cancel Asynchronous Operations after a Timeout

You can specify a cancelWaitsOnNonFatalErrors property in your BaseSyncConfiguration object.

When true, the Node.js SDK cancels asynchronous operations, such as waiting for uploads or downloads, when a timeout period elapses. You can define the timeout for this option on the AppConfiguration. For an example, refer to Configure a Timeout for the App Client.

This property defaults to false if you do not specify a value.