Instrument Node.js HTTP requests

instrumentRequests.js

// Based on <https://blog.bearer.sh/http-api-instrumentation-nodejs/>
const http = require('http')
const https = require('https')
const { URL } = require('url')
const Metrics = require('metrics')
const { performance } = require('perf_hooks')

const metrics = {}

const metricTypes = {
  counter: () => new Metrics.Counter(),
  histogram: () => Metrics.Histogram.createUniformHistogram(),
}

function getServiceName(url) {
  const { host } = new URL(url)
  return host.replace(/\./g, '-')
}

function getMetric(url, type, key) {
  const service = getServiceName(url)

  metrics[service] ??= {}
  metrics[service][key] ??= metricTypes[type]()

  return metrics[service][key]
}

function patchModule(module) {
  const original = module.request

  function newRequest(outgoing) {
    // Using perf.now() instead of Date.now() not because we need the resolution
    // but because we can avoid being bound to the system's time and its quirks.
    const startTime = performance.now()
    const request = original.apply(this, arguments)
    const { emit } = request

    request.emit = function (event, response) {
      if (event === 'response') {
        response.on('end', () => {
          const endTime = performance.now()

          const { statusCode } = response
          const time = Math.ceil(endTime - startTime)
          const host = outgoing.host || outgoing.hostname

          getMetric(host, 'counter', `status_${statusCode}.count`).inc(1)
          getMetric(host, 'histogram', `status_${statusCode}.response_time`).update(time)
        })
      }

      return emit.apply(this, arguments)
    }

    return request
  }

  module.request = newRequest
}

function instrumentRequests() {
  patchModule(http)
  patchModule(https)
}

module.exports = instrumentRequests