Merge pull request #455 from prometheus/beorn7/desc

Add a DescribeByCollect helper
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Björn Rabenstein 2018-09-05 14:55:05 +02:00 committed by GitHub
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2 changed files with 93 additions and 0 deletions

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@ -61,6 +61,37 @@ type Collector interface {
Collect(chan<- Metric) Collect(chan<- Metric)
} }
// DescribeByCollect is a helper to implement the Describe method of a custom
// Collector. It collects the metrics from the provided Collector and sends
// their descriptors to the provided channel.
//
// If a Collector collects the same metrics throughout its lifetime, its
// Describe method can simply be implemented as:
//
// func (c customCollector) Describe(ch chan<- *Desc) {
// DescribeByCollect(c, ch)
// }
//
// However, this will not work if the metrics collected change dynamically over
// the lifetime of the Collector in a way that their combined set of descriptors
// changes as well. The shortcut implementation will then violate the contract
// of the Describe method. If a Collector sometimes collects no metrics at all
// (for example vectors like CounterVec, GaugeVec, etc., which only collect
// metrics after a metric with a fully specified label set has been accessed),
// it might even get registered as an unchecked Collecter (cf. the Register
// method of the Registerer interface). Hence, only use this shortcut
// implementation of Describe if you are certain to fulfill the contract.
func DescribeByCollect(c Collector, descs chan<- *Desc) {
metrics := make(chan Metric)
go func() {
c.Collect(metrics)
close(metrics)
}()
for m := range metrics {
descs <- m.Desc()
}
}
// selfCollector implements Collector for a single Metric so that the Metric // selfCollector implements Collector for a single Metric so that the Metric
// collects itself. Add it as an anonymous field to a struct that implements // collects itself. Add it as an anonymous field to a struct that implements
// Metric, and call init with the Metric itself as an argument. // Metric, and call init with the Metric itself as an argument.

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@ -0,0 +1,62 @@
// 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
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package prometheus
import "testing"
type collectorDescribedByCollect struct {
cnt Counter
gge Gauge
}
func (c collectorDescribedByCollect) Collect(ch chan<- Metric) {
ch <- c.cnt
ch <- c.gge
}
func (c collectorDescribedByCollect) Describe(ch chan<- *Desc) {
DescribeByCollect(c, ch)
}
func TestDescribeByCollect(t *testing.T) {
goodCollector := collectorDescribedByCollect{
cnt: NewCounter(CounterOpts{Name: "c1", Help: "help c1"}),
gge: NewGauge(GaugeOpts{Name: "g1", Help: "help g1"}),
}
collidingCollector := collectorDescribedByCollect{
cnt: NewCounter(CounterOpts{Name: "c2", Help: "help c2"}),
gge: NewGauge(GaugeOpts{Name: "g1", Help: "help g1"}),
}
inconsistentCollector := collectorDescribedByCollect{
cnt: NewCounter(CounterOpts{Name: "c3", Help: "help c3"}),
gge: NewGauge(GaugeOpts{Name: "c3", Help: "help inconsistent"}),
}
reg := NewPedanticRegistry()
if err := reg.Register(goodCollector); err != nil {
t.Error("registration failed:", err)
}
if err := reg.Register(collidingCollector); err == nil {
t.Error("registration unexpectedly succeeded")
}
if err := reg.Register(inconsistentCollector); err == nil {
t.Error("registration unexpectedly succeeded")
}
if _, err := reg.Gather(); err != nil {
t.Error("gathering failed:", err)
}
}