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Atlas App Services

Tutorial: Atlas Device Sync for Kotlin

On this page

  • Learning Objectives
  • Prerequisites
  • Start with the Template
  • Set up the Template App
  • Open the App
  • Explore the App Structure
  • Run the App
  • Check the Backend
  • Modify the Application
  • Add a New Property
  • Add a New Property to the Model
  • Set the Priority when Creating a New Item
  • Run and Test
  • Change the Subscription
  • Update the subscription
  • Run and Test
  • Conclusion
  • What's Next?

Estimated time to complete: 30 minutes, depending on your experience with Kotlin

Realm provides a Kotlin SDK that allows you to create an Android mobile application with Kotlin using Jetpack Compose. This tutorial is based on the Kotlin Flexible Sync Template App, named kotlin.todo.flex, which illustrates the creation of a To-do Item List management application. This application enables users to:

  • Register their email as a new user account.

  • Sign in to their account with their email and password (and sign out later).

  • View, create, modify, and delete their own tasks.

  • View all tasks, even where the user is not the owner.

The template app also provides a toggle that simulates the device being in "Offline Mode." This toggle lets you quickly test Device Sync functionality on the simulator, emulating the user having no internet connection. However, you would likely remove this toggle in a production application.

This tutorial adds functionality to the Template App. You will add a new Priority field to the existing Item model and update the Flexible Sync subscription to only show items within a range of priorities. This example illustrates how you might adapt the template app for your own needs.

This tutorial illustrates how you might adapt the template app for your own needs.

In this tutorial, you will learn how to:

  • Update a Realm object model with a non-breaking change.

  • Update a Device Sync subscription.

  • Add a queryable field to the Device Sync configuration on the server to change which data is synchronized.


Check Out the Quick Start

If you prefer to get started with your own application rather than follow a guided tutorial, check out the Kotlin Quick Start. It includes copyable code examples and the essential information that you need to set up an Atlas App Services backend.

  • Android Studio Bumblebee 2021.1.1 or higher.

  • JDK 11 or higher.

  • Kotlin Plugin for Android Studio, version 1.6.10 or higher.

  • An Android Virtual Device (AVD) using a supported CPU architecture.

  • This tutorial starts with a Template App. You need an Atlas Account, an API key, and App Services CLI to create a Template App.

    • You can learn more about creating an Atlas account in the Atlas Getting Started documentation. For this tutorial, you need an Atlas account with a free-tier cluster.

    • You also need an Atlas API key for the MongoDB Cloud account you wish to log in with. You must be a Project Owner to create a Template App using App Services CLI.

    • To learn more about installing App Services CLI, see Install App Services CLI. After installing, run the login command using the API key for your Atlas project.

This tutorial is based on the Kotlin SDK Flexible Sync Template App named kotlin.todo.flex. We start with the default app and build new features on it.

To learn more about the Template Apps, see Template Apps.

If you don't already have an Atlas account, sign-up to deploy a Template App.

Follow the procedure described in the Create an App guide, and select Create App from Template. Select the Real-time Sync template. This creates an App Services App pre-configured to use with one of the Device Sync template app clients.

After you create a template app, the UI displays a modal labeled Get the Front-end Code for your Template. This modal provides instructions for downloading the template app client code as a .zip file or using App Services CLI to get the client.

After selecting the .zip or App Services CLI method, follow the on-screen instructions to get the client code. For this tutorial, select the Kotlin (Android) client code.


The default Windows ZIP utility may show the .zip file as empty. If you encounter this, use one of the third-party zip programs that are available.

The appservices apps create command sets up the backend and creates a Kotlin template app for you to use as a base for this tutorial.

Run the following command in a terminal window to create an app named "MyTutorialApp" that is deployed in the US-VA region with its environment set to "development" (instead of production or QA).

appservices app create \
--name MyTutorialApp \
--template kotlin.todo.flex \
--deployment-model global \
--environment development

The command creates a new directory in your current path with the same name as the value of the --name flag.

You can fork and clone a GitHub repository that contains the Device Sync client code. The Kotlin client code is available at

If you use this process to get the client code, you must create a template app to use with the client. Follow the instructions at Create a Template App to use the Atlas App Services UI, App Services CLI, or Admin API to create a Device Sync template app.


