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The StatsigEvaluationsDataAdapter is the default EvaluationsDataAdapter used by a StatsigClient instance. It handles fetching and caching values from Statsig’s servers. If required, you can create your own custom EvaluationsDataAdapter and provide it to the Statsig client via StatsigOptions.dataAdapter.

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Overview

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Synchronous Behavior (Cache)

When calling StatsigClient.initializeSync, the StatsigEvaluationsDataAdapter will load values from Cache and provide them to the client. The client will also call refresh in the background via StatsigEvaluationsDataAdapter.getDataAsync, leading to values lagging until the next time initializeSync or updateUserSync is called. If you do not want this background refresh to happen, you can pass in disableBackgroundCacheRefresh in the options when calling initializeSync. In practice (Unless an additional update call is made), this means that for the very first session of a given user, there will be no values, and not until the next session will the cache have values.

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Asynchronous Behavior (Network)

When calling StatsigClient.initializeAsync, the StatsigEvaluationsDataAdapter will load values from Cache and provide them to the client. The client will then block on a call to StatsigEvaluationsDataAdapter.getDataAsync, allowing the ability to await the latest values from Statsig. If you want the latest values, but do not want to await the asynchronous call, you may call initializeAsync and simply .catch the promise. Note that this could lead to values changing mid-session as newer values arrive from the network.

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Advanced Usage

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Getting the Data Adapter

You can access the data adapter from your Statsig client instance: 
const client = new StatsigClient('client-key', { userID: 'user-123' });
await client.initializeAsync();

const dataAdapter = client.dataAdapter;
```python

[View full example on GitHub](https://github.com/statsig-io/js-client-monorepo/blob/main/samples/react/src/samples/precomputed-client/sample-precomp-get-data-adapter.tsx)

### Bootstrapping

Bootstrapping allows you to provide data for a given `StatsigUser`. This can be useful if you are are running a
Statsig Server SDK on your backend and do not wish to make unnecessary network calls (See [statsig-node](/server/nodejsServerSDK#bootstrap)'s getClientInitializeResponse).

This approach can also be useful if you are building a mobile app and want to bundle values with your application. Then you can load the values from a local file at startup and provide them to the data adapter.

```typescript
import { StatsigClient } from '@statsig/js-client';

const client = new StatsigClient('client-key', { userID: 'user-123' });

// Bootstrap with data from server
const bootstrapData = /* data from server or local file */;
await client.dataAdapter.setData(bootstrapData);

await client.initializeSync();
```python

[View full example on GitHub](https://github.com/statsig-io/js-client-monorepo/blob/main/samples/react/src/samples/precomputed-client/sample-precomp-bootstrap.tsx)

If you are using a server SDK to bootstrap your js/react app, you will need to make some updates to how your server SDK generates values.  One of the optimizations we made with this new js-client SDK was to remove the `sha256` library for hashing gate/experiment names.  Instead, we use a `djb2` hash.  By default, all server SDKs generate `sha256` hashes of gate/experiment names in the `getClientInitializeResponse` method.  You will need to set the hash algorithm parameter to that method call to `"djb2"` instead in order to bootstrap this new client SDK.  One of the benefits to this hashing algorithm is it will make the entire payload smaller as well, so its a net win on package size, speed, and payload size for the SDK.

For example, if you are bootstrapping from a nodejs app, you will need to do:

```js
statsig.getClientInitializeResponse(
  user,
  '[client-key]',
  {
    hash: 'djb2',
  },
);
```text

### Prefetching

You can manually trigger a prefetch for a user before calling `updateUser`:

```typescript
const client = new StatsigClient('client-key', { userID: 'user-123' });
await client.initializeAsync();

// Prefetch data for a different user
const newUser = { userID: 'user-456' };
await client.dataAdapter.prefetchData(newUser);

// Update to the new user (will use prefetched data)
await client.updateUserAsync(newUser);
```python

[View full example on GitHub](https://github.com/statsig-io/js-client-monorepo/blob/main/samples/react/src/samples/precomputed-client/sample-precomp-prefetch.tsx)

### Custom Implementation

If you would like to customize when and how data is fetched, as well as where it is stored, you can create your own class the conforms to the `EvaluationsDataAdapter` type.

The `EvaluationsDataAdapter` type outlines the following functions:

`attach` - Called when the `EvaluationsDataAdapter` is passed into a `StatsigClient` via `StatsigOptions`. This allows the `EvaluationsDataAdapter` to use
  the same SDK Key and `StatsigOptions` as the `StatsigClient` instance.

```typescript
attach: (
  sdkKey: string, 
  options: StatsigOptionsCommon | null
) => void
```sql

---

`getDataSync` - Synchronously get evaluation data for the given user. Called during initializeSync and/or updateUserSync. It is also called during
  async update operations before StatsigDataAdapter.getDataAsync is called.

```typescript
getDataSync: (user: StatsigUser) => DataAdapterResult | null;
```text

---

`getDataAsync` - Asynchronously get evaluation data for the given user. Called during initializeAsync and/or updateUserAsync.
```typescript
getDataAsync: (
  current: DataAdapterResult | null, 
  user: StatsigUser
) => Promise<DataAdapterResult | null>;
```text

---

`prefetchData` - Manually trigger a fetch for new evaluations data for the given user.
```typescript
prefetchData: (user: StatsigUser) => Promise<void>;
```python

---

`setData` - Manually set the evaluations data from a JSON string received from a Statsig Server SDK's `getClientInitializeResponse` method.

```typescript
setData: (data: string) => Promise<void>;
```text

<Note>
You can use `setDataLegacy` if your Server SDK is outdated.
</Note>

<Accordion title="setData is supported by the following Statsig Server SDKs">
- node-js-server-sdk@5.20.0
- java-server-sdk@1.18.0
- ruby-sdk@1.34.0
- dotnet-sdk@1.25.0
- php-sdk@3.2.0
</Accordion>


---

`setDataLegacy` - Manually set evaluations data for the given user.

This method is **deprecated** and is provided only to support older versions of Statsig server SDKs.

```typescript
setDataLegacy: (data: string, user: StatsigUser) => void;