VWO FullStack vs FE

A comprehensive guide to understanding the differences and migrating from VWO FullStack to Feature Experimentation (FE).

Introduction

DEPRECATED VWO FullStack (FS) — The original server-side SDK for A/B testing and feature management. Only receiving critical bug fixes and security patches.

CURRENT VWO FE (Feature Experimentation) — The actively developed replacement with a modernized API and improved developer experience.

Same Purpose, New Philosophy

Both SDKs enable server-side A/B testing and feature management. The key difference lies in their design philosophy:

FullStack	FE
Campaign-Centric — Everything revolves around campaigns. You activate users into campaigns and track goals per campaign.	Feature-Flag-Centric — Features are the primary unit. You evaluate flags and track events independently of campaigns.

Installation

Aspect	FullStack	FE
Package Name	`vwo-node-sdk`	`vwo-fme-node-sdk`
Install Command	`npm install vwo-node-sdk`	`npm install vwo-fme-node-sdk`
Node.js Version	>= 6.10.0	>= 12.0

Initialization

FullStack: Two-Step Process

const vwoSDK = require('vwo-node-sdk');

// Step 1: Fetch settings file
const settingsFile = await vwoSDK.getSettingsFile(
  accountId,
  sdkKey
);

// Step 2: Launch SDK
const vwoClient = vwoSDK.launch({
  settingsFile: settingsFile
});

FE: Single-Step Process

const { init } = require('vwo-fme-node-sdk');

// Single initialization call
const vwoClient = await init({
  accountId: '123456',
  sdkKey: 'your-sdk-key',
});

// Optional: auto-refresh settings
const vwoClient = await init({
  accountId: '123456',
  sdkKey: 'your-sdk-key',
  pollInterval: 60000,
});

Key Difference: FE handles settings file fetching internally. No need for a separate getSettingsFile() call.

Running an A/B Test

FullStack Approach (Campaign-Centric)

const vwoSDK = require('vwo-node-sdk');

// Initialize
const settingsFile = await vwoSDK.getSettingsFile(accountId, sdkKey);
const vwoClient = vwoSDK.launch({ settingsFile });

// Activate user in campaign - returns variation name
const variation = vwoClient.activate(
  'checkout-button-campaign',  // campaignKey
  'user-123',                   // userId
  {
    customVariables: { plan: 'premium' },
    userAgent: req.headers['user-agent']
  }
);

// Handle variations by name
if (variation === 'Control') {
  buttonColor = 'blue';
} else if (variation === 'Variation-1') {
  buttonColor = 'green';
} else {
  // User not in campaign (null returned)
  buttonColor = 'blue';
}

// Track conversion (requires campaignKey)
vwoClient.track(
  'checkout-button-campaign',  // campaignKey
  'user-123',                   // userId
  'purchase-completed'          // goalIdentifier
);

FE Approach (Feature-Flag-Centric)

const { init } = require('vwo-fme-node-sdk');

// Initialize
const vwoClient = await init({
  accountId: '123456',
  sdkKey: 'your-sdk-key',
});

// Create user context once, reuse it
const userContext = {
  id: 'user-123',
  customVariables: { plan: 'premium' },
  userAgent: req.headers['user-agent']
};

// Get feature flag - contains all variation data
const flag = await vwoClient.getFlag('checkout-button', userContext);

// Check if enabled and get variables
if (flag.isEnabled()) {
  buttonColor = flag.getVariable('buttonColor', 'blue');
  buttonText = flag.getVariable('buttonText', 'Buy Now');
}

// Track event (no campaign key needed)
vwoClient.trackEvent('purchase-completed', userContext);

Key Differences:

User ID: FS passes it per method; FE uses a reusable userContext object

Variation access: FS returns variation name to check; FE provides variables directly via getVariable()

Tracking: FS requires campaignKey; FE tracks events independently

Variables Over Variations

One of the most important changes in FE is the shift from variation-name-based logic to variable-driven logic. This isn't just a syntax change—it's a fundamental improvement in how you architect experiments.