In Android Studio, open the kotlin.todo.flex folder located in the frontend folder of the template app.

If you downloaded the client as a .zip file or cloned the client GitHub repository, you must manually insert the App Services App ID in the appropriate place in your client. Follow the Configuration instructions in the client to learn where to insert your App ID.


Take a few minutes to explore the project organization while Android Studio indexes your project. Within the app/java/ directory, you can see a few files worth noting:

Activity class that defines the layout and provides functionality for opening a realm, writing Items to the realm, logging a user out, and closing a realm.
Activity class that defines the layout and provides functionality for registering a user and logging a user in.
Class that initializes the App Services App.

In this tutorial, you'll be working in the following files:

Located in the domain directory. Defines the Realm object we store in the database.
Located in the ui/tasks directory. Contains the composable function that defines the layout used when adding an item.
Located in the presentation/tasks directory. The view model that contains business logic and manages state when adding an item.
Located in the data directory. Repository used to access Realm Sync and defines the Flexible Sync subscription.
Located in the res/values directory. Defines the text string resources used in the app.

Without making any changes to the code, you should be able to run the app on an Android Emulator using Android Studio or on a physical device.

Run the app, register a new user account, and then add a new Item to your todo list.


Log in to Atlas App Services. In the Data Services tab, click on Browse Collections. In the list of databases, find and expand the todo database, and then the Item collection. You should see the document you created in this collection.


Now that you have confirmed everything is working as expected, we can add changes. In this tutorial, we have decided that we want to add a "priority" property to each Item so that we can filter Items by their priority level. The priority property will be mapped to a PriorityLevel enum to constrain the possible values, and we will use the ordinal of each enum to correspond to the priority integer so we can query based on a numeric priority level later.

To do this, follow these steps:

  1. Within the app/java/ folder, open the Item class file.

  2. Add a PriorityLevel enum to constrain the possible values. Also add a priority property to the Item class, which sets the default priority to 3, indicating that it is a low-priority todo item:

    // ... imports
    enum class PriorityLevel() {
    Severe, // priority 0
    High, // priority 1
    Medium, // priority 2
    Low // priority 3
    class Item() : RealmObject {
    var _id: ObjectId = ObjectId.create()
    var isComplete: Boolean = false
    var summary: String = ""
    var owner_id: String = ""
    var priority: Int = PriorityLevel.Low.ordinal
    constructor(ownerId: String = "") : this() {
    owner_id = ownerId
    // ... equals() and hashCode() functions
  1. From the ui/tasks folder, open the AddItem.kt file. This file defines the composable functions for the UI that is displayed when a user clicks the '+' button to add a new todo item.

  2. First, add the following imports below the package

    import androidx.compose.material3.DropdownMenuItem
    import androidx.compose.material3.ExposedDropdownMenuBox
    import androidx.compose.ui.Modifier
    import androidx.compose.ui.unit.dp
  3. Now we can add a dropdown field to the AddItemPrompt composable function that will enable the user to pick a priority level from a list using the PriorityLevel enums as available values:

    // ... imports
    fun AddItemPrompt(viewModel: AddItemViewModel) {
    containerColor = Color.White,
    onDismissRequest = {
    title = { Text(stringResource(R.string.add_item)) },
    text = {
    Column {
    colors = ExposedDropdownMenuDefaults.textFieldColors(containerColor = Color.White),
    value = viewModel.taskSummary.value,
    maxLines = 2,
    onValueChange = {
    label = { Text(stringResource(R.string.item_summary)) }
    val priorities = PriorityLevel.values()
    modifier = Modifier.padding(16.dp),
    expanded = viewModel.expanded.value,
    onExpandedChange = { },
    ) {
    readOnly = true,
    value =,
    onValueChange = {},
    label = { Text(stringResource(R.string.item_priority)) },
    trailingIcon = { ExposedDropdownMenuDefaults.TrailingIcon(expanded = viewModel.expanded.value) },
    colors = ExposedDropdownMenuDefaults.textFieldColors(),
    modifier = Modifier
    expanded = viewModel.expanded.value,
    onDismissRequest = { viewModel.close() }
    ) {
    priorities.forEach {
    text = { Text( },
    onClick = {
    // ... buttons

    Android Studio will identify several errors. We'll correct these in the next steps by adding the related functions.

