Experiment Node.js SDK

Official documentation for Amplitude Experiment's server-side Node.js SDK implementation.

SDK Resources

GitHub · Releases · API Reference

This documentation has separate sections for remote and local evaluation:

Remote evaluation
Local evaluation

Remote evaluation¶

Implements fetching variants for a user via remote evaluation.

Install¶

Node version compatibility

The Node Server SDK works with Node 10+.

Install the Node.js Server SDK with npm or yarn.

npmyarn

npm install --save @amplitude/experiment-node-server

yarn add @amplitude/experiment-node-server

Quick Start

Initialize the experiment client
Fetch variants for the user
Access a flag's variant

// (1) Initialize the experiment client
const experiment = Experiment.initializeRemote('<DEPLOYMENT_KEY>', config: {
    fetchTimeoutMillis: 500,
    fetchRetries: 1,
    fetchRetryBackoffMinMillis: 0,
    fetchRetryTimeoutMillis: 500,
});

// (2) Fetch variants for a user
const user = {
    user_id: 'user@company.com',
    device_id: 'abcdefg',
    user_properties: {
        'premium': true,
    },
};
const variants = await experiment.fetch(user);

// (3) Access a flag's variant
const variant = variants['YOUR-FLAG-KEY'];
if (variant?.value === 'on') {
    // Flag is on
} else {
    // Flag is off
}

Initialize remote¶

The SDK client should be initialized in your server on startup. The deployment key argument passed into the apiKey parameter must live within the same project that you are sending analytics events to.

initializeRemote(apiKey: string, config?: RemoteEvaluationConfig): RemoteEvaluationClient

Parameter	Requirement	Description
`apiKey`	required	The deployment key which authorizes fetch requests and determines which flags should be evaluated for the user.
`config`	optional	The client configuration used to customize SDK client behavior.

Timeout & Retry Configuration

The default timeout and retry configuration options are too high for most server environments. Configure the timeout and retry options to best fit your performance requirements. If remote evaluation performance is too slow, consider using local evaluation.

const experiment = Experiment.initializeRemote('<DEPLOYMENT_KEY>', config: {
    fetchTimeoutMillis: 500,
    fetchRetries: 1,
    fetchRetryBackoffMinMillis: 0,
    fetchRetryTimeoutMillis: 500,
});

Configuration¶

The SDK client can be configured on initialization.

Configuration Options

Name	Description	Default Value
`debug`	Enable additional debug logging.	`false`
`serverUrl`	The host to fetch variants from.	`https://api.lab.amplitude.com`
`fetchTimeoutMillis`	The timeout for fetching variants in milliseconds. This timeout only applies to the initial request, not subsequent retries	`10000`
`fetchRetries`	The number of retries to attempt if a request to fetch variants fails.	`8`
`fetchRetryBackoffMinMillis`	The minimum (initial) backoff after a request to fetch variants fails. This delay is scaled by the `fetchRetryBackoffScalar`	`500`
`fetchRetryBackoffMaxMillis`	The maximum backoff between retries. If the scaled backoff becomes greater than the max, the max is used for all subsequent requests	`10000`
`fetchRetryBackoffScalar`	Scales the minimum backoff exponentially.	`1.5`
`fetchRetryTimeoutMillis`	The request timeout for retrying variant fetches.	`10000`

Fetch¶

Fetches variants for a user and returns the results. This function remote evaluates the user for flags associated with the deployment used to initialize the SDK client.

fetch(user: ExperimentUser): Promise<Variants>

Parameter	Requirement	Description
`user`	required	The user to remote fetch variants for.

const user = {
    user_id: 'user@company.com',
    device_id: 'abcdefg',
    user_properties: {
        'premium': true,
    },
};
const variants = await experiment.fetch(user);

After fetching variants for a user, you may to access the variant for a specific flag.

const variant = variants['YOUR-FLAG-KEY'];
if (variant?.value === 'on') {
    // Flag is on
} else {
    // Flag is off
}

Local evaluation¶

Implements evaluating variants for a user via local evaluation. If you plan on using local evaluation, you should understand the tradeoffs.

