Sync Changes Between Devices - Node.js SDK

Prerequisites

Before you can access a synced realm from the client, you must:

1. Enable sync in the Realm UI.
2. Initialize the app
3. Authenticate a user in your client project.
4. Open a Flexible Synced Realm or Open a Partition-Based Synced Realm

const config = {
schema: [DogSchema], // predefined schema
sync: {
   user: app.currentUser,
   partitionValue: "MyPartitionValue",
   ...rest of SyncConfiguration
},
};

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".

const OpenRealmBehaviorConfiguration = {
  type: "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.

const config = {
  schema: [DogSchema], // predefined schema
  sync: {
    user: app.currentUser,
    partitionValue: "MyPartitionValue",
    // The behavior to use when this is the first time opening a realm.
    newRealmFileBehavior: OpenRealmBehaviorConfiguration,
    // The behavior to use when a realm file already exists locally,
    // i.e. you have previously opened the realm.
    existingRealmFileBehavior: OpenRealmBehaviorConfiguration,
  },
};

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);

Pause or Resume a Sync Session

Opening a synced realm starts a sync session. You can pause and resume the sync session on the realm. Pausing a sync session only pauses that realm's sync session. If you have more than one open realm, pause does not affect the sync sessions for other realms.

To pause synchronization, use the syncSession.pause() method. To resume synchronization, use the syncSession.resume() method.

const OpenRealmBehaviorConfiguration = {
  type: "openImmediately",
};
var config = {
  schema: [DogSchema], // predefined schema
  sync: {
    user: app.currentUser,
    partitionValue: "MyPartitionValue",
    newRealmFileBehavior: OpenRealmBehaviorConfiguration,
    existingRealmFileBehavior: OpenRealmBehaviorConfiguration,
  },
};
let realm = await Realm.open(config);
const syncSession = realm.syncSession;
// Pause synchronization
syncSession.pause();
// Later, resume synchronization
syncSession.resume();

Check Upload & Download Progress for a Sync Session

To check the upload and download progress for a sync session, add a progress notification using the syncSession.addProgressNotification() method.

The syncSession.addProgressNotification() method takes in the following three parameters:

• A direction parameter. Set to "upload" to register notifications for uploading data. Set to "download" to register notifications for downloading data.
• A mode parameter. Set to "reportIndefinitely" for the notifications to continue until the callback is unregistered using syncSession.removeProgressNotification(). Set to "forCurrentlyOutstandingWork" for the notifications to continue until only the currently transferable bytes are synced.
• A callback function parameter that has the arguments transferred and transferable. transferred is the current number of bytes already transferred. transferable is the total number of bytes already transferred plus the number of bytes pending transfer.

const OpenRealmBehaviorConfiguration = {
  type: "openImmediately",
};
var config = {
  schema: [DogSchema], // predefined schema
  sync: {
    user: app.currentUser,
    partitionValue: "MyPartitionValue",
    newRealmFileBehavior: OpenRealmBehaviorConfiguration,
    existingRealmFileBehavior: OpenRealmBehaviorConfiguration,
  },
};
let realm = await Realm.open(config);
const syncSession = realm.syncSession;
syncSession.addProgressNotification(
  "upload",
  "reportIndefinitely",
  (transferred, transferable) => {
    console.log(`${transferred} bytes has been transferred`);
    console.log(
      `There are ${transferable} total transferable bytes, including the ones that have already been transferred`
    );
  }
);
// Upload something
let dog;
realm.write(() => {
  dog = realm.create("Dog", {
    name: "Fido",
    age: 2,
  });
});
// use dog
// remember to unregister the progress notifications
syncSession.removeProgressNotification((transferred, transferable) => {
  console.log(`There was ${transferable} total transferable bytes`);
  console.log(`${transferred} bytes were transferred`);
});

Check the Network Connection

To check the current state of the connection to the server, call the syncSession.connectionState() method.

var config = {
  schema: [DogSchema], // predefined schema
  sync: {
    user: app.currentUser,
    partitionValue: "MyPartitionValue",
  },
};
let realm = await Realm.open(config);
const syncSession = realm.syncSession;
const connectionState = syncSession.connectionState();

Set the Client Log Level

You can set the realm Sync client log level by calling Realm.App.Sync.setLogLevel() with your Realm.App.

Realm.App.Sync.setLogLevel(app, "debug");

Multiplex Sync Sessions

Enable session multiplexing to consolidate multiple sync sessions of a Realm app. Only use session multiplexing if you see errors about reaching the file descriptor limit, and you know you are using many sync sessions.

To enable session multiplexing, call Realm.App.Sync.enableSessionMultiplexing() with your Realm.App.

Realm.App.Sync.enableSessionMultiplexing(app);

Handle Sync Errors

While developing an application that uses Realm Sync, you should set an error handler. This error handler will detect and respond to any failed sync-related API calls.

Set an error handler by registering an error callback as part of the SyncConfiguration.

var config = {
  schema: [DogSchema], // predefined schema
  sync: {
    user: app.currentUser,
    partitionValue: "MyPartitionValue",
    error: (_session, error) => {
      (error) => {
        console.log(error.name, error.message);
      };
    },
  },
};
const realm = await Realm.open(config);

Perform a Client Reset

You can customize behavior in the event of a client reset error with a custom error handler function:

let realm = await Realm.open(config);
function errorSync(_session, error) {
  if (realm) {
    if (error.name === "ClientReset") {
      const realmPath = "<Your Realm Path>";
      realm.close();
      console.log(`Error ${error.message}, need to reset ${realmPath}…`);
      Realm.App.Sync.initiateClientReset(app, realmPath); // pass your realm app instance, and realm path to initiateClientReset()
      console.log(`Creating backup from ${error.config.path}…`);
      // Move backup file to a known location for a restore
      fs.renameSync(error.config.path, realmPath + "~");
      // Discard the reference to the realm instance
      realm = null;
    } else {
      console.log(`Received error ${error.message}`);
    }
  }
}
var config = {
  schema: [DogSchema], // predefined schema
  sync: {
    user: app.currentUser,
    partitionValue: "MyPartitionValue",
    error: errorSync,
  },
};