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express prometheus bundle

Express middleware with popular prometheus metrics in one bundle. It's also compatible with koa v1 and v2 (see below).

This library uses prom-client v15+ as a peer dependency. See:

If you need a support for older versions of prom-client (v12-v14), downgrade to express-prom-bundle v6.6.0

Included metrics:

  • up: normally is just 1
  • http_request_duration_seconds: http latency histogram/summary labeled with status_code, method and path


npm install prom-client express-prom-bundle

Sample Usage

const promBundle = require("express-prom-bundle");
const app = require("express")();
const metricsMiddleware = promBundle({includeMethod: true});

app.use(/* your middleware */);


The order in which the routes are registered is important, since only the routes registered after the express-prom-bundle will be measured

You can use this to your advantage to bypass some of the routes. See the example below.


Which labels to include in http_request_duration_seconds metric:

  • includeStatusCode: HTTP status code (200, 400, 404 etc.), default: true
  • includeMethod: HTTP method (GET, PUT, ...), default: false
  • includePath: URL path (see important details below), default: false
  • customLabels: an object containing extra labels, e.g. {project_name: 'hello_world'}. Most useful together with transformLabels callback, otherwise it's better to use native Prometheus relabeling.
  • includeUp: include an auxiliary "up"-metric which always returns 1, default: true
  • metricsPath: replace the /metrics route with a regex or exact string. Note: it is highly recommended to just stick to the default
  • metricType: histogram/summary selection. See more details below
  • httpDurationMetricName: Allows you change the name of HTTP duration metric, default: http_request_duration_seconds.

metricType option

Two metric types are supported for http_request_duration_seconds metric:

Additional options for histogram:

  • buckets: buckets used for the http_request_duration_seconds histogram

Additional options for summary:

  • percentiles: percentiles used for http_request_duration_seconds summary
  • ageBuckets: ageBuckets configures how many buckets we have in our sliding window for the summary
  • maxAgeSeconds: the maxAgeSeconds will tell how old a bucket can be before it is reset
  • pruneAgedBuckets: When enabled, timed out buckets will be removed entirely. By default, buckets are reset to 0.

Transformation callbacks

  • normalizePath: function(req) or Array
    • if function is provided, then it should generate path value from express req
    • if array is provided, then it should be an array of tuples [regex, replacement]. The regex can be a string and is automatically converted into JS regex.
    • ... see more details in the section below
  • urlValueParser: options passed when instantiating url-value-parser. This is the easiest way to customize which parts of the URL should be replaced with "#val". See the docs of url-value-parser module for details.
  • formatStatusCode: function(res) producing final status code from express res object, e.g. you can combine 200, 201 and 204 to just 2xx.
  • transformLabels: function(labels, req, res) transforms the labels object, e.g. setting dynamic values to customLabels
  • urlPathReplacement: replacement string for the values (default: "#val")

Other options

  • autoregister: if /metrics endpoint should be registered (default: true)

  • promClient: options for promClient startup, e.g. collectDefaultMetrics. This option was added to keep express-prom-bundle runnable using confit (e.g. with kraken.js) without writing any JS code, see advanced example

  • promRegistry: Optional promClient.Registry instance to attach metrics to. Defaults to global promClient.register.

  • metricsApp: Allows you to attach the metrics endpoint to a different express app. You probably want to use it in combination with autoregister: false.

  • bypass: An object that takes onRequest and onFinish callbacks that determines whether the given request should be excluded in the metrics. Default:

      onRequest: (req) => false,
      onFinish: (req, res) => false

    onRequest is run directly in the middleware chain, before the request is processed. onFinish is run after the request has been processed, and has access to the express response object in addition to the request object. Both callbacks are optional, and if one or both returns true the request is excluded.

    As a shorthand, just the onRequest callback can be used instead of the object.

More details on includePath option

Let's say you want to have latency statistics by URL path, e.g. separate metrics for /my-app/user/, /products/by-category etc.