Local Evaluation Mode

The local evaluation client can only evaluation flags which are set to local evaluation mode.

Install¶

Install the Node.js Server SDK with npm or yarn.

npmyarn

$ npm install --save @amplitude/experiment-node-server

$ yarn add @amplitude/experiment-node-server

Quick Start

Initialize the local evaluation client.
Start the local evaluation client.
Evaluate a user.

// (1) Initialize the local evaluation client with a server deployment key.
const experiment = Experiment.initializeLocal('<DEPLOYMENT_KEY>');

// (2) Start the local evaluation client.
await experiment.start();

// (2) Evaluate a user.
const user = { device_id: 'abcdefg' };
const variants = await experiment.evaluate(user);

Initialize Local¶

Initializes a local evaluation client.

Server Deployment Key

You must initialize the local evaluation client with a server deployment key to get access to local evaluation flag configs.

initializeLocal(apiKey: string, config?: LocalEvaluationConfig): LocalEvaluationClient

Parameter	Requirement	Description
`apiKey`	required	The server deployment key which authorizes fetch requests and determines which flags should be evaluated for the user.
`config`	optional	The client configuration used to customize SDK client behavior.

Flag Polling Interval

Use the flagConfigPollingIntervalMillis configuration to determine the time flag configs take to update once modified (default 30s).

Configuration¶

You can configure the SDK client on initialization.

Configuration Options

Name	Description	Default Value
`debug`	Set to `true` to enable debug logging.	`false`
`serverUrl`	The host to fetch flag configurations from.	`https://api.lab.amplitude.com`
`bootstrap`	Bootstrap the client with a map of flag key to flag configuration	`{}`
`flagConfigPollingIntervalMillis`	The interval (in milliseconds) to poll for updated flag configs after calling `start()`	`30000`

Start¶

Start the local evaluation client, pre-fetching local evaluation mode flag configs for evaluation and starting the flag config poller at the configured interval.

start(): Promise<void>

You should await the result of start() to ensure that flag configs are ready to be used before calling evaluate()

await experiment.start();

Evaluate¶

Executes the evaluation logic using the flags pre-fetched on start(). You must give evaluate a user object argument. You can optionally pass an array of flag keys if you require only a specific subset of required flag variants.

evaluate(user: ExperimentUser, flagKeys?: string[]): Promise<Variants>

Parameter	Requirement	Description
`user`	required	The user to evaluate.
`flagKeys`	optional	Specific flags or experiments to evaluate. If undefined, null, or empty, all flags and experiments are evaluated.

// The user to evaluate
const user = { device_id: 'abcdefg' };

// Evaluate all flag variants
const allVariants = await experiment.evaluate(user);

// Evaluate a specific subset of flag variants
const specificVariants = await experiment.evaluate(user, [
  'my-local-flag-1',
  'my-local-flag-2',
]);

Access Amplitude cookies¶

If you're using the Amplitude Analytics SDK on the client-side, the Node.js server SDK provides an AmplitudeCookie class with convenience functions for parsing and interacting with the Amplitude identity cookie. This is useful for ensuring that the Device ID on the server matches the Device ID set on the client, especially if the client hasn't yet generated a Device ID.

import { AmplitudeCookie } from '@amplitude/experiment-node-server';

app.use((req, res, next) => {
  const { query, cookies, url, path, ip, host } = req

  // grab amp device id if present
  const ampCookieName = AmplitudeCookie.cookieName('amplitude-api-key');
  let deviceId = null;
  if (cookies[ampCookieName]) {
    deviceId = AmplitudeCookie.parse(cookies[ampCookieName]).device_id;
  }
  if (!deviceId) {
    // deviceId doesn't exist, set the Amplitude Cookie
    deviceId = random22CharBase64String();
    const ampCookieValue = AmplitudeCookie.generate(deviceId);
    res.cookie(ampCookieName, ampCookieValue, {
      domain: '.yourdomain.com', // this should be the same domain used by the Amplitude JS SDK
      maxAge: 365 * 24 * 60 * 60 * 1000, // this is currently the same as the default in the Amplitude JS SDK, can be modified
      sameSite: 'Lax'
    });
  }

  next()
});