Unleash OpenFeature Provider for Node.js

An OpenFeature provider backed by the official Unleash Node.js SDK (unleash-client), for server-side Node.js applications.

Adheres to OpenFeature specification v0.8.0 via @openfeature/server-sdk >=1.14.0 (the first release of the server SDK implementing spec v0.8.0). All required provider behavior is implemented: typed flag resolution, evaluation context handling, lifecycle (initialization/shutdown), and provider events. Of the spec's optional capabilities, provider hooks and the tracking API (spec section 6) are not implemented — see Out of scope.

The provider constructs and owns the Unleash client: it is created and started when the provider is initialized, and destroyed (with a final metrics flush) when OpenFeature shuts down. Flags are evaluated locally by the Unleash SDK against the translated evaluation context, so Unleash usage metrics keep working as usual.

Installation

npm install @unleash/openfeature-node-provider @openfeature/server-sdk unleash-client

If you cloned without submodules, initialize the verifier harness:

git submodule update --init --recursive

The contract tests use the verifier submodule. To refresh it:

git submodule update --remote --merge verifier

Usage

import { OpenFeature } from '@openfeature/server-sdk';
import { UnleashProvider } from '@unleash/openfeature-node-provider';

const provider = new UnleashProvider({
  // any UnleashConfig accepted by the Unleash Node SDK
  url: 'https://app.unleash-hosted.com/demo/api/',
  appName: 'my-app',
  customHeaders: { Authorization: '<your-api-token>' },
});

// Resolves once the Unleash client is ready and synchronized
await OpenFeature.setProviderAndWait(provider);

const client = OpenFeature.getClient();

const enabled = await client.getBooleanValue('my-flag', false, { targetingKey: 'user-123' });
const variantText = await client.getStringValue('my-copy-experiment', 'default text', {
  targetingKey: 'user-123',
  sessionId: 'session-1',
  region: 'EMEA', // custom keys become Unleash properties
});

The underlying Unleash client is available as an escape hatch via provider.unleashClient.

Concept mapping

Flag evaluation

OpenFeature call	Unleash mechanism
getBooleanValue / getBooleanDetails	isEnabled()
getStringValue / getStringDetails	variant payload of type string or csv
getNumberValue / getNumberDetails	variant payload of type number
getObjectValue / getObjectDetails	variant payload of type json (parsed)

If the variant payload type does not match the requested type, the evaluation returns the default value with errorCode: TYPE_MISMATCH. A flag that does not exist in Unleash returns the default value with errorCode: FLAG_NOT_FOUND (note that this differs from the raw Unleash SDK, which treats unknown flags as disabled). Disabled flags resolve variant evaluations to the default value with reason DISABLED; enabled flags without an assigned variant (or without a payload) resolve to the default value with reason DEFAULT.

The assigned variant name is reported in variant, and flagMetadata carries featureEnabled and payloadType.

Evaluation context

OpenFeature context key	Unleash context field
targetingKey	userId (takes precedence over an explicit userId key)
userId, sessionId, remoteAddress, environment, appName	same field
currentTime (Date or ISO string)	currentTime
any other key	properties.<key>

Strings and numbers pass into properties unchanged; booleans and dates are stringified. Nested objects and arrays are dropped (with a debug log), since Unleash constraints cannot evaluate them.

Stickiness

Unleash resolves default stickiness for gradual rollouts and variant assignment internally, falling back through userId → sessionId → random. The provider does not replicate or alter this chain — it only fills in the fields. targetingKey fills userId; if you want session-based stickiness, set sessionId in the evaluation context. A missing targetingKey is never an error: evaluation falls through Unleash's normal chain and ends at random stickiness. Custom stickiness on a custom context field works via properties.

Events and lifecycle

Unleash client state	OpenFeature provider event
ready + synchronized	PROVIDER_READY
configuration changed	PROVIDER_CONFIGURATION_CHANGED
fetch error while cached flags are served	PROVIDER_STALE
error before any flag data is available	PROVIDER_ERROR
recovery after an error	PROVIDER_READY

initialize() rejects on the first Unleash error instead of waiting forever, so setProviderAndWait() cannot hang on a misconfigured connection. The Unleash client keeps retrying in the background and the provider recovers (emitting PROVIDER_READY) once a fetch succeeds.

Reasons

Unleash does not expose which strategy matched an evaluation, so reasons are best-effort: TARGETING_MATCH for enabled flags, DISABLED for disabled ones, SPLIT for assigned variants, DEFAULT when the default value is used, and ERROR alongside an error code.

Out of scope

The OpenFeature tracking API is not implemented. Unleash impression data and usage metrics are unaffected — they are handled by the underlying SDK.

Development

npm install
npm test        # vitest: unit + offline integration tests (bootstrap data, no server)
npm run build   # tsup: ESM + CJS + type declarations

License

Apache-2.0