A/B Testing AWS Lambda@Edge demo.

ab-testing-lambda.js

/**
 * A/B Testing AWS Lambda@Edge demo.
 * 
 * Serves content from server hosts, based on an A/B Testing
 * header value.
 *
 * NOTE: For every new request, an A/B Testing header is added
 *       and a test value is selected and set. This header should
 *       be returned in the response and cached by the client, so
 *       it can be re-submitted with every consecutive API request.
 */

'use strict';

// Lambda entry point
exports.handler = (event, context, callback) => {
    // Get the HTTP request headers
    const request = event.Records[0].cf.request;
    const headers = request.headers;

    // Check for an A/B Testing header
    let abHeader = headers['x-ab-testing'];

    // If there is none, pick an test value and add the header
    if (!abHeader) {
        headers['x-ab-testing'] = [{
            key: 'X-AB-Testing',
            value: Math.random() < 0.7 ? 'A' : 'B'
        }];
        abHeader = headers['x-ab-testing'];
    }

    // Route the traffic to the correct server
    const abValue = abHeader[0].value;
    switch (abValue) {
        case 'A':
            headers.host[0].value = 'a.example.com';
            break;

        case 'B':
            headers.host[0].value = 'b.example.com';
            break;

        default:
            throw new Error(`Unsupported A/B Testing value: ${abValue}`);
    }

    // Propagate the request
    callback(null, request);
};