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Configure & Open a Realm - Swift SDK

On this page

  • Open a Realm Without Sync
  • Open a Default Realm or Realm at a File URL
  • Open an In-Memory Realm
  • Open a Realm with Swift Concurrency Features
  • Close a Realm
  • Handle Errors When Accessing a Realm
  • Provide a Subset of Classes to a Realm
  • Initialize Properties Using Realm APIs
  • Use Realm When the Device Is Locked

When you open a realm, you can pass a Realm.Configuration that specifies additional details about how to configure the realm file. This includes things like:

  • Pass a fileURL or in-memory identifier to customize how the realm is stored on device

  • Provide a logged-in user and Sync details to use Sync with the realm

  • Specify the realm use only a subset of your app's classes

  • Whether and when to compact a realm to reduce its file size

  • Pass an encryption key to encrypt a realm

  • Provide a schema version or migration block when making schema changes


See also:

This page covers how to open a realm file that does not sync data. If you'd like to use Device Sync to sync data with other devices, see: Configure & Open a Synced Realm.

You can open a non-synced local realm with several different configuration options:

  • No configuration - i.e. default configuration

  • Specify a file URL for the realm

  • Open the realm only in memory, without saving a file to the file system

  • Copy a synced realm to use without Sync

You can 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.


When all in-memory realm instances with a particular identifier go out of scope, Realm Database deletes all data in that realm. To avoid this, hold onto a strong reference to any in-memory realms during your app's lifetime.

You can use Swift's async/await syntax to open a MainActor-isolated realm, or specify an actor when opening a realm asynchronously:

func mainThreadFunction() async throws {
// These are identical: the async init produces a
// MainActor-isolated Realm if no actor is supplied
let realm1 = try await Realm()
let realm2 = try await Realm(actor: MainActor.shared)
try await useTheRealm(realm: realm1)

Or you can define a custom realm actor to manage all of your realm operations:

actor RealmActor {
// An implicitly-unwrapped optional is used here to let us pass `self` to
// `Realm(actor:)` within `init`
var realm: Realm!
init() async throws {
realm = try await Realm(actor: self)
var count: Int {
func createTodo(name: String, owner: String, status: String) async throws {
try await realm.asyncWrite {
realm.create(Todo.self, value: [
"_id": ObjectId.generate(),
"name": name,
"owner": owner,
"status": status
func updateTodo(_id: ObjectId, name: String, owner: String, status: String) async throws {
try await realm.asyncWrite {
realm.create(Todo.self, value: [
"_id": _id,
"name": name,
"owner": owner,
"status": status
], update: .modified)
func deleteTodo(todo: Todo) async throws {
try await realm.asyncWrite {
func close() {
realm = nil

An actor-isolated realm may be used with either local or global actors.

// A simple example of a custom global actor
@globalActor actor BackgroundActor: GlobalActor {
static var shared = BackgroundActor()
func backgroundThreadFunction() async throws {
// Explicitly specifying the actor is required for anything that is not MainActor
let realm = try await Realm(actor: BackgroundActor.shared)
try await realm.asyncWrite {
_ = realm.create(Todo.self, value: [
"name": "Pledge fealty and service to Gondor",
"owner": "Pippin",
"status": "In Progress"
// Thread-confined Realms would sometimes throw an exception here, as we
// may end up on a different thread after an `await`
let todoCount = realm.objects(Todo.self).count
print("The number of Realm objects is: \(todoCount)")
func mainThreadFunction() async throws {
try await backgroundThreadFunction()

For more information about working with actor-isolated realms, refer to Actor-Isolated Realms - Swift SDK.

There is no need to manually close a realm in Swift or Objective-C. When a realm goes out of scope and is removed from memory due to ARC, the realm is closed.


Operating with Low Memory Constraints

Some applications, such as watchOS apps and iOS app extensions, have tight constraints on their memory footprints. To optimize your data model for low-memory environments, open the realm with a subset of classes.

You might define properties whose values are initialized using Realm Database APIs. For example:

class SomeSwiftType {
let persons = try! Realm().objects(Person.self)
// ...

If this initialization code runs before you set up your Realm configurations, you might get unexpected behavior. For example, if you set a migration block for the default realm configuration in applicationDidFinishLaunching(), but you create an instance of SomeSwiftType before applicationDidFinishLaunching(), you might be accessing your realm before it has been correctly configured.

To avoid such issues, consider doing one of the following:

  • Defer instantiation of any type that eagerly initializes properties using Realm Database APIs until after your app has completed setting up its realm configurations.

  • Define your properties using Swift's lazy keyword. This allows you to safely instantiate such types at any time during your application's lifecycle, as long as you do not attempt to access your lazy properties until after your app has set up its realm configurations.

  • Only initialize your properties using Realm APIs that explicitly take in user-defined configurations. You can be sure that the configuration values you are using have been set up properly before they are used to open realms.

By default, iOS 8 and above encrypts app files using NSFileProtection whenever the device is locked. If your app attempts to access a realm while the device is locked, you might see the following error:

open() failed: Operation not permitted

To handle this, downgrade the file protection of the folder containing the Realm files. A less strict protection level like NSFileProtectionCompleteUntilFirstUserAuthentication allows file access even when the device is locked.


If you reduce iOS file encryption, consider using Realm's built-in encryption to secure your data instead.

This example shows how to apply a less strict protection level to the parent directory of the default realm.

let realm = try! Realm()
// Get the realm file's parent directory
let folderPath = realm.configuration.fileURL!.deletingLastPathComponent().path
// Disable file protection for this directory after the user has unlocked the device once
try! FileManager.default.setAttributes([FileAttributeKey.protectionKey: FileProtectionType.completeUntilFirstUserAuthentication],
ofItemAtPath: folderPath)

Realm may create and delete auxiliary files at any time. Instead of downgrading file protection on the files, apply it to the parent folder. This way, the file protection applies to all relevant files regardless of creation time.

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