Viewer Request lambda@edge function

cf-origin-switch_viewer-request.js

'use strict';

const AWS = require('aws-sdk');
const ssm = new AWS.SSM({ region: 'eu-central-1' });

const SOURCE_COOKIE_NAME = 'myapp-origin';
const LATEST_ORIGIN_GIT_HASH = 'myapp_latest-origin-git-hash';

exports.handler = async (event, context, callback) => {
    console.group('[INFO] lambda@edge Viewer Request')
    const request = event.Records[0].cf.request;

    const latestOriginHash = await ssm.getParameter({
        Name: LATEST_ORIGIN_GIT_HASH,
        WithDecryption: false,
    }).promise();

    const hash = latestOriginHash.Parameter.Value;

    if (request.headers.cookie) {
        console.log(request.headers.cookie);
        const cookieObj = cookieStringToObj(request.headers.cookie[0].value);

        if (cookieObj.hasOwnProperty(SOURCE_COOKIE_NAME)) {
            const sourceCookieValue = cookieObj[SOURCE_COOKIE_NAME];
            console.log(`Request has source cookie with value: ${sourceCookieValue}`)

            if (sourceCookieValue !== hash) {
                console.log(`Source cookie hash (${sourceCookieValue}) is different, update with latest ${hash}.`);
                cookieObj[SOURCE_COOKIE_NAME] = hash;

                request.headers.cookie.push({ key: 'Cookie', value: objToCookieString(cookieObj) });
            }

            return request;
        }
    }

    console.log(`NO source cookie found -> cookies: ${JSON.stringify(request.headers.cookie)}, uri: ${request.uri}`);

    const cookieObj = { [SOURCE_COOKIE_NAME]: hash };
    request.headers.cookie = [{ key: 'Cookie', value: objToCookieString(cookieObj) }]

    console.groupEnd()

    return request;
};

function cookieStringToObj(cookieString) {
    return Object.fromEntries(cookieString.split(/; *!/).map(c => {
        const [key, ...v] = c.split('=');
        return [key, decodeURIComponent(v.join('='))];
    }));
}

function objToCookieString(cookieObj) {
    return Object.entries(cookieObj).reduce((res, [key, value]) => res += `${key}=${encodeURIComponent(value)}; `, '').trim();
}