Realms - React Native SDK

Realms are the core data structure used to organize data in Realm Database. At its core, a realm is a collection of the objects that you use in your application, called Realm objects, as well as additional metadata that describe the objects.

Learn How to Work With a Realm

See Open & Close a Local Realm for code examples that show how to configure and open a realm in the React Native SDK.

The Realm data model is similar to both relational and document databases but has distinct differences from both. To underscore these differences, it's helpful to highlight what a realm is not:

A realm is not a single, application-wide database.
Applications based on other database systems generally store all of their data in a single database. Realm apps often split data across multiple realms to organize data more efficiently and to enforce access controls.
A realm is not a relational table.
Normalized tables in relational databases only store one type of information, such as street addresses or items in a store inventory. A realm can contain any number of object types that are relevant to a given domain.
A realm is not a collection of schemaless documents.
Document databases don't necessarily enforce a strict schema for the data in each collection. While similar to documents in form, every Realm object conforms to a schema for a specific object type in the realm. An object cannot contain a property that is not described by its schema.

Every Realm object conforms to a specific object type, which is essentially a class that defines the properties and relationships for objects of that type using a pre-defined schema. Realm guarantees that all objects in a realm conform to the schema for their object type and validates objects whenever they're created, modified, or deleted.

Realm objects are fundamentally similar to a common JavaScript object but they also bring along a few additional features like schema validation and live queries. The React Native SDK memory maps Realm objects directly to native JavaScript objects, which means there's no need to use a special data access library, such as an ORM. Instead, you can work with Realm objects as you would any other object.


The following schema defines a Student object type with a string name and a list of integer grades.

const studentSchema = {
name: "Student",
properties: {
name: "string",
grades: "int[]"
Learn How to Define an Object Schema

For code examples that show how to define a Realm object schema in the React Native SDK, see Define a Realm Object Schema.

A realm schema is a list of valid object schemas that a realm may contain. Every Realm object must conform to an object type that's included in its realm's schema.

If a realm already contains data when you open it, Realm Database validates each object to ensure that an object schema was provided for its type and that it meets all of the constraints specified in the schema.


A realm that contains basic data about books in libraries might use a schema like the following:

"type": "Library",
"properties": {
"address": "string",
"books": "Book[]"
"type": "Book",
"primaryKey": "isbn",
"properties": {
"isbn": "string",
"title": "string",
"author": "string",
"numberOwned": { "type": "int?", "default": 0 },
"numberLoaned": { "type": "int?", "default": 0 }

Realm Database stores a binary encoded version of every object and type in a realm in a single .realm file. The file is located at a specific path that you define when you open the realm.

Auxiliary Realm Files

Realm Database creates a few additional files for each realm. These files are primarily for internal use and in general you can ignore them.

  • .realm.lock: The lock file enables synchronization between writes.
  • .realm.note: The note file is a named pipe for inter-thread and inter-process notifications.
  • The management directory contains internal state management files.
Use Caution When Deleting Realm Files

In some circumstances, such as a client reset scenario, you might need to delete a realm file and its auxiliary files.

If you delete a realm file or any of its auxiliary files while one or more instances of the realm are open, you might corrupt the realm or disrupt sync.

You may safely delete these files when all instances of a realm are closed. Before you delete a realm file, make sure that you back up any important objects as you will lose all unsynced data in the realm.

You can also open a realm entirely in memory, which will not create a .realm file or its associated auxiliary files. Instead the SDK stores objects in memory while the realm is open and discards them immediately when all instances are closed.

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 a MongoDB Realm app 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 Realm 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 receieved out of order.

Learn How to Configure and Use Sync

For more information on synced realms, including directions on how to set up sync in a MongoDB Realm app, see Realm Sync Overview.

For code examples that show how to work with a synced realm in the React Native SDK, see Sync Changes Between Devices.

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On this page

  • Realm vs Other Databases
  • Object Types & Schemas
  • Realm Schema
  • Realm Files
  • In-Memory Realms
  • Synced Realms