npm package: @openfeature/multi-provider
Description: > **⚠️ DEPRECATED**: This package is deprecated. The Multi-Provider is now included in `@openfeature/server-sdk`. Please migrate to use the SDK directly. See [Multi-Provider in @openfeature/server-sdk](https://github.com/open-feature/js-sdk/blob/main/pack
Latest version: 0.1.3
Deprecated: This package is deprecated. The Multi-Provider is now included in @openfeature/server-sdk. Please migrate to use the SDK directly.

# OpenFeature Multi-Provider

> **⚠️ DEPRECATED**: This package is deprecated. The Multi-Provider is now included in `@openfeature/server-sdk`. Please migrate to use the SDK directly. See [Multi-Provider in @openfeature/server-sdk](https://github.com/open-feature/js-sdk/blob/main/packages/server/README.md#multi-provider).

The Multi-Provider allows you to use multiple underlying providers as sources of flag data for the OpenFeature server SDK.
When a flag is being evaluated, the Multi-Provider will consult each underlying provider it is managing in order to determine
the final result. Different evaluation strategies can be defined to control which providers get evaluated and which result is used.

The Multi-Provider is a powerful tool for performing migrations between flag providers, or combining multiple providers into a single
feature flagging interface. For example:

- *Migration*: When migrating between two providers, you can run both in parallel under a unified flagging interface. As flags are added to the
new provider, the Multi-Provider will automatically find and return them, falling back to the old provider if the new provider does not have
- *Multiple Data Sources*: The Multi-Provider allows you to seamlessly combine many sources of flagging data, such as environment variables,
local files, database values and SaaS hosted feature management systems.

## Installation

```bash
$ npm install @openfeature/multi-provider
```

> [!TIP]
> This provider is designed to be used with the [Node.js SDK](https://openfeature.dev/docs/reference/technologies/server/javascript/).

## Usage

The Multi-Provider is initialized with an array of providers it should evaluate:

```typescript
import { MultiProvider } from '@openfeature/multi-provider'
import { OpenFeature } from '@openfeature/server-sdk'

const multiProvider = new MultiProvider([
  { provider: new ProviderA() },
  { provider: new ProviderB() }
])

await OpenFeature.setProviderAndWait(multiProvider)

const client = OpenFeature.getClient()

console.log("Evaluating flag")
console.log(await client.getBooleanDetails("my-flag", false));
```

By default, the Multi-Provider will evaluate all underlying providers in order and return the first successful result. If a provider indicates
it does not have a flag (FLAG_NOT_FOUND error code), then it will be skipped and the next provider will be evaluated. If any provider throws
or returns an error result, the operation will fail and the error will be thrown. If no provider returns a successful result, the operation
will fail with a FLAG_NOT_FOUND error code.

To change this behaviour, a different "strategy" can be provided:

```typescript
import { MultiProvider, FirstSuccessfulStrategy } from '@openfeature/multi-provider'

const multiProvider = new MultiProvider(
    [
      { provider: new ProviderA() },
      { provider: new ProviderB() }
    ], 
    new FirstSuccessfulStrategy()
)
```

The Multi-Provider comes with three strategies out of the box:

- `FirstMatchStrategy` (default): Evaluates all providers in order and returns the first successful result. Providers that indicate FLAG_NOT_FOUND error will be skipped and the next provider will be evaluated. Any other error will cause the operation to fail and the set of errors to be thrown.
- `FirstSuccessfulStrategy`: Evaluates all providers in order and returns the first successful result. Any error will cause that provider to be skipped.
If no successful result is returned, the set of errors will be thrown.
- `ComparisonStrategy`: Evaluates all providers in parallel. If every provider returns a successful result with the same value, then that result is returned.
Otherwise, the result returned by the configured "fallback provider" will be used. When values do not agree, an optional callback will be executed to notify
you of the mismatch. This can be useful when migrating between providers that are expected to contain identical configuration. You can easily spot mismatches
in configuration without affecting flag behaviour.

This strategy accepts several arguments during initialization:

```typescript
import { MultiProvider, ComparisonStrategy } from '@openfeature/multi-provider'

const providerA = new ProviderA()
const multiProvider = new MultiProvider(
  [
    { provider: providerA },
    { provider: new ProviderB() }
  ],
  new ComparisonStrategy(providerA, (details) => {
      console.log("Mismatch detected", details)
  })
)
```

The first argument is the "fallback provider" whose value to use in the event that providers do not agree. It should be the same object reference as one of the providers in the list. The second argument is a callback function that will be executed when a mismatch is detected. The callback will be passed an object containing the details of each provider's resolution, including the flag key, the value returned, and any errors that were thrown.