The Problem with Variation Names (FS Approach)

In FullStack, you typically check which variation a user is in and branch your code accordingly:

// FullStack: Variation-name-based logic (NOT recommended)
const variation = vwoClient.activate('checkout-experiment', userId);

if (variation === 'Control') {
  buttonColor = 'blue';
  buttonText = 'Buy Now';
  showBadge = false;
} else if (variation === 'Variation-1') {
  buttonColor = 'green';
  buttonText = 'Buy Now';
  showBadge = true;
} else if (variation === 'Variation-2') {
  buttonColor = 'orange';
  buttonText = 'Add to Cart';
  showBadge = true;
}

Problems with this approach:

Tight coupling: Your code is tightly coupled to variation names defined in VWO

Code changes for experiments: Adding a new variation requires a code deployment

Hard to maintain: Variation logic is scattered across your codebase

Rigid: Non-developers cannot iterate on experiments without code changes

The Solution: Variable-Driven Logic (FE Approach)

FE encourages you to define variables (buttonColor, buttonText, showBadge) in the VWO dashboard, with each variation specifying different values. Your code simply reads these variables:

// FE: Variable-driven logic (RECOMMENDED)
const flag = await vwoClient.getFlag('checkout-experiment', userContext);

if (flag.isEnabled()) {
  // Variables are configured in VWO dashboard, not hardcoded
  const buttonColor = flag.getVariable('buttonColor', 'blue');
  const buttonText = flag.getVariable('buttonText', 'Buy Now');
  const showBadge = flag.getVariable('showBadge', false);

  renderButton(buttonColor, buttonText, showBadge);
}

Benefits of variable-driven logic:

Decoupled: Code doesn't know or care about variation names

No deployments: Add new variations or modify existing ones entirely in VWO

Single source of truth: All experiment configuration lives in the dashboard

Team empowerment: Product managers can iterate without engineering

Side-by-Side Comparison

FullStack (Variation-Based):

// Code knows about variations
if (variation === 'Variation-1') {
  // Hardcoded behavior
  showNewCheckout();
} else {
  showOldCheckout();
}

// Adding Variation-2 requires:
// 1. Code change
// 2. Code review
// 3. Deployment

FE (Variable-Based):

// Code only knows about variables
const useNewFlow = flag.getVariable(
  'useNewCheckout', false
);

if (useNewFlow) {
  showNewCheckout();
} else {
  showOldCheckout();
}

// Adding variations:
// Just update VWO dashboard!

When You Need Variation Names

Sometimes you genuinely need to know which variation a user is in (for logging, analytics, or debugging). In FE, the recommended approach is to create an explicit variation_name variable:

// Create a 'variationName' variable in VWO dashboard
// Control: variationName = "control"
// Variation-1: variationName = "green-button"
// Variation-2: variationName = "orange-button"

const flag = await vwoClient.getFlag('checkout-experiment', userContext);
const variationName = flag.getVariable('variationName', 'control');

// Use for logging/analytics only, not branching logic
analytics.track('experiment_viewed', { variation: variationName });

Best Practice: Treat variation names as configuration artifacts, not API contracts. Your code should depend on variables (what to do), not variations (which bucket the user is in). This aligns with OpenFeature best practices.

API Method Mapping

Purpose	FullStack	FE
Initialize SDK	`getSettingsFile()` + `launch()`	`init()`
Get variation	`activate(campaignKey, userId)`	`getFlag(featureKey, userContext)`
Check if enabled	`isFeatureEnabled(campaignKey, userId)`	`flag.isEnabled()`
Get variable value	`getFeatureVariableValue(campaignKey, variableKey, userId)`	`flag.getVariable(key, defaultValue)`
Get all variables	N/A (call per variable)	`flag.getVariables()`
Track conversion	`track(campaignKey, userId, goalId)`	`trackEvent(eventName, userContext)`
Push attributes	`push(tagKey, tagValue, userId)`	`setAttribute(name, value, userContext)`

Conceptual Differences

Aspect	FullStack	FE
Primary Unit	Campaign	Feature Flag
User Identification	`userId` string passed to each method	`userContext` object created once, reused
Async Pattern	Callback-based with opt-in promises (`returnPromiseFor: { activate: true }`)	Native async/await by default
Event Tracking	Tied to campaigns (requires `campaignKey`)	Event-based, campaign-agnostic
Custom Variables	Passed in `options` per method call	Part of `userContext.customVariables`
Settings Management	Manual: fetch, store, refresh via webhooks/polling	Automatic with `pollInterval` option

