gather / export prometheus metrics easily in nodejs
Last updated 7 days ago by balena.io .
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$ cnpm install @balena/node-metrics-gatherer 
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gather and expose prometheus metrics with a simple syntax


diagram of module

Basic Usage:

Writing metrics

Basic usage involves adding a single import and a single line.

import { metrics } from '@balena/node-metrics-gatherer';


// inside some loop which checks sensors...
metrics.gauge('temperature', 28);

Exporting metrics (simple)

Then, you need to make sure the metrics can be served when a request arrives, either on an existing express app, or creating a dedicated express app listening on a given port:

// create a request handler to respond to prometheus pulls, given an
// already-existing express app
app.use('/metrics', metrics.requestHandler());

// OR, create our own app (using port 9337, arbitrarily)
metrics.exportOn(9337, '/metrics');

Optionally, you can provide a function which validates whether the requeest should be served or given a 403, by returning a boolean (an "authFunc"):

const authFunc = (req) => req.get('Authorization') === 'Basic 123456');
app.use('/metrics', metrics.requestHandler(authFunc));

// OR, if creating our own app (using port 9337, arbitrarily)
metrics.exportOn(9337, '/metrics', metrics.requestHandler(authFunc));

Exporting metrics (cluster)

If an application forks several child workers and they each listen on :80/metrics, each worker will have a random chance of being hit, only exporting their own metrics each time, causing instability and confusion. Thankfully, prom-client provides a way to handle this, with a registry you create just after you fork. Internally, it handles message-passing between the workers and the main process, where the metrics are aggregated. The examples/ folder in that repo has (as of this writing) an example. This requires a separate express app to be created, listening on a port which isn't participating in the worker cluster pooling.

See below an example usage:

(port 9337 chosen arbitrarily)

if (cluster.isMaster) {
	for (let i = 0; i < 4; i++) {
	metrics.listenAndExport(9337, '/cluster_metrics', metrics.aggregateRequestHandler());

A warning about metrics.describe and Node's cluster module

When you make a call to metrics.describe.counter() (or metrics.describe.histogram(), etc.) , a global registry object is updated with the metric's description. If you call metrics.describe.counter() in the master before forking, the registries inside the workers will not have the metric definition, and because you can write to a metric without first describing it, this means it will simply have all defaults, and not your custom description, labels, buckets, percentiles, etc...

To avoid this, metrics should be described inside the code path that the workers will follow, so that the global registry objects in the workers will have the metric descriptions.

Metric types

See the prometheus documentation

The metric types available as method calls (eg., metrics.summary) correspond to the Prometheus metric types (eg., Prometheus's "Summary")


Used to record a value which can vary over time, like temperature.

metrics.gauge('greenhouse_temperature', temp [, labelObject ]);


Used to record increases to a monotonic counter, like requests served.

metrics.counter('requests_served_total', 1 [, labelObject ]);


Used to calculate (pre-defined) quantiles on a stream of data.

metrics.summary('db_query_duration_milliseconds', queryTime, [, labelObject ]);


Used to calculate a histogram on a stream of data.

metrics.histogram('db_query_duration_milliseconds', queryTime, [, labelObject ]);


There's a convenience method to observe both a histogram and a summary, which will suffix _hist and _summary to the metrics.

metrics.histogramSummary('db_query_duration_milliseconds', queryTime, [, labelObject ]);


Labels can be used to add some more granularity to a metric:

metrics.counter('application_deployments_total', 1, { app: userRequest.appId, method: 'git-push' });
metrics.counter('application_deployments_total', 1, { app: userRequest.appId, method: 'balena-cli-push' });

You must use the full set of labels you intend to attach with each call; for example, you can't have one line add the labels { method: 'git-push' } and another line add the label { result: 'success' }. Labels should be used for things which apply to the metric every time it's invoked (clever use of values like 'unknown' or 'default' can handle cases where you might think you shouldn't add the label).

Labels and time-series cardinality

Use labels sparingly - there will be one time-series created for every pair of label names / values which you create. It would be bad to create a label to track source IP, for example, unless you were really sure that the tsdb could handle it.

As an example, let's say we have two labels, A and B, and they can have values [x, y], and [q, r, s], respectively. Then there would be the following time series:

A=x, B=q
A=x, B=r
A=x, B=s
A=y, B=q
A=y, B=r
A=y, B=s


Descriptions can be used to inform prometheus / the client library about the metric:

Help text

Metrics description calls specify the type of the metric, its name, and a description, at minimum:

    'histogram of total time taken to service requests to the api including queue wait (all queues and all userAgents together)',

Defining the label-set

This is where labels would be specified as well:

    'histogram of total time taken to service requests to the api including queue wait (all queues and all userAgents together)',
        labelNames: ['requestType']

Histogram buckets and Summary percentiles

You can also declare buckets for histogram-type metrics or percentiles for summary-type metrics, if you'd like them to differ from the defaults:

histogram buckets

    'histogram of total time taken to service requests to the api including queue wait',
        buckets: [4, 10, 100, 500, 1000, 5000, 15000, 30000],
        labelNames: ['queue', 'userAgent'],

summary percentiles

    'summary of total time taken to service requests to the api including queue wait',
        percentiles: [0.9, 0.99, 0.999, 0.9999, 0.99999],
        labelNames: ['queue', 'userAgent'],

Clustered aggregation strategy

There are several aggregation strategies which prom-client's AggregatorRegistry can use to combine the metrics recorded by cluster workers. They are:

  • sum
  • first
  • min
  • max
  • average
  • omit

(You can see how they work in the source of prom-client/lib/metricAggregators.js)

In order to control the aggregation strategy (which defaults to 'sum' if unspecified) for a given metric, you can supply an aggregator property to describe() which will determine the method used by the aggregator registry when multiple clustered workers write to that metric.

Let's say we forked 4 workers, each of which observed a given number of connections being active at a time, writing to a Gauge-type metric:

on('connect', () => {
    metrics.gauge('active_connections', nConnections);
on('disconnect', () => {
    metrics.gauge('active_connections', nConnections);

We would probably be content with the default aggregation behaviour (which is to sum the individual results from each worker) to find out the total number of active connections.

However, if we wanted to also know the number of connections which the most-busy worker was handling, to get a measure of how far the busiest worker deviated from the average (which would be total / nWorkers), we could also record a gauge which had been described with the max aggregation strategy:

    'the number of connections being handled by the busiest worker',
        aggregator: 'max'


on('connect', () => {
    metrics.gauge('active_connections', nConnections);
    metrics.gauge('active_connections_max', nConnections);
on('disconnect', () => {
    metrics.gauge('active_connections', nConnections);
    metrics.gauge('active_connections_max', nConnections);

Internal errors

Errors occuring in calls to this library are caught, logged to stderr if the DEBUG env var is defined, and a public property of the MetricsGatherer object will be incremented: internalErrorCount.

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