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// Copyright 2018 The Prometheus Authors
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
// Package testutil provides helpers to test code using the prometheus package
// of client_golang.
// While writing unit tests to verify correct instrumentation of your code, it's
// a common mistake to mostly test the instrumentation library instead of your
// own code. Rather than verifying that a prometheus.Counter's value has changed
// as expected or that it shows up in the exposition after registration, it is
// in general more robust and more faithful to the concept of unit tests to use
// mock implementations of the prometheus.Counter and prometheus.Registerer
// interfaces that simply assert that the Add or Register methods have been
// called with the expected arguments. However, this might be overkill in simple
// scenarios. The ToFloat64 function is provided for simple inspection of a
// single-value metric, but it has to be used with caution.
// End-to-end tests to verify all or larger parts of the metrics exposition can
// be implemented with the CollectAndCompare or GatherAndCompare functions. The
// most appropriate use is not so much testing instrumentation of your code, but
// testing custom prometheus.Collector implementations and in particular whole
// exporters, i.e. programs that retrieve telemetry data from a 3rd party source
// and convert it into Prometheus metrics.
package testutil
import (
dto ""
// ToFloat64 collects all Metrics from the provided Collector. It expects that
// this results in exactly one Metric being collected, which must be a Gauge,
// Counter, or Untyped. In all other cases, ToFloat64 panics. ToFloat64 returns
// the value of the collected Metric.
// The Collector provided is typically a simple instance of Gauge or Counter, or
// – less commonly – a GaugeVec or CounterVec with exactly one element. But any
// Collector fulfilling the prerequisites described above will do.
// Use this function with caution. It is computationally very expensive and thus
// not suited at all to read values from Metrics in regular code. This is really
// only for testing purposes, and even for testing, other approaches are often
// more appropriate (see this package's documentation).
// A clear anti-pattern would be to use a metric type from the prometheus
// package to track values that are also needed for something else than the
// exposition of Prometheus metrics. For example, you would like to track the
// number of items in a queue because your code should reject queuing further
// items if a certain limit is reached. It is tempting to track the number of
// items in a prometheus.Gauge, as it is then easily available as a metric for
// exposition, too. However, then you would need to call ToFloat64 in your
// regular code, potentially quite often. The recommended way is to track the
// number of items conventionally (in the way you would have done it without
// considering Prometheus metrics) and then expose the number with a
// prometheus.GaugeFunc.
func ToFloat64(c prometheus.Collector) float64 {
var (
m prometheus.Metric
mCount int
mChan = make(chan prometheus.Metric)
done = make(chan struct{})
go func() {
for m = range mChan {
if mCount != 1 {
panic(fmt.Errorf("collected %d metrics instead of exactly 1", mCount))
pb := &dto.Metric{}
if pb.Gauge != nil {
return pb.Gauge.GetValue()
if pb.Counter != nil {
return pb.Counter.GetValue()
if pb.Untyped != nil {
return pb.Untyped.GetValue()
panic(fmt.Errorf("collected a non-gauge/counter/untyped metric: %s", pb))
// CollectAndCompare registers the provided Collector with a newly created
// pedantic Registry. It then does the same as GatherAndCompare, gathering the
// metrics from the pedantic Registry.
func CollectAndCompare(c prometheus.Collector, expected io.Reader, metricNames ...string) error {
reg := prometheus.NewPedanticRegistry()
if err := reg.Register(c); err != nil {
return fmt.Errorf("registering collector failed: %s", err)
return GatherAndCompare(reg, expected, metricNames...)
// GatherAndCompare gathers all metrics from the provided Gatherer and compares
// it to an expected output read from the provided Reader in the Prometheus text
// exposition format. If any metricNames are provided, only metrics with those
// names are compared.
func GatherAndCompare(g prometheus.Gatherer, expected io.Reader, metricNames ...string) error {
got, err := g.Gather()
if err != nil {
return fmt.Errorf("gathering metrics failed: %s", err)
if metricNames != nil {
got = filterMetrics(got, metricNames)
var tp expfmt.TextParser
wantRaw, err := tp.TextToMetricFamilies(expected)
if err != nil {
return fmt.Errorf("parsing expected metrics failed: %s", err)
want := internal.NormalizeMetricFamilies(wantRaw)
return compare(got, want)
// compare encodes both provided slices of metric families into the text format,
// compares their string message, and returns an error if they do not match.
// The error contains the encoded text of both the desired and the actual
// result.
func compare(got, want []*dto.MetricFamily) error {
var gotBuf, wantBuf bytes.Buffer
enc := expfmt.NewEncoder(&gotBuf, expfmt.FmtText)
for _, mf := range got {
if err := enc.Encode(mf); err != nil {
return fmt.Errorf("encoding gathered metrics failed: %s", err)
enc = expfmt.NewEncoder(&wantBuf, expfmt.FmtText)
for _, mf := range want {
if err := enc.Encode(mf); err != nil {
return fmt.Errorf("encoding expected metrics failed: %s", err)
if wantBuf.String() != gotBuf.String() {
return fmt.Errorf(`
metric output does not match expectation; want:
%s`, wantBuf.String(), gotBuf.String())
return nil
func filterMetrics(metrics []*dto.MetricFamily, names []string) []*dto.MetricFamily {
var filtered []*dto.MetricFamily
for _, m := range metrics {
for _, name := range names {
if m.GetName() == name {
filtered = append(filtered, m)
return filtered
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