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Initializing SDKs

The first step in using a Statsig SDK is calling initialize(), which retrieves the values you need to evaluate experiments & send events. Before initialization, Statsig SDKs won't have latest values in-memory, and therefore may return stale values or none at all.

Unlike Server SDK initialization, which happens at Server startup, Client SDK initialization happens when a screen is rendered - meaning initialization can have more impact on user experience. Statsig offers several client initialization methods to tune performance to your needs.

General Initialization Flow​

initialize will take an SDK key and StatsigUser object. The SDK will then:

  1. Check local storage for cached values. The SDK caches the previous evaluations locally so they are available on the next session if there isn't a successful network call
  2. Create a STATSIG_STABLE_ID - an ID that stays consistent per-device, which can often be helpful for logged-out experiments.
  3. Set the SDK as initialized so checks won't throw errors - they will either return cached values or defaults.
  4. Issue a network request to Statsig to get the latest values for all gates/experiments/configs/layers/autotunes for the given user. If the project definition does not change from the most recent cached values, this request may succeed without returning new data.
  5. Resolve the asynchronous initialize call. If the request to the server failed, the SDK will have the cached values or return defaults for this session.

Depending on when you check a gate or experiment after initializing, its possible that you may not have retrieved fresh values yet. Awaiting initialization solves this, with some performance downsides (discussed below).

Client Initialization Strategies​

Below are the various strategies summarized at a high level, ordered from most common to least common:

  • Asynchronous Initialization (Awaited): Wait for the initialization network call to finish before rendering content.
  • Bootstrap Initialization: Generate the assignment values on your own server, and pass them down with other request, resulting in zero-latency rendering. Best of both worlds for latency and availability of fresh assignments, but requires additional engineering effort.
  • Asynchronous Initialization (Not Awaited): Do not await the return of the initialization network call. This ensures immediate rendering, but in a state that reflects stale assignments or no assignments available.
    • After initialization, the client will then fetch fresh assignments over the network from Statsig. Subsequent calls to check assignments may result in different assignments than the initial state and therefore render a different treatment (this is referred to as "flicker"). This mimics the old behavior of StatsigProvider.waitForInitialization=false.
  • Synchronous Initialization: Renders immediately, but with stale or no assignments available. First-visit users will never be assigned to gates and experiments. These users will only see updated assignments after they do a hard-refresh of the website. Effectively, all assignment information is 1 page-load out-of-date..
MethodSpeed-to-render?Render consistency?Latest content?Engineering Complexity?
ExplanationWhen I visit the webpage, how fast does the content appear?Does the content ever change/flicker?Does the user ever see an out-of-date config value?How easy is this to implement?
Await InitializeAsync()❌ Slow✅ Good✅ Yes✅ Easy
InitializeAsync()✅ Fast❌ Poor✅ Yes✅ Easy
InitializeSync()✅ Fast✅ Good❌ No✅ Easy
BootstrapInit✅ Fast✅ Good✅ Yes❌ Extra Effort

1. Asynchronous Initialization - Awaited​

Ensures latest assignments but requires a loading state

When calling StatsigClient.initializeAsync, the client loads values from the cache and fetches the latest values from the network. This approach waits for the latest values before rendering, which means it is not immediate but ensures the values are up-to-date.

Example:

...

2. Bootstrap Initialization​

Ensures both latest assignments with no rendering latency

Bootstrapping allows you to initialize the client with a JSON string. This approach ensures that values are immediately available without the client making any network requests. Note that you will be responsible for keeping these values up to date. With this approach, your server will be responsible for serving the configuration payload to your client app on page load (for web implementations) or during app launch (for mobile implementations).

This architecture requires running a server SDK that supports the getClientInitializeResponse method. Your server SDK will maintain a fresh configuration in memory and when a request hits your route handler, you should call getClientInitializeResponse(<user>), passing in a StatsigUser Object to generate the configuration object that gets passed to the client SDK for synchronous initialization.

Implementation Notes:​

  • Bootstrapping requires a user object both on the client and server side, and it's important that they're kept in sync. This may seem redundant, but is important for other SDK operations, and provides a layer of safeguarding should you accidentally attempt to perform checks on a user object different than the one you created values for (if you do, the SDK will throw warnings). Client SDKs also generate their own StableID for the user when one is not provided, which can sometimes result in out-of-sync user objects. See keeping StableID consistent across client and server.
  • The initializeValues option should be an Object - except in our js SDK, where its expected to be a string. Calling .stringify() on the object should work.
  • If you're using Statsig's new javascript SDK (@statsig/js-client), ensure that you set the hashing algorithm as DJB2 when calling getClientInitializeResponse(), as the new SDK uses DJB2 by default.

Example:

...

3. Asynchronous Initialization - Not Awaited​

If you want to fetch the latest values without awaiting the asynchronous call, you can call initializeAsync and catch the promise. This approach provides immediate rendering with cached values initially, which will update to the latest values mid-session.

caution

Be aware that the values may switch when checked a second time after the latest values have been loaded.

Example:

...

4. Synchronous Initialization​

Ensures immediate rendering but uses cached assignments (when available)

When calling StatsigClient.initializeSync, the client uses cached values if they are available. The client fetches new values in the background and updates the cache. This approach provides immediate rendering, but the values might be stale or absent during the first session.

Example:

...

These strategies help you balance the need for the latest gate / experiment assignment information with the need for immediate application rendering based on your specific requirements.

/Initialize Response Schema​

Provided for reference if you're implementing Bootstrapping - the job of your server is to provide the values that Statsig's servers other wise would when they call /initialize. Statsig's getClientInitializeResponse function provides this payload.

/** Specs for Dynamic Configs */
dynamic_configs: {
  [key: string]: {
    name: string;
    rule_id: string | null;
    value: { [key: string]: unknown };
    group?: string;
    secondary_exposures?: {
      gate: string,
      gateValue: string,
      ruleID: string,
    }[];
    undelegated_secondary_exposures?: {
      gate: string,
      gateValue: string,
      ruleID: string,
    }[];
    is_device_based?: boolean;
    is_user_in_experiment?: boolean;
    is_experiment_active?: boolean;
    explicit_parameters?: string[];
    is_in_layer?: boolean;
    allocated_experiment_name?: string;
  };
};
/** Specs for Feature Gates */
feature_gates: {
  [key: string]: {
    name: string;
    value: boolean;
    rule_id: string | null;
    secondary_exposures?: {
      gate: string,
      gateValue: string,
      ruleID: string,
    }[];
  };
};
/** Specs for Layer Configs */
layer_configs: {
  [key: string]: {
    name: string;
    rule_id: string | null;
    value: { [key: string]: unknown };
    group?: string;
    secondary_exposures?: {
      gate: string,
      gateValue: string,
      ruleID: string,
    }[];
    undelegated_secondary_exposures?: {
      gate: string,
      gateValue: string,
      ruleID: string,
    }[];
    is_device_based?: boolean;
    is_user_in_experiment?: boolean;
    is_experiment_active?: boolean;
    explicit_parameters?: string[];
    is_in_layer?: boolean;
    allocated_experiment_name?: string;
  };
};
/** Whether the response contains updates from the sinceTime */
has_updates: boolean;
/** Name of the service that generated the response */
generator: string;
/** Timestamp of response */
time: number;
/** Timestamp of company's last config update time */
company_lcut: number;
/** The user keys evaluated */
evaluated_keys: {
  userID?: string;
  stableID?: string;
  customIDs?: Record<string, string>;
};
/** The hashing algorithm used */
hash_used: string;