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// Copyright 2016 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 prometheus
import (
//lint:ignore SA1019 Need to keep deprecated package for compatibility.
dto ""
// Desc is the descriptor used by every Prometheus Metric. It is essentially
// the immutable meta-data of a Metric. The normal Metric implementations
// included in this package manage their Desc under the hood. Users only have to
// deal with Desc if they use advanced features like the ExpvarCollector or
// custom Collectors and Metrics.
// Descriptors registered with the same registry have to fulfill certain
// consistency and uniqueness criteria if they share the same fully-qualified
// name: They must have the same help string and the same label names (aka label
// dimensions) in each, constLabels and variableLabels, but they must differ in
// the values of the constLabels.
// Descriptors that share the same fully-qualified names and the same label
// values of their constLabels are considered equal.
// Use NewDesc to create new Desc instances.
type Desc struct {
// fqName has been built from Namespace, Subsystem, and Name.
fqName string
// help provides some helpful information about this metric.
help string
// constLabelPairs contains precalculated DTO label pairs based on
// the constant labels.
constLabelPairs []*dto.LabelPair
// variableLabels contains names of labels for which the metric
// maintains variable values.
variableLabels []string
// id is a hash of the values of the ConstLabels and fqName. This
// must be unique among all registered descriptors and can therefore be
// used as an identifier of the descriptor.
id uint64
// dimHash is a hash of the label names (preset and variable) and the
// Help string. Each Desc with the same fqName must have the same
// dimHash.
dimHash uint64
// err is an error that occurred during construction. It is reported on
// registration time.
err error
// NewDesc allocates and initializes a new Desc. Errors are recorded in the Desc
// and will be reported on registration time. variableLabels and constLabels can
// be nil if no such labels should be set. fqName must not be empty.
// variableLabels only contain the label names. Their label values are variable
// and therefore not part of the Desc. (They are managed within the Metric.)
// For constLabels, the label values are constant. Therefore, they are fully
// specified in the Desc. See the Collector example for a usage pattern.
func NewDesc(fqName, help string, variableLabels []string, constLabels Labels) *Desc {
d := &Desc{
fqName: fqName,
help: help,
variableLabels: variableLabels,
if !model.IsValidMetricName(model.LabelValue(fqName)) {
d.err = fmt.Errorf("%q is not a valid metric name", fqName)
return d
// labelValues contains the label values of const labels (in order of
// their sorted label names) plus the fqName (at position 0).
labelValues := make([]string, 1, len(constLabels)+1)
labelValues[0] = fqName
labelNames := make([]string, 0, len(constLabels)+len(variableLabels))
labelNameSet := map[string]struct{}{}
// First add only the const label names and sort them...
for labelName := range constLabels {
if !checkLabelName(labelName) {
d.err = fmt.Errorf("%q is not a valid label name for metric %q", labelName, fqName)
return d
labelNames = append(labelNames, labelName)
labelNameSet[labelName] = struct{}{}
// ... so that we can now add const label values in the order of their names.
for _, labelName := range labelNames {
labelValues = append(labelValues, constLabels[labelName])
// Validate the const label values. They can't have a wrong cardinality, so
// use in len(labelValues) as expectedNumberOfValues.
if err := validateLabelValues(labelValues, len(labelValues)); err != nil {
d.err = err
return d
// Now add the variable label names, but prefix them with something that
// cannot be in a regular label name. That prevents matching the label
// dimension with a different mix between preset and variable labels.
for _, labelName := range variableLabels {
if !checkLabelName(labelName) {
d.err = fmt.Errorf("%q is not a valid label name for metric %q", labelName, fqName)
return d
labelNames = append(labelNames, "$"+labelName)
labelNameSet[labelName] = struct{}{}
if len(labelNames) != len(labelNameSet) {
d.err = errors.New("duplicate label names")
return d
xxh := xxhash.New()
for _, val := range labelValues {
} = xxh.Sum64()
// Sort labelNames so that order doesn't matter for the hash.
// Now hash together (in this order) the help string and the sorted
// label names.
for _, labelName := range labelNames {
d.dimHash = xxh.Sum64()
d.constLabelPairs = make([]*dto.LabelPair, 0, len(constLabels))
for n, v := range constLabels {
d.constLabelPairs = append(d.constLabelPairs, &dto.LabelPair{
Name: proto.String(n),
Value: proto.String(v),
return d
// NewInvalidDesc returns an invalid descriptor, i.e. a descriptor with the
// provided error set. If a collector returning such a descriptor is registered,
// registration will fail with the provided error. NewInvalidDesc can be used by
// a Collector to signal inability to describe itself.
func NewInvalidDesc(err error) *Desc {
return &Desc{
err: err,
func (d *Desc) String() string {
lpStrings := make([]string, 0, len(d.constLabelPairs))
for _, lp := range d.constLabelPairs {
lpStrings = append(
fmt.Sprintf("%s=%q", lp.GetName(), lp.GetValue()),
return fmt.Sprintf(
"Desc{fqName: %q, help: %q, constLabels: {%s}, variableLabels: %v}",
strings.Join(lpStrings, ","),
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