Sync Data

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Realm Sync is currently in Beta.

Realm Sync lets you share data across devices, between Realm clients and a synced MongoDB Atlas cluster and is a solution for offline-first app development. To learn more about Realm Sync, see Sync Overview.

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

To open a synced realm for a given partition value, initialize a Realm by passing a configuration object to "". The config object should have a sync field that is an object with the authenticated user, and the partition value as fields:

const config = {
schema: [MySchema],
sync: {
user: user,
partitionValue: partitionValue,
try {
let realm = await;
} catch (error) {

The partition value specifies which subset of your data to sync. This is typically a user ID, project ID, store ID, or some other category identifier in your app that has particular relevance to the current user.

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The syntax to read, write, and watch for changes on a synced realm is identical to the syntax for non-synced realms. While you work with local data, Realm efficiently integrates, uploads, and downloads changesets.

The following code reads a collection of Task objects, watches that collection for changes, then writes a new Task to the realm:

// open a realm with the config object with a synced field
let realm = await;
const syncTasks = realm.objects("Task");
// Watch for changes. No special syntax required for synced realms.
realm.addListener("change", () => {
console.log("sync tasks - ", syncTasks);
realm.write(() => {
// Write to the realm. No special syntax required for synced realms.
realm.write(() => {
realm.create("Task", {
name: "buy groceries",
description: "go to store and buy milk, eggs and bread",
// remove listeners and close the synced realm when done observing

Some Realm applications, such as event handlers, require the ability to open large numbers of realms. Normally, each realm uses a single websocket connection to synchronize data. However, applications that open a large number of realms can consume a disproportionate amount of network resources to synchronize all of those realms simultaneously.

To reduce this network load, you can enable session multiplexing in your application. Session multiplexing uses a single connection for all realms synchronized by your application. To enable session multiplexing, call the Realm.App.Sync.enableSessionMultiplexing() static method on startup before instantiating any realm connections.

  • Open a synced realm by creating a config object with a sync sub-object that has a partition value.
  • The syntax for reading from, writing to, or watching objects on a synced realm is the same as the syntax for a non-synced realm
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