Logging with Cloudflare and Logtail

Collect logs using a Cloudflare Worker. Set up everything in 5 minutes or less.

1.Go to the Cloudflare dashboard  → Select your domain → Workers

2.Click on Manage WorkersCreate a Worker

3.Replace the script with the following code and click on Save and Deploy

const CF_APP_VERSION = '1.0.0'

const logtailApiURL = "https://in.logtail.com/";
const apiKey = "YOUR_LOGTAIL_SOURCE_TOKEN";

const headers = [
  "rMeth",
  "rUrl",
  "uAgent",
  "cfRay",
  "cIP",
  "statusCode",
  "contentLength",
  "cfCacheStatus",
  "contentType",
  "responseConnection",
  "requestConnection",
  "cacheControl",
  "acceptRanges",
  "expectCt",
  "expires",
  "lastModified",
  "vary",
  "server",
  "etag",
  "date",
  "transferEncoding",
]

const options = {
  metadata: headers.map(value => ({ field: value })),
}

const sleep = ms => {
  return new Promise(resolve => {
    setTimeout(resolve, ms)
  })
}

const makeid = length => {
  let text = ""
  const possible = "ABCDEFGHIJKLMNPQRSTUVWXYZ0123456789"
  for (let i = 0; i < length; i += 1) {
    text += possible.charAt(Math.floor(Math.random() * possible.length))
  }
  return text
}

const buildLogMessage = (request, response) => {
  const logDefs = {
    rMeth: request.method,
    rUrl: request.url,
    uAgent: request.headers.get("user-agent"),
    cfRay: request.headers.get("cf-ray"),
    cIP: request.headers.get("cf-connecting-ip"),
    statusCode: response.status,
    contentLength: response.headers.get("content-length"),
    cfCacheStatus: response.headers.get("cf-cache-status"),
    contentType: response.headers.get("content-type"),
    responseConnection: response.headers.get("connection"),
    requestConnection: request.headers.get("connection"),
    cacheControl: response.headers.get("cache-control"),
    acceptRanges: response.headers.get("accept-ranges"),
    expectCt: response.headers.get("expect-ct"),
    expires: response.headers.get("expires"),
    lastModified: response.headers.get("last-modified"),
    vary: response.headers.get("vary"),
    server: response.headers.get("server"),
    etag: response.headers.get("etag"),
    date: response.headers.get("date"),
    transferEncoding: response.headers.get("transfer-encoding"),
  }

  const logArray = []
  options.metadata.forEach(entry => logArray.push(logDefs[entry.field]))
  return logArray.join(" | ")
}

const buildMetadataFromHeaders = headers => {
  const responseMetadata = {}
  Array.from(headers).forEach(([key, value]) => {
    responseMetadata[key.replace(/-/g, "_")] = value
  })
  return responseMetadata
}

// Batching
const BATCH_INTERVAL_MS = 500
const MAX_REQUESTS_PER_BATCH = 100
const WORKER_ID = makeid(6)

let workerTimestamp

let batchTimeoutReached = true
let logEventsBatch = []

// Backoff
const BACKOFF_INTERVAL = 10000
let backoff = 0

async function addToBatch(body, connectingIp, event) {
  logEventsBatch.push(body)

  if (logEventsBatch.length >= MAX_REQUESTS_PER_BATCH) {
    event.waitUntil(postBatch(event))
  }

  return true
}

async function handleRequest(event) {
  const { request } = event

  const requestMetadata = buildMetadataFromHeaders(request.headers)

  const t1 = Date.now()
  const response = await fetch(request)
  const originTimeMs = Date.now() - t1

  const rUrl = request.url
  const rMeth = request.method
  const rCf = request.cf
  delete rCf.tlsClientAuth
  delete rCf.tlsExportedAuthenticator

  const responseMetadata = buildMetadataFromHeaders(response.headers)

  const eventBody = {
    message: buildLogMessage(request, response),
    dt: new Date().toISOString(),
    metadata: {
      response: {
        headers: responseMetadata,
        origin_time: originTimeMs,
        status_code: response.status,
      },
      request: {
        url: rUrl,
        method: rMeth,
        headers: requestMetadata,
        cf: rCf,
      },
      cloudflare_worker: {
        version: CF_APP_VERSION,
        worker_id: WORKER_ID,
        worker_started: workerTimestamp,
      },
    },
  }
  event.waitUntil(
    addToBatch(eventBody, requestMetadata.cf_connecting_ip, event),
  )

  return response
}

const fetchAndSetBackOff = async (lfRequest, event) => {
  if (backoff <= Date.now()) {
    const resp = await fetch(logtailApiURL, lfRequest)
    if (resp.status === 403 || resp.status === 429) {
      backoff = Date.now() + BACKOFF_INTERVAL
    }
  }

  event.waitUntil(scheduleBatch(event))

  return true
}

const postBatch = async event => {
  const batchInFlight = [...logEventsBatch]
  logEventsBatch = []
  const rHost = batchInFlight[0].metadata.request.headers.host
  const body = JSON.stringify(batchInFlight)
  const request = {
    method: "POST",
    headers: {
      "Authorization": `Bearer ${apiKey}`,
      "Content-Type": "application/json",
      "User-Agent": `Cloudflare Worker via ${rHost}`,
    },
    body,
  }
  event.waitUntil(fetchAndSetBackOff(request, event))
}

const scheduleBatch = async event => {
  if (batchTimeoutReached) {
    batchTimeoutReached = false
    await sleep(BATCH_INTERVAL_MS)
    if (logEventsBatch.length > 0) {
      event.waitUntil(postBatch(event))
    }
    batchTimeoutReached = true
  }
  return true
}

addEventListener("fetch", event => {
  event.passThroughOnException()

  if (!workerTimestamp) {
    workerTimestamp = new Date().toISOString()
  }

  event.waitUntil(scheduleBatch(event))
  event.respondWith(handleRequest(event))
})
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4.Go back to Workers → Click on Add route

5.Set the route pattern to *.your-domain.com/* and select the new Logtail worker

6.Click on Save and you're all set 🎉