  4. Next, we'll define the dropdown field label as a string resource. Open the res/values/strings.xml file, and add the following before the closing of the 'resource' element:

    <string name="item_priority">Item Priority</string>
  5. Now within the presentation/tasks folder, open the AddItemViewModel.kt file. Here we will add the business logic related to our new dropdown field.

    Add the PriorityLevel import below the package, then add the variables and functions to the AddItemViewModel class needed to handle the state changes within the dropdown:

    // ... imports
    // ... events
    class AddItemViewModel(
    private val repository: SyncRepository
    ) : ViewModel() {
    private val _addItemPopupVisible: MutableState<Boolean> = mutableStateOf(false)
    val addItemPopupVisible: State<Boolean>
    get() = _addItemPopupVisible
    private val _taskSummary: MutableState<String> = mutableStateOf("")
    val taskSummary: State<String>
    get() = _taskSummary
    private val _taskPriority: MutableState<PriorityLevel> = mutableStateOf(PriorityLevel.Low)
    val taskPriority: State<PriorityLevel>
    get() = _taskPriority
    private val _expanded: MutableState<Boolean> = mutableStateOf(false)
    val expanded: State<Boolean>
    get() = _expanded
    private val _addItemEvent: MutableSharedFlow<AddItemEvent> = MutableSharedFlow()
    val addItemEvent: Flow<AddItemEvent>
    get() = _addItemEvent
    fun openAddTaskDialog() {
    _addItemPopupVisible.value = true
    fun closeAddTaskDialog() {
    fun updateTaskSummary(taskSummary: String) {
    _taskSummary.value = taskSummary
    fun updateTaskPriority(taskPriority: PriorityLevel) {
    _taskPriority.value = taskPriority
    fun open() {
    _expanded.value = true
    fun close() {
    _expanded.value = false
    // addTask() and cleanUpAndClose() functions

    Now update the addTask() and cleanUpAndClose() functions to include the new taskPriority parameter, update the message with the priority information, and reset the priority field to low once the Add Item view is closed:

    fun addTask() {
    CoroutineScope(Dispatchers.IO).launch {
    runCatching {
    repository.addTask(taskSummary.value, taskPriority.value)
    }.onSuccess {
    withContext(Dispatchers.Main) {
    _addItemEvent.emit(AddItemEvent.Info("Task '$taskSummary' with priority '$taskPriority' added successfully."))
    }.onFailure {
    withContext(Dispatchers.Main) {
    _addItemEvent.emit(AddItemEvent.Error("There was an error while adding the task '$taskSummary'", it))
    private fun cleanUpAndClose() {
    _taskSummary.value = ""
    _taskPriority.value = PriorityLevel.Low
    _addItemPopupVisible.value = false
  6. Finally, from the data folder, open the SyncRepository.kt file to reflect the same changes in the addTask() function, which writes the Item to the realm.

    First, add the PriorityLevel import below the package, then update the addTask() functions to pass the taskPriority as a parameter and write the priority field to the realm as an integer (using the enum ordinal):

    // ... imports
    interface SyncRepository {
    // ... Sync functions
    suspend fun addTask(taskSummary: String, taskPriority: PriorityLevel)
    // ... Sync functions
    class RealmSyncRepository(
    onSyncError: (session: SyncSession, error: SyncException) -> Unit
    ) : SyncRepository {
    // ... variables and SyncConfiguration initializer
    // ... Sync functions
    override suspend fun addTask(taskSummary: String, taskPriority: PriorityLevel) {
    val task = Item().apply {
    owner_id =
    summary = taskSummary
    priority = taskPriority.ordinal
    realm.write {
    override suspend fun updateSubscriptions(subscriptionType: SubscriptionType) {
    realm.subscriptions.update {
    val query = when (subscriptionType) {
    SubscriptionType.MINE -> getQuery(realm, SubscriptionType.MINE)
    SubscriptionType.ALL -> getQuery(realm, SubscriptionType.ALL)
    // ... additional Sync functions
    class MockRepository : SyncRepository {
    override fun getTaskList(): Flow<ResultsChange<Item>> = flowOf()
    override suspend fun toggleIsComplete(task: Item) = Unit
    override suspend fun addTask(taskSummary: String, taskPriority: PriorityLevel) = Unit
    override suspend fun updateSubscriptions(subscriptionType: SubscriptionType) = Unit
    override suspend fun deleteTask(task: Item) = Unit
    override fun getActiveSubscriptionType(realm: Realm?): SubscriptionType = SubscriptionType.ALL
    override fun pauseSync() = Unit
    override fun resumeSync() = Unit
    override fun isTaskMine(task: Item): Boolean = task.owner_id == MOCK_OWNER_ID_MINE
    override fun close() = Unit
    // ... companion object

