> ## Documentation Index
> Fetch the complete documentation index at: https://statsig-4b2ff144-serverless-cloudflare.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# Client Persistent Assignment

Persistent assignment allows you to ensure that a user's variant stays consistent while an experiment is running, regardless of changes to allocation or targeting.

## Persistent Storage

Persistent storage takes an "adapter" approach, allowing you to plug in a storage solution of your choice to store assignments, which can then be referenced later to ensure a user stays in the same bucket. You can implement an adapter that uses Local Storage, or one that uses remote storage, to enable persistence across multiple devices.
The user persistent storage interface consists of just a `load`/`loadAsync`, `save`, `delete` API for read/write operations.

<Info>
  Persistent Storage is currently supported on:

  * The modern [`Javascript,`](/client/javascript-sdk) [`React`](/client/React), and [`React Native`](/client/ReactNative) SDKs, including on-device evaluation
  * [`Android, on-device evaluation`](/client/androidOnDeviceEvaluationSDK) only
  * [`iOS, on-device evaluation`](/client/swiftOnDeviceEvaluationSDK) only
  * See [below](#support-in-ios-and-android-sdks) for more information on Android and iOS Support
</Info>

### Persistent Storage Logic

* Providing a storage adapter on Statsig initialization will give the SDK access to read & write on your custom storage
* Providing user persisted values to `get_experiment` will inform the SDK to
  * **save** the evaluation of the current user **on first evaluation**
  * **load** the previously saved evaluation of a persisted user **on subsequent evaluations**
  * **delete** the previously saved evaluation of a persisted user if the experiment is no longer active
* Not providing user persisted values to `get_experiment` will **delete** a previously saved evaluation

### Example usage

<Tabs>
  <Tab title="Javascript">
    ```typescript theme={null}
    import { StatsigClient } from '@statsig/js-client';
    import { UserPersistentOverrideAdapter } from '@statsig/js-user-persisted-storage';

    // Custom storage implementation using localStorage
    class LocalStorageUserPersistedStorage {
      load(key) {
        return JSON.parse(localStorage.getItem(key) ?? '{}');
      }

      save(key, experiment, data) {
        const values = JSON.parse(localStorage.getItem(key) ?? '{}');
        values[experiment] = JSON.parse(data);
        localStorage.setItem(key, JSON.stringify(values));
      }

      delete(key, experiment) {
        const data = JSON.parse(localStorage.getItem(key) ?? '{}');
        delete data[experiment];
        localStorage.setItem(key, JSON.stringify(data));
      }
    }

    const storage = new LocalStorageUserPersistedStorage();
    const adapter = new UserPersistentOverrideAdapter(storage);
    const client = new StatsigClient('client-key', { overrideAdapter: adapter });

    await client.initializeAsync({ userID: "123" });

    const user = { userID: "123" };
    const userPersistedValues = adapter.loadUserPersistedValues(user, 'userID');

    const experiment = client.getExperiment('active_experiment', { userPersistedValues });
    console.log(experiment.getGroupName()); // 'Control'

    // Switch to different user - will maintain same experiment group due to persistence
    const newUser = { userID: "456" };
    const newExperiment = client.getExperiment('active_experiment', { userPersistedValues });
    console.log(newExperiment.getGroupName()); // Still 'Control'
    ```
  </Tab>

  <Tab title="React">
    The Syntax for react matches vanilla Javascript - for a full implementation example, you can refer to our [Persistent Storage Example](https://github.com/statsig-io/js-client-monorepo/tree/main/samples/next-js/src/app/persisted-user-storage-example) in Next.js.
  </Tab>

  <Tab title="Android On-Device Eval">
    #### Synchronous Persistent Evaluations

    The `UserPersistentStorageInterface` exposes two methods for synchronous persistent storage, which will be called by default when evaluating an experiment.

    ```
    interface UserPersistentStorageInterface {
        suspend fun load(key: String): PersistedValues
        fun save(key: String, experimentName: String, data: String)
        fun delete(key: String, experiment: String)
        ...
    }
    ```

    The `key` string is a combination of ID and ID Type: e.g. "123:userID" or "abc:stableID" which the SDK will construct and call `get` and `set` on by default

    You can use this interface to persist evaluations synchronously to local storage.  If you need an async interface, read on.

    #### Asynchronous Persistent Evaluations

    The `UserPersistentStorageInterface` exposes two methods for asynchronous persistent evaluations. Because the `getExperiment` call is synchronous, you must load the value first, and pass it in as `userPersistedValues`

    ```kotlin theme={null}
    interface UserPersistentStorageInterface {
        fun loadAsync(key: String, callback: IPersistentStorageCallback)
        fun save(key: String, experimentName: String, data: String)
        fun delete(key: String, experiment: String)
        ...
    }
    interface IPersistentStorageCallback {
        fun onLoaded(values: PersistedValues)
    }
    ```

    For your convenience, we've created a top level method to load the value for a given user and ID Type:

    ```kotlin theme={null}
    // Asynchronous load values
    val userPersistedValues = Statsig.client.loadUserPersistedValuesAsync(
      user: StatsigUser,
      idType: string, // userID, stableID, customIDxyz, etc
      callback: IPersistentStorageCallback
    );

    // Synchronous load values 
    val userPersistedvalues = Statsig.client.loadUserPersistedValues(
        user: StatsigUser,
      idType: string, // userID, stableID, customIDxyz, etc
    )
    ```

    Putting it all together, assuming you have implemented the `UserPersistentStorageInterface` and set it on `StatsigOptions`, your call site will look like this:

    ```kotlin theme={null}
    // Asynchronous 
    val callback = object: IPersistentStorageCallback {
        @override
        fun onLoaded(values: PersistedValues) {
            Statsig.getExperiment(user, "sample_experiment", GetExperimentOptions(userPersistedValues = values))
        }
    }
    val userValues = Statsig.client.loadUserPersistedValuesAsync(user, "userID", callback)

    // Synchronous
    val user = StatsigUser(userID = "user123")
    val userValues = Statsig.client.loadUserPersistedValues(user, 'userID');
    const experiment = statsig.getExperiment({userID: "123"}, 'the_allocated_experiment', { userPersistedValues: userValues });
    ```

    If you are using java, you can only override loadAsync function and ignore load function as empty.
  </Tab>

  <Tab title="JS On-Device Eval">
    ```typescript theme={null}
    const storage = new CustomStorageAdapter(); // Need to implement
    const adapter = new UserPersistentOverrideAdapter(storage);
    const client = new StatsigOnDeviceEvalClient(
      'client-key', 
      { overrideAdapter: adapter }
    );
    await client.initializeAsync();

    const userInControl = { userID: "123" }
    const userInUnknown = { userID: "unknown" }
    const userPersistedValues = adapter.loadUserPersistedValues(user, 'userID');

    let experiment = client.getExperiment('active_experiment', user, { userPersistedValues });
    console.log(experiment.getGroupName()) // 'Control'

    experiment = client.getExperiment('active_experiment', userInUnknown, { userPersistedValues });
    console.log(experiment.getGroupName()) // 'Control'
    ```

    For a full implementation example, you can refer to our [Persistent Storage On-Device Eval Example](https://github.com/statsig-io/js-client-monorepo/tree/main/samples/next-js/src/app/persisted-user-storage-example-on-device) in Next.js.
  </Tab>

  <Tab title="JS (legacy)">
    ```typescript theme={null}
    await statsig.initialize(
      'client-key',
      { userPersistentStorage: new CustomStorageAdapter() } // Need to implement
    );

    const userInControl = { userID: "123" }
    const userInUnknown = { userID: "unknown" }
    const userPersistedValues = await statsig.loadUserPersistedValuesAsync(userInControl, 'userID');

    let experiment = statsig.getExperiment(userInControl, 'active_experiment', { userPersistedValues });
    console.log(experiment.getGroupName()) // 'Control'

    experiment = statsig.getExperiment(userInUnknown, 'active_experiment', { userPersistedValues });
    console.log(experiment.getGroupName()) // 'Control'
    ```
  </Tab>
</Tabs>

## Support in iOS and Android SDKs

Android and iOS SDKs offer a simplified version of Persistent Storage called `keepDeviceValues` that relies on on-device storage. While this is less flexible its simple to leverage - with a simple boolean flag. When enabled, the SDK (under the hood in the getExperiment/getLayer call) will check for previous stored value, and use those instead, even if allocation or targeting has changed. When the experiment ends - the SDK will stop persisting values.

<Tabs>
  <Tab title="iOS">
    #### Swift:

    ```swift theme={null}
    // With an Experiment:
    let titleExperiment = Statsig.getExperiment("new_user_promo_title", true) // <-- "true" flag sets keep device values
    // Use the experiment like normal:
    let promoTitle = titleExperiment.getValue(forKey: "title", defaultValue: "Welcome to Statsig!")

    // Or a Layer:
    let layer = Statsig.getLayer("user_promo_experiments", true) // <-- "true" flag sets keep device values
    // Use the layer like normal:
    let promoTitle = layer.getValue(forKey: "title", defaultValue: "Welcome to Statsig!")
    ```

    #### Objective C:

    ```objc theme={null}
    // With an Experiment:
    DynamicConfig *expConfig = [Statsig getExperimentForName:@"new_user_promo_title" true]; // <-- "true" flag sets keep device values
    // Use the experiment like normal:
    NSString *promoTitle = [expConfig getStringForKey:@"title" defaultValue:@"Welcome to Statsig! Use discount code WELCOME10OFF for 10% off your first purchase!"];
    double discount = [expConfig getDoubleForKey:@"discount" defaultValue:0.1];

    double price = msrp * (1 - discount);
    ```
  </Tab>

  <Tab title="Android">
    #### Java:

    ```Java theme={null}
    // With an Experiment:
    DynamicConfig titleExperiment = Statsig.getExperiment("new_user_promo_title", true); // <-- "true" flag sets keep device values
    // Use the experiment like normal:
    String promoTitle = titleExperiment.getString("title", "Welcome to Statsig!");

    // Or a Layer:
    Layer layer = Statsig.getLayer("user_promo_experiments", true) // <-- "true" flag sets keep device values
    // Use the layer like normal:
    String promoTitle = layer.getString("title", "Welcome to Statsig!");
    ```

    #### Kotlin:

    ```kotlin theme={null}
    // With an Experiment:
    val titleExperiment = Statsig.getExperiment("new_user_promo_title", true) // <-- "true" flag sets keep device values
    // Use the experiment like normal:
    val promoTitle = titleExperiment.getString("title", "Welcome to Statsig!")

    // Or a Layer:
    val layer = Statsig.getLayer("user_promo_experiments", true) // <-- "true" flag sets keep device values
    // Use the layer like normal:
    val promoTitle = layer.getString("title", "Welcome to Statsig!")

    ```
  </Tab>
</Tabs>