## Tracking Support

The Multi-Provider supports tracking events across multiple providers. When you call the `track` method, it will by default send the tracking event to all underlying providers that implement the `track` method.

```typescript
import { OpenFeature } from '@openfeature/server-sdk'
import { MultiProvider } from '@openfeature/multi-provider'

const multiProvider = new MultiProvider([
  { provider: new ProviderA() },
  { provider: new ProviderB() }
])

await OpenFeature.setProviderAndWait(multiProvider)
const client = OpenFeature.getClient()

// Tracked events will be sent to all providers by default
client.track('purchase', { targetingKey: 'user123' }, { value: 99.99, currency: 'USD' })
```

### Tracking Behavior

- **Default**: All providers receive tracking calls by default
- **Error Handling**: If one provider fails to track, others continue normally and errors are logged
- **Provider Status**: Providers in `NOT_READY` or `FATAL` status are automatically skipped
- **Optional Method**: Providers without a `track` method are gracefully skipped

### Customizing Tracking with Strategies

You can customize which providers receive tracking calls by overriding the `shouldTrackWithThisProvider` method in your custom strategy:

```typescript
import { BaseEvaluationStrategy, StrategyPerProviderContext } from '@openfeature/multi-provider'

class CustomTrackingStrategy extends BaseEvaluationStrategy {
  shouldTrackWithThisProvider(
    strategyContext: StrategyPerProviderContext,
    context: EvaluationContext,
    trackingEventName: string,
    trackingEventDetails: TrackingEventDetails,
  ): boolean {
    // Only track with the primary provider
    if (strategyContext.providerName === 'primary-provider') {
      return true;
    }
    
    // Skip tracking for analytics events on backup providers
    if (trackingEventName.startsWith('analytics.')) {
      return false;
    }
    
    return super.shouldTrackWithThisProvider(strategyContext, context, trackingEventName, trackingEventDetails);
  }
}
```

## Custom Strategies

It is also possible to implement your own strategy if the above options do not fit your use case. To do so, create a class which implements the "BaseEvaluationStrategy":

```typescript
export abstract class BaseEvaluationStrategy {
    public runMode: 'parallel' | 'sequential' = 'sequential';

    abstract shouldEvaluateThisProvider(strategyContext: StrategyPerProviderContext, evalContext: EvaluationContext): boolean;

    abstract shouldEvaluateNextProvider<T extends FlagValue>(
        strategyContext: StrategyPerProviderContext,
        context: EvaluationContext,
        result: ProviderResolutionResult<T>,
    ): boolean;

    abstract shouldTrackWithThisProvider(
        strategyContext: StrategyPerProviderContext,
        context: EvaluationContext,
        trackingEventName: string,
        trackingEventDetails: TrackingEventDetails,
    ): boolean;

    abstract determineFinalResult<T extends FlagValue>(
        strategyContext: StrategyEvaluationContext,
        context: EvaluationContext,
        resolutions: ProviderResolutionResult<T>[],
    ): FinalResult<T>;
}
```

The `runMode` property determines whether the list of providers will be evaluated sequentially or in parallel.

The `shouldEvaluateThisProvider` method is called just before a provider is evaluated by the Multi-Provider. If the function returns `false`, then
the provider will be skipped instead of being evaluated. The function is called with details about the evaluation including the flag key and type.
Check the type definitions for the full list.

The `shouldEvaluateNextProvider` function is called after a provider is evaluated. If it returns `true`, the next provider in the sequence will be called,
otherwise no more providers will be evaluated. It is called with the same data as `shouldEvaluateThisProvider` as well as the details about the evaluation result. This function is not called when the `runMode` is `parallel`.

The `shouldTrackWithThisProvider` method is called before sending a tracking event to each provider. Return `false` to skip tracking with that provider. By default, it only tracks with providers that are in a ready state (not `NOT_READY` or `FATAL`). Override this method to implement custom tracking logic based on the tracking event name, details, or provider characteristics.

The `determineFinalResult` function is called after all providers have been called, or the `shouldEvaluateNextProvider` function returned false. It is called
with a list of results from all the individual providers' evaluations. It returns the final decision for evaluation result, or throws an error if needed.

## Building

Run `nx package providers-multi-provider` to build the library.

## Running unit tests

Run `nx test providers-multi-provider` to execute the unit tests via [Jest](https://jestjs.io).