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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).

Internally it uses prom-client. See: https://github.com/siimon/prom-client

Included metrics:

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


npm install 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 importent 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.

Extra transformation callbacks:

  • normalizePath: function(req) generates path values from express req (see details below)
  • 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

Other options:

  • buckets: buckets used for http_request_duration_seconds histogram
  • autoregister: if /metrics endpoint should be registered. (Default: true)


  • whitelist, blacklist: array of strings or regexp specifying which metrics to include/exclude (there are only 2 metrics)
  • excludeRoutes: array of strings or regexp specifying which routes should be skipped for http_request_duration_seconds metric. It uses req.originalUrl as subject when checking. You want to use express or meddleware features instead of this option.
  • httpDurationMetricName: name of the request duration histogram metric. (Default: http_request_duration_seconds)

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.

You can override this magical behavior and define your own function by providing an optional callback using normalizePath option. You can also replace the default normalizePath function globally.

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 (no matter which file) as a single path
  return path.match(/^\/docs/) ? '/docs/*' : path;

For more details:

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 kraken.js

Here is meddleware config sample, which can be used in a standard kraken.js application:

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


  • 3.2.0

    • added options customLabels, transformLabels
    • upgrade prom-client to 10.1.0
  • 3.1.0

    • upgrade prom-client to 10.0.0
  • 3.0.0

    • upgrade dependencies, most notably prom-client to 9.0.0
    • switch to koa v2 in koa unittest
    • only node v6 or higher is supported (stop supporting node v4 and v5)
    • switch to npm5 and use package-lock.json
    • options added: includeStatusCode, formatStatusCode
  • 2.1.0

    • deprecate excludeRoutes, use req.originalUrl instead of req.path
  • 2.0.0

  • 1.2.1

    • upgrade prom-client to 6.1.2
    • add options: includeMethod, includePath, keepDefaultMetrics



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