Quick Migration Checklist

Detailed Migration Guide

Step 1: Update Package

# Remove old package
npm uninstall vwo-node-sdk

# Install new package
npm install vwo-fme-node-sdk --save

Step 2: Update Import Statement

Before (FS):

const vwoSDK = require('vwo-node-sdk');

After (FE):

const { init } = require('vwo-fme-node-sdk');

Step 3: Refactor Initialization

Before (FS):

const settingsFile = await vwoSDK.getSettingsFile(accountId, sdkKey);

const vwoClient = vwoSDK.launch({
  settingsFile,
  isDevelopmentMode: false,
  logging: { level: 'ERROR' }
});

After (FE):

const vwoClient = await init({
  accountId: '123456',
  sdkKey: 'your-sdk-key',
  logger: { level: 'ERROR' },
  pollInterval: 60000  // auto-refresh settings
});

Step 4: Create User Context

Before (FS) — User ID and options passed to each method:

const userId = 'user-123';
const options = {
  customVariables: { plan: 'premium' },
  userAgent: req.headers['user-agent'],
  userIpAddress: req.ip
};

vwoClient.activate(campaignKey, userId, options);
vwoClient.track(campaignKey, userId, goalId, options);

After (FE) — User context created once and reused:

const userContext = {
  id: 'user-123',
  customVariables: { plan: 'premium' },
  userAgent: req.headers['user-agent'],
  ipAddress: req.ip
};

const flag = await vwoClient.getFlag(featureKey, userContext);
vwoClient.trackEvent(eventName, userContext);

Step 5: Migrate A/B Test Logic

Before (FS):

const variation = vwoClient.activate('button-test', userId, options);

if (variation === 'Control') {
  showBlueButton();
} else if (variation === 'Variation-1') {
  showGreenButton();
}

After (FE):

const flag = await vwoClient.getFlag('button-test', userContext);

if (flag.isEnabled()) {
  const color = flag.getVariable('buttonColor', 'blue');
  showButton(color);
} else {
  showBlueButton();
}

Step 6: Migrate Event Tracking

Before (FS):

// Track requires campaign key
vwoClient.track('button-test', userId, 'button-clicked');

// Track across multiple campaigns
vwoClient.track(['campaign-1', 'campaign-2'], userId, 'purchase');

// Revenue tracking
vwoClient.track('campaign', userId, 'revenue-goal', {
  revenueValue: 99.99
});

After (FE):

// Track event (no campaign key needed)
vwoClient.trackEvent('button-clicked', userContext);

// Same event works across all campaigns automatically
vwoClient.trackEvent('purchase', userContext);

// Event with properties
vwoClient.trackEvent('revenue-goal', userContext, {
  revenue: 99.99
});

Step 7: Migrate Variable Access

Before (FS):

// Check feature and get variables separately
const isEnabled = vwoClient.isFeatureEnabled('new-checkout', userId);

if (isEnabled) {
  const headerText = vwoClient.getFeatureVariableValue(
    'new-checkout', 'headerText', userId
  );
  const showBanner = vwoClient.getFeatureVariableValue(
    'new-checkout', 'showBanner', userId
  );
}

After (FME):

// Get flag once, access all variables
const flag = await vwoClient.getFlag('new-checkout', userContext);

if (flag.isEnabled()) {
  const headerText = flag.getVariable('headerText', 'Welcome');
  const showBanner = flag.getVariable('showBanner', false);

  // Or get all variables at once
  const allVars = flag.getVariables();
}

Summary

Aspect	FullStack (Deprecated)	FE (Current)
Package	`vwo-node-sdk`	`vwo-fme-node-sdk`
Node.js	>= 6.10.0	>= 12.0
Init Pattern	Two-step (fetch + launch)	Single `init()` call
User ID	String per method	`userContext` object
Get Variation	`activate()` → string	`getFlag()` → flag object
Variables	One call per variable	`flag.getVariable()` or `getVariables()`
Tracking	Requires `campaignKey`	Event-based, campaign-agnostic
Async	Opt-in promises	Native async/await
Status	Critical fixes only	Actively developed

Need Help? For assistance migrating from FullStack to FE, contact VWO support at [email protected]