Just taking req.path as a label value won't work as IDs are often part of the URL, like /user/12352/profile. So what we actually need is a path template. The module tries to figure out what parts of the path are values or IDs, and what is an actual path. The example mentioned before would be normalized to /user/#val/profile and that will become the value for the label. These conversions are handled by normalizePath function.

You can extend this magical behavior by providing additional RegExp rules to be performed, or override normalizePath with your own function.

Example 1 (add custom RegExp):

  normalizePath: [
    // collect paths like "/customer/johnbobson" as just one "/custom/#name"
    ['^/customer/.*', '/customer/#name'],

    // collect paths like "/bobjohnson/order-list" as just one "/#name/order-list"
    ['^.*/order-list', '/#name/order-list']
  urlValueParser: {
    minHexLength: 5,
    extraMasks: [
      'ORD[0-9]{5,}' // replace strings like ORD1243423, ORD673562 as #val

Example 2 (override normalizePath function):

app.use(promBundle(/* options? */));

// let's reuse the existing one and just add some
// functionality on top
const originalNormalize = promBundle.normalizePath;
promBundle.normalizePath = (req, opts) => {
  const path = originalNormalize(req, opts);
  // count all docs as one path, but /docs/login as a separate one
  return (path.match(/^\/docs/) && !path.match(/^\/login/)) ? '/docs/*' : path;

For more details:

Example 3 (return express route definition):

app.use(promBundle(/* options? */));

promBundle.normalizePath = (req, opts) => {
  // Return the path of the express route (i.e. /v1/user/:id or /v1/timer/automated/:userid/:timerid")
  return req.route?.path ?? "NULL";

express example

setup std. metrics but exclude up-metric:

const express = require("express");
const app = express();
const promBundle = require("express-prom-bundle");

// calls to this route will not appear in metrics
// because it's applied before promBundle
app.get("/status", (req, res) => res.send("i am healthy"));

// register metrics collection for all routes
// ... except those starting with /foo
app.use("/((?!foo))*", promBundle({includePath: true}));

// this call will NOT appear in metrics,
// because express will skip the metrics middleware
app.get("/foo", (req, res) => res.send("bar"));

// calls to this route will appear in metrics
app.get("/hello", (req, res) => res.send("ok"));


See an advanced example on github

koa v2 example

const promBundle = require("express-prom-bundle");
const Koa = require("koa");
const c2k = require("koa-connect");
const metricsMiddleware = promBundle({/* options */ });

const app = new Koa();

app.use(/* your middleware */);

using with cluster

You'll need to use an additional clusterMetrics() middleware.

In the example below the master process will expose an API with a single endpoint /metrics which returns an aggregate of all metrics from all the workers.

const cluster = require('cluster');
const promBundle = require('express-prom-bundle');
const promClient = require('prom-client');
const numCPUs = Math.max(2, require('os').cpus().length);
const express = require('express');

if (cluster.isMaster) {
    for (let i = 1; i < numCPUs; i++) {

    const metricsApp = express();
    metricsApp.use('/metrics', promBundle.clusterMetrics());

    console.log('cluster metrics listening on 9999');
    console.log('call localhost:9999/metrics for aggregated metrics');
} else {
    new promClient.AggregatorRegistry();
    const app = express();
        autoregister: false, // disable /metrics for single workers
        includeMethod: true
    app.use((req, res) => res.send(`hello from pid ${}\n`));
    console.log(`worker ${} listening on 3000`);

using with kraken.js

Here is meddleware config sample, which can be used in a standard kraken.js application. In this case the stats for URI paths and HTTP methods are collected separately, while replacing all HEX values starting from 5 characters and all IP addresses in the path as #val.

  "middleware": {
    "expressPromBundle": {
      "route": "/((?!status|favicon.ico|robots.txt))*",
      "priority": 0,
      "module": {
        "name": "express-prom-bundle",
        "arguments": [
            "includeMethod": true,
            "includePath": true,
            "buckets": [0.1, 1, 5],
            "promClient": {
              "collectDefaultMetrics": {
            "urlValueParser": {
              "minHexLength": 5,
              "extraMasks": [



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