Example of a Cloudflare Worker and Worker KV storage used to transfer/migrate traffic to alternatively infrastructure

worker.js

addEventListener('fetch', event => {
  event.passThroughOnException()
  event.respondWith(handleRequest(event.request))
})

async function handleRequest(request) {
  const maxBizId = await MIGRATION_SETTINGS.get('maxbizid')
  const currentBusinessId = getBusinessIdFromCookie(request)

  if (!maxBizId || maxBizId == 0 || !currentBusinessId) {
    return await fetch(request)
  }

  const isTestBiz = await businessInKey(currentBusinessId, 'testbizids')
  if (isTestBiz) {
    // They're whitelisted as a test business so redirect them
    return await redirect(request)
  }

  const isException = await businessInKey(currentBusinessId, 'bizidexceptions')
  if (!isException && Number(currentBusinessId) <= Number(maxBizId)) {
    // This biz is under the current max and is not an exception (i.e. VIPs) so redirect it
    return await redirect(request)
  }

  return await fetch(request)
}

function getBusinessIdFromCookie(request) {
  if (!request.headers) return null

  let bizId = null
  const cookies = request.headers.get('Cookie') || ""
  if (cookies.includes(';')) {
    // There's multiple cookies
    const splitCookies = cookies.split(';')
    splitCookies.forEach(cookie => {
      if (cookie.includes('t-business')) {
        bizId = cookie.split('=')[1]
      }
    })
  } else {
    if (cookies.includes('t-business')) {
      bizId = cookie.split('=')[1]
    }
  }

  return bizId
}

async function businessInKey(bizId, keyName) {
  if (!bizId || bizId == 0) {
    return false
  }

  const ids = await MIGRATION_SETTINGS.get(keyName)
  return ids.split(',').indexOf(bizId.toString()) > -1
}

function requiresRewritingLinks(request, response){
  // Change the string below to match whatever you need
  return request.url.includes('[entermypartialurlstringhere]') && 
    response.headers.get("Content-Type").includes("application/json")
}

async function rewriteResponseBody(response, target, replacement){
  let bodyText = await response.text()
  bodyText = bodyText.replace(new RegExp(target, 'g'), replacement)
  return new Response(bodyText, {
    headers: response.headers
  })
}

async function redirect(request){
  try {
    const originalHost = 'app.myurl.com' // enter original URL here
    const redirectHost = 'myapp.azurewebsites.net' // enter URL of what you're redirecting to
    const redirectUrl = request.url.replace(originalHost, redirectHost)

    let init = {
      method: request.method,
      headers: request.headers,
      body: request.body,
      redirect  :  "manual" // The stupidly important redirect parameter
    }
    
    let response = await fetch(new Request(redirectUrl, init))

    if(requiresRewritingLinks(request, response)){
      response = await rewriteResponseBody(response, redirectHost, originalHost)
    }

    let finalResponse = new Response(response.body, response)
    finalResponse.headers.set('t-redirected', 'true') // Flag we added so it was clear your response was redirected

    return finalResponse

  } catch (e) {
    return new Response(e)
  }
}