// Copyright 2014 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_test

import "github.com/prometheus/client_golang/prometheus"

// ClusterManager is an example for a system that might have been built without
// Prometheus in mind. It models a central manager of jobs running in a
// cluster. To turn it into something that collects Prometheus metrics, we
// simply add the two methods required for the Collector interface.
//
// An additional challenge is that multiple instances of the ClusterManager are
// run within the same binary, each in charge of a different zone. We need to
// make use of ConstLabels to be able to register each ClusterManager instance
// with Prometheus.
type ClusterManager struct {
	Zone         string
	OOMCountDesc *prometheus.Desc
	RAMUsageDesc *prometheus.Desc
	// ... many more fields
}

// ReallyExpensiveAssessmentOfTheSystemState is a mock for the data gathering a
// real cluster manager would have to do. Since it may actually be really
// expensive, it must only be called once per collection. This implementation,
// obviously, only returns some made-up data.
func (c *ClusterManager) ReallyExpensiveAssessmentOfTheSystemState() (
	oomCountByHost map[string]int, ramUsageByHost map[string]float64,
) {
	// Just example fake data.
	oomCountByHost = map[string]int{
		"foo.example.org": 42,
		"bar.example.org": 2001,
	}
	ramUsageByHost = map[string]float64{
		"foo.example.org": 6.023e23,
		"bar.example.org": 3.14,
	}
	return
}

// Describe simply sends the two Descs in the struct to the channel.
func (c *ClusterManager) Describe(ch chan<- *prometheus.Desc) {
	ch <- c.OOMCountDesc
	ch <- c.RAMUsageDesc
}

// Collect first triggers the ReallyExpensiveAssessmentOfTheSystemState. Then it
// creates constant metrics for each host on the fly based on the returned data.
//
// Note that Collect could be called concurrently, so we depend on
// ReallyExpensiveAssessmentOfTheSystemState to be concurrency-safe.
func (c *ClusterManager) Collect(ch chan<- prometheus.Metric) {
	oomCountByHost, ramUsageByHost := c.ReallyExpensiveAssessmentOfTheSystemState()
	for host, oomCount := range oomCountByHost {
		ch <- prometheus.MustNewConstMetric(
			c.OOMCountDesc,
			prometheus.CounterValue,
			float64(oomCount),
			host,
		)
	}
	for host, ramUsage := range ramUsageByHost {
		ch <- prometheus.MustNewConstMetric(
			c.RAMUsageDesc,
			prometheus.GaugeValue,
			ramUsage,
			host,
		)
	}
}

// NewClusterManager creates the two Descs OOMCountDesc and RAMUsageDesc. Note
// that the zone is set as a ConstLabel. (It's different in each instance of the
// ClusterManager, but constant over the lifetime of an instance.) Then there is
// a variable label "host", since we want to partition the collected metrics by
// host. Since all Descs created in this way are consistent across instances,
// with a guaranteed distinction by the "zone" label, we can register different
// ClusterManager instances with the same registry.
func NewClusterManager(zone string) *ClusterManager {
	return &ClusterManager{
		Zone: zone,
		OOMCountDesc: prometheus.