Express Prometheus Middleware

This is a middleware for express servers, that expose metrics for prometheus.

The metrics exposed allows to calculate common RED (Request, Error rate, Duration of requests), and USE (Utilisation, Error rate, and Saturation), metrics

Install

yarn add express-prometheus-middleware
# or
npm i --save express-prometheus-middleware

Usage

Options

Name	Description	Default
metricsPath	Url route that will expose the metrics for scraping.	/metrics
metricsApp	Express app that will expose metrics endpoint, if an app is provided, use it, instead of instantiating a new one	null
collectDefaultMetrics	Whether or not to collect prom-client default metrics. These metrics are usefull for collecting saturation metrics, for example.	true
collectGCMetrics	Whether or not to collect garbage collection metrics via module prometheus-gc-stats. Dependency prometheus-gc-stats is marked as optional, hence if this option is set to true but npm/yarn could not install the dependency, no garbage collection metric will be collected.	false
requestDurationBuckets	Buckets for the request duration metrics (in milliseconds) histogram	Uses prom-client utility: Prometheus.exponentialBuckets(0.05, 1.75, 8)
extraMasks	Optional, list of regexes to be used as argument to url-value-parser, this will cause extra route params, to be replaced with a #val placeholder.	no extra masks: []
authenticate	Optional authentication callback, the function should receive as argument, the req object and return truthy for sucessfull authentication, or falsy, otherwise. This option supports Promise results.	null
prefix	Optional prefix for the metrics name	no prefix added
customLabels	Optional Array containing extra labels, used together with transformLabels	no extra labels: []
transformLabels	Optional function(labels, req, res) adds to the labels object dynamic values for each label in customLabels	null

Example

const express = require('express');
const promMid = require('express-prometheus-middleware');
const app = express();

const PORT = 9091;
app.use(promMid({
  metricsPath: '/metrics',
  collectDefaultMetrics: true,
  requestDurationBuckets: [0.1, 0.5, 1, 1.5],
  /**
   * Uncomenting the `authenticate` callback will make the `metricsPath` route
   * require authentication. This authentication callback can make a simple
   * basic auth test, or even query a remote server to validate access.
   * To access /metrics you could do:
   * curl -X GET user:password@localhost:9091/metrics
   */
  // authenticate: req => req.headers.authorization === 'Basic dXNlcjpwYXNzd29yZA==',
  /**
   * Uncommenting the `extraMasks` config will use the list of regexes to
   * reformat URL path names and replace the values found with a placeholder value
  */
  // extraMasks: [/..:..:..:..:..:../],
  /**
   * The prefix option will cause all metrics to have the given prefix.
   * E.g.: `app_prefix_http_requests_total`
   */
  // prefix: 'app_prefix_',
  /**
   * Can add custom labels with customLabels and transformLabels options
   */
  // customLabels: ['contentType'],
  // transformLabels(labels, req) {
  //   // eslint-disable-next-line no-param-reassign
  //   labels.contentType = req.headers['content-type'];
  // },
}));

// curl -X GET localhost:9091/hello?name=Chuck%20Norris
app.get('/hello', (req, res) => {
  console.log('GET /hello');
  const { name = 'Anon' } = req.query;
  res.json({ message: `Hello, ${name}!` });
});

app.listen(PORT, () => {
  console.log(`Example api is listening on http://localhost:${PORT}`);
});

Metrics exposed

• Default metrics from prom-client
• http_requests_total: Counter for total requests received, has labels route, method, status
• http_request_duration_seconds: - Duration of HTTP requests in seconds, has labels route, method, status

The labels route and status are normalized:

• route: will normalize id like route params
• status: will normalize to status code family groups, like 2XX or 4XX.

Example prometheus queries

In the examples below, Suppose you tagged your application as "myapp", in the prometheus scrapping config.

Running instances

sum(up{app="myapp"})

Overall error rate

Rate of http status code 5XX responses

sum(rate(http_requests_total{status="5XX", app="myapp"}[5m]))

95% of requests served within seconds

histogram_quantile(0.95, sum(rate(http_request_duration_seconds_bucket{app="myapp"}[5m])) by (le))

Average response time in seconds

sum(rate(http_request_duration_seconds_sum{app="myapp"}[5m])) by (instance) / sum(rate(http_request_duration_seconds_count{app="myapp"}[5m])) by (instance)

Overall Request rate

sum(rate(http_requests_total{app="myapp"}[5m])) by (instance)

Request rate by route

In this example we are removing some health/status-check routes, replace them with your needs.

sum(rate(http_requests_total{app="myapp", route!~"/|/healthz"}[5m])) by (instance, route)

CPU usage

rate(process_cpu_system_seconds_total{app="myapp"}[5m])
rate(process_cpu_user_seconds_total{app="myapp"}[5m])

Memory usage

nodejs_heap_size_total_bytes{app="myapp"}
nodejs_heap_size_used_bytes{app="myapp"}