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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.

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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.
Persistent Storage is currently supported on:

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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    • delete the previously saved evaluation of a persisted user if the experiment is no longer active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

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Example usage

  • Javascript
  • React
  • Android On-Device Eval
  • JS On-Device Eval
  • JS (legacy)
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'

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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.
  • iOS
  • Android

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Swift:

// 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!")

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Objective C:

// 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);
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