Cloudflare Workers / Segment Smart Proxy — serve data collection assets and endpoints from your own domain

worker.js

/**
 * Steps to use:
 *  1. Create CF Worker, copy and paste this in
 *  2. (Optional) Update configuration defaults
 *     - If you want to manage in code, do so below under "Static Configuration"
 *     - If you want dynamic custom config: Create CFW KV namespace, link them, and add reference below
 *
 *      - You can overwrite default path prefix for loading analytics.js (<yourdomain>/ajs)
 *        (corresponding KV entry: `script_path_prefix`)
 *      - You can overwrite default path prefix for handling first-party data collection (<yourdomain>/data)
 *        (corresponding KV entry: `collection_api_path_prefix`)
 *      - You can overwrite default cookie name for the edge-side anonymous ID
 *        (corresponding KV entry: `cookie_name`)
 *      - You can overwrite default integration list path prefix (/int-list)
 *        (corresponding KV entry: `integration_list_path_prefix`)
 *      - You can overwrite the default refresh trigger if you want to more regularly update the anonymousId
 *        (corresponding KV entry: `refresh_threshold`)
 *      - You can set a path for echoing the session ID
 *        (corresponding KV entry: `default_write_key`)
 *      - You can set a default write key if you just want to use one globally and want to omit it from your site code
 *        (corresponding KV entry: `write_key`)
 *      - You can set an error collection endpoint if you have a logging service that accepts webhooks
 *        (corresponding KV entry: `write_key`)
 *
 *  3. (If needed) If you use it for Consent Management, update any conditional destination loading logic to pull the integration list from your host + integration list path prefix
 *        eg. If using Segment Consent Manager or https://gist.github.com/sperand-io/4725e248a35d5005d68d810d8a8f7b29
 *        ...instead of fetch(`https://cdn.segment.com/v1/projects/${writeKey}/integrations`)
 *        ...replace with fetch(`${location.origin}/ilist/${writeKey}`) or fetch(`${location.origin}/ilist/}`)
 *  3. (REQUIRED) Deploy and configure the worker to serve for your desired domain/subdomain and at your desired path
 *  4. (REQUIRED) Update your segment snippet to load from your host + script path prefix
 *      (eg find n.src="https://cdn.segment.com/analytics.js/v1/"+t+"/analytics.min.js" in snippet and ...)
 *      (replace with n.src=`${location.origin}/ajs` if you have a default write key set)
 *      (or with n.src=`${location.origin}/ajs/${t}` if not)
 */

let KV_NAMESPACE

// START STATIC CONFIGURATION
const STATIC_CONFIG = {
  COOKIE_NAME: '__anonymous_session_id',
  SCRIPT_PATH_PREFIX: 'ajs',
  COLLECTION_API_PATH_PREFIX: 'data',
  INTEGRATION_LIST_PATH_PREFIX: 'ilist',
  ANONYMOUS_ID_ECHO_PATH: '',
  REFRESH_THRESHOLD: 45,
  DEFAULT_WRITE_KEY: '3K4xZlUgQFAa3MRdnRRKvbvDEukDCWeu',
  ERROR_ENDPOINT: 'https://enj0zt42hq1y.x.pipedream.net'
}
// END STATIC CONFIGUATION. Editing below this line is discouraged.

/**
 * Attach top-level responder.
 */
addEventListener('fetch', event => {
  event.respondWith(handleErr(event))
})

/**
 * Top level event handler.
 *
 * Wraps our request handler in an error handler,
 * optionally forward errors to a logging service.
 *
 * @param {Event} event
 */
async function handleErr(event) {
  try {
    const res = await handleEvent(event)
    return res
  } catch (err) {
    let endpoint = KV_NAMESPACE && (await KV_NAMESPACE.get('error_endpoint'))
    if (!endpoint) endpoint = STATIC_CONFIG['ERROR_ENDPOINT']
    if (endpoint) event.waitUntil(log(endpoint, err, event.request))
    return new Response(err.message || 'An error occurred!', {
      status: err.statusCode || 500
    })
  }
}