At this point, you can rerun the application. Log in using the account you created earlier in this tutorial. You will see the one Item you previously created. Add a new Item, and you will see that you can now set the priority. Choose High for the priority and save the Item.

Now switch back to the Atlas data page in your browser, and refresh the Item collection. You should now see the new Item with the priority field added and set to 1. The existing Item does not have a priority field.

Two items in a collection
click to enlarge


Why Didn't This Break Sync?

Adding a property to a Realm object is not a breaking change and therefore does not require a client reset. The template app has Development Mode enabled, so changes to the client Realm object are reflected in the server-side schema. For more information, see Development Mode and Update Your Data Model.


Within the app/java/ folder, open the SyncRepository.kt file, where we define the Flexible Sync subscription. The subscription defines which documents we sync with the user's device and account. Find the getQuery() function. You can see that we are currently subscribing to two subscriptions:

  • MINE: All documents where the ownerId property matches the authenticated user.

  • ALL: All documents from all users.

We want to update the MINE subscription to only sync Items that are marked as High or Severe priority.

As you may recall, the priority field is of type int, where the highest priority ("Severe") has a value of 0, and the lowest priority ("Low") has a value of 3. We can make direct comparisons between an integer and the priority property. To do so, edit the RQL statement to include documents where the priority is equal to or less than PriorityLevel.High (or 1), as shown here:

private fun getQuery(realm: Realm, subscriptionType: SubscriptionType): RealmQuery<Item> =
when (subscriptionType) {
SubscriptionType.MINE -> realm.query("owner_id == $0 AND priority <= ${PriorityLevel.High.ordinal}",
SubscriptionType.ALL -> realm.query()

We'll also force the subscription query to recalculate which documents to sync every time we open the app.

To do this, find the SyncConfiguration.Builder().initialSubscriptions() function that our application calls on start. First add the reRunOnOpen parameter set to true, then set updateExisting to true, which allows the existing query to be updated.

config = SyncConfiguration.Builder(currentUser, setOf(Item::class))
.initialSubscriptions(rerunOnOpen = true) { realm ->
// Subscribe to the active subscriptionType - first time defaults to MINE
val activeSubscriptionType = getActiveSubscriptionType(realm)
add(getQuery(realm, activeSubscriptionType),, updateExisting = true)
.errorHandler { session: SyncSession, error: SyncException ->
onSyncError.invoke(session, error)

Run the application again. Log in using the account you created earlier in this tutorial.

After an initial moment when Realm re-syncs the document collection, you will see the new Item of High priority that you created.


Changing Subscriptions with Developer Mode Enabled

In this tutorial, when you change the subscription and query on the priority field for the first time, the field is automatically added to the Device Sync Collection Queryable Fields. This occurs because the template app has Development Mode enabled by default. If Development Mode was not enabled, you would have to manually add the field as a queryable field to use it in a client-side Sync query.

For more information, refer to Queryable Fields.

If you want to test the functionality further, you can create Items of various priorities. You'll note that if you try to add an Item with a priority lower than High, you will get a Toast message indicating you do not have permission. And if you check your logs using Logcat, you will see a message indicating the item was "added successfully", followed by a sync error:

ERROR "Client attempted a write that is outside of permissions or query
filters; it has been reverted"

That's because, in this scenario, Realm creates the Item locally, syncs it with the backend, and then reverts the write because it doesn't meet the subscription rules.

You'll note, too, that the document you initially created is not synced, because it has a priority of null. If you want this Item to be synced, you can edit the document in the Atlas UI and add a value for the priority field.

Adding a property to an existing Realm object is a non-breaking change, and Development Mode ensures that the schema change is reflected server-side.


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