/**
 * Respond to the request
 *
 * Provides special handling for Segment requests against the configured || default paths.
 *
 * @param {Event} event
 */
async function handleEvent(event) {
  const config = KV_NAMESPACE ? await hydrateConfig(KV_NAMESPACE) : STATIC_CONFIG
  const {
    COOKIE_NAME,
    SCRIPT_PATH_PREFIX,
    COLLECTION_API_PATH_PREFIX,
    INTEGRATION_LIST_PATH_PREFIX,
    ANONYMOUS_ID_ECHO_PATH
  } = config
  const cache = caches.default

  const { request } = event
  const url = new URL(request.url)

  // extract cookie information
  const cookieData = getCookieData(request, COOKIE_NAME)

  // serve analytics.js
  if (startsWith(url, SCRIPT_PATH_PREFIX))
    return await handleScript(event, cache, cookieData, config)

  // serve first party data collection pings
  if (startsWith(url, COLLECTION_API_PATH_PREFIX))
    return await handleDataCollection(request, cookieData, config)

  // serve first party data collection pings
  if (startsWith(url, INTEGRATION_LIST_PATH_PREFIX))
    return await handleIntegrationListing(request, config)

  // serve anonymousId echo
  if (ANONYMOUS_ID_ECHO_PATH && startsWith(url, ANONYMOUS_ID_ECHO_PATH))
    return await handleEcho(event, cookieData)
  
  // passthrough everything else
  return await fetch(request)
}

/**
 * Serve analytics.js
 *
 * Serves a modified analytics.js for (default || passed) writeKey at (default || configured) (path || path prefix)
 * Mods:
 *  If writeKey is omitted, get the default script
 *  Updates data collection api host in the script itself
 *  If needed, sets an HTTPOnly anonymous session cookie (and corresponding set-at cookie)
 *
 * @param {Event} event
 * @param {Cache} cache
 * @param {Object} cookieData
 * @param {String} cookieData.anonymousId
 * @param {Date} cookieData.expires
 * @param {Object} config
 */
async function handleScript(
  event,
  cache,
  { anonymousId, expires },
  {
    SCRIPT_PATH_PREFIX,
    DEFAULT_WRITE_KEY,
    COLLECTION_API_PATH_PREFIX,
    COOKIE_NAME,
    REFRESH_THRESHOLD
  }
) {
  const { request } = event
  const { pathname, hostname } = new URL(request.url)
  let [_, writeKey] = pathname.split(`/${SCRIPT_PATH_PREFIX}/`)
  if (!writeKey) writeKey = DEFAULT_WRITE_KEY
  let response
  const cached = await cache.match(request)

  if (cached) {
    response = cached
  } else {
    const endpoint = `https://cdn.segment.com/analytics.js/v1/${writeKey}/analytics.min.js`
    const originalResponse = await fetch(new Request(endpoint, request))
    const newResponse = originalResponse.clone()
    const analyticsjs = await originalResponse.text()
    const modifiedAnalyticsjs = analyticsjs.replace(
      /\api\.segment\.io\/v1/g,
      `${hostname}/${COLLECTION_API_PATH_PREFIX}`
    )

    response = new Response(modifiedAnalyticsjs, newResponse)
    event.waitUntil(cache.put(request, response.clone()))
  }

  if (!anonymousId || expiresSoon(expires, REFRESH_THRESHOLD)) {
    const oneYearFromNow = new Date()
    oneYearFromNow.setFullYear(oneYearFromNow.getFullYear() + 1)
    response.headers.append(
      'Set-Cookie',
      createCookie(COOKIE_NAME, uuid(), oneYearFromNow)
    )
    response.headers.append(
      'Set-Cookie',
      createCookie(`${COOKIE_NAME}_set`, oneYearFromNow.toUTCString(), oneYearFromNow)
    )
  }

  return response
}

/**
 * Serve first party data collection API
 *
 * Serves a handler to modify and forward events to Segment at the default || configured path prefix
 * Mods:
 *  If present in the request cookie, overwrites anonymousId with edge-side cookie value
 *
 * @param {Request} request
 * @param {Object} cookieData
 * @param {String} cookieData.anonymousId
 * @param {Object} config
 */
async function handleDataCollection(
  request,
  { anonymousId },
  { COLLECTION_API_PATH_PREFIX }
) {
  const originalRequest = request.clone()
  const body = JSON.stringify({
    ...(await request.json()),
    ...(anonymousId ? { anonymousId } : {})
  })

  const { pathname, hostname } = new URL(request.url)
  const correctPath = pathname.replace(COLLECTION_API_PATH_PREFIX, 'v1')

  const newRequest = new Request(
    `https://api.segment.io${correctPath}`,
    new Request(originalRequest, { body })
  )

  newRequest.headers.append('origin', `https://${hostname}`)
    
  return await fetch(newRequest)
}

/**
 * Serve first party integration list API
 *
 * Serves a handler to passthrough list requests for default || passed writeKey at the default || configured path prefix
 *
 * @param {Request} request
 * @param {Object} config
 */
async function handleIntegrationListing(
  request,
  { INTEGRATION_LIST_PATH_PREFIX, DEFAULT_WRITE_KEY }
) {
  const { pathname } = new URL(request.url)
  let [_, writeKey] = pathname.split(`/${INTEGRATION_LIST_PATH_PREFIX}/`)
  if (!writeKey) writeKey = DEFAULT_WRITE_KEY
  const endpoint = `https://cdn.segment.com/v1/projects/${writeKey}/integrations`
  return await fetch(new Request(endpoint, new Request(request, { body })))
}

/**
 * Serve first party anonymousID echo API
 *
 * @param {Request} request
 * @param {Object} config
 */
async function handleEcho(request, { anonymousId }) {
  if (anonymousId) {
    return new Response(JSON.stringify({ anonymousId }), {
      headers: new Headers({
        'Content-Type': 'application/json'
      })
    })
  }
  new Response('No AnonymousId', { status: 404 })
}

/**
 * HELPERS
 */

/**
 * Check if url path begins with a specified prefix
 * @param {NAMESPACE} KV
 */
async function hydrateConfig(KV) {
  const keys = [
    'cookie_name',
    'script_path_prefix',
    'collection_api_path_prefix',
    'integration_list_path_prefix',
    'refresh_threshold',
    'default_write_key'
  ]
  return Promise.all(
    keys.map(async k => {
      return { [k.toUpperCase()]: (await KV.get(k)) || '' }
    })
  ).reduce((config, { key, storedKValue }) => {
    if (storedKValue) {
      config[key] = storedKValue
    }
    return config
  }, STATIC_CONFIG)

}

/**
 * Check if url path begins with a specified prefix
 */
function startsWith(url, prefix) {
  if (url.pathname.startsWith(`/${prefix}`)) return true
  return false
}

/**
 * Check if the anonymousId is due to be refreshed
 * (ie. is our expiration closer than our threshold window allows?)
 */
function expiresSoon(when, REFRESH_THRESHOLD) {
  // eg. 45 days from now
  const threshold = new Date()
  threshold.setDate(threshold.getDate() + REFRESH_THRESHOLD)
  // is expiration in less than eg. 45 days?
  if (when < threshold) return true
  else return false
}

/**
 * Encode a cookie string suited for our use case
 */
function createCookie(name, value, expires) {
  return `${encodeURIComponent(name)}=${encodeURIComponent(
    value
  )}; Expires=${expires.toUTCString()}; SameSite=Strict; Secure; HttpOnly`
}

/**
 * Generate a spec-compliant uuid-v4
 * adapted from: https://gist.github.com/bentranter/ed524091170137a72c1d54d641493c1f
 */
function uuid() {
  const bytes = crypto.getRandomValues(new Uint8Array(16))

  bytes[6] = (bytes[6] & 0x0f) | 0x40
  bytes[8] = (bytes[8] & 0xbf) | 0x80

  const chars = [...bytes].map(byte => byte.toString(16))

  const insertionPoints = [4, 6, 8, 10]
  return chars.reduce((uuid, char, index) => {
    if (insertionPoints.includes(index)) {
      return (uuid += `-${char}`)
    } else {
      return (uuid += char)
    }
  })
}

/**
 * Grabs the anonymousId and expiration time from the cookies in the request header
 *
 * Adapted from: https://developers.cloudflare.com/workers/templates/pages/cookie_extract/
 *
 * @param {Request} request incoming Request
 * @param {string} name of the edge-side cookie
 */
function getCookieData(request, name) {
  let anonymousId = null
  let expires = null
  let cookieString = request.headers.get('Cookie')
  if (cookieString) {
    let cookies = cookieString.split(';')
    cookies.forEach(cookie => {
      let cookieName = cookie.split('=')[0].trim()
      if (cookieName === name) {
        anonymousId = cookie.split('=')[1]
      }
      if (cookieName === `${name}_set`) {
        expires = new Date(decodeURIComponent(cookie.split('=')[1]))
      }
    })
  }
  return { anonymousId, expires }
}

/**
 * Ship the error with some helpful request context as JSON to the specified endpoint
 *
 * ADAPTED from https://github.com/bustle/cf-sentry/
 *
 * @param {String} endpoint
 * @param {Error} err the error
 * @param {Request} request incoming Request
 */
async function log(endpoint, err, request) {
  const body = JSON.stringify(errToJson(err, request))
  const res = await fetch(endpoint, {
    method: 'POST',
    headers: {
      'Content-Type': 'application/json'
    },
    body
  })
  if (res.status === 200) {
    return
  }
  // We couldn't send to error endpoint, try to log the response at least
  console.error({ httpStatus: res.status, ...(await res.json()) }) // eslint-disable-line no-console
}

/**
 * Encode the parsed and formatted error as JSON
 *
 * ADAPTED from https://github.com/bustle/cf-sentry/
 *
 * @param {Error} err the error
 * @param {Request} request incoming Request
 */
function errToJson(err, request) {
  const errType = err.name || (err.contructor || {}).name
  const frames = parse(err)
  const extraKeys = Object.keys(err).filter(
    key => !['name', 'message', 'stack'].includes(key)
  )
  return {
    message: errType + ': ' + (err.message || '<no message>'),
    exception: {
      values: [
        {
          type: errType,
          value: err.message,
          stacktrace: frames.length ? { frames: frames.reverse() } : undefined
        }
      ]
    },
    extra: extraKeys.length
      ? {
          [errType]: extraKeys.reduce((obj, key) => ({ ...obj, [key]: err[key] }), {})
        }
      : undefined,
    platform: 'worker',
    timestamp: Date.now() / 1000,
    request:
      request && request.url
        ? {
            method: request.method,
            url: request.url,
            query_string: request.query,
            headers: request.headers,
            data: request.body
          }
        : undefined
  }
}

/**
 * Parse errors.
 *
 * ADAPTED from https://github.com/bustle/cf-sentry/
 *
 * @param {Error} err the error\
 */
function parse(err) {
  return (err.stack || '')
    .split('\n')
    .slice(1)
    .map(line => {
      if (line.match(/^\s*[-]{4,}$/)) {
        return { filename: line }
      }

      // From https://github.com/felixge/node-stack-trace/blob/1ec9ba43eece124526c273c917104b4226898932/lib/stack-trace.js#L42
      const lineMatch = line.match(
        /at (?:(.+)\s+\()?(?:(.+?):(\d+)(?::(\d+))?|([^)]+))\)?/
      )
      if (!lineMatch) {
        return
      }

      return {
        function: lineMatch[1] || undefined,
        filename: lineMatch[2] || undefined,
        lineno: +lineMatch[3] || undefined,
        colno: +lineMatch[4] || undefined,
        in_app: lineMatch[5] !== 'native' || undefined
      }
    })
    .filter(Boolean)
}

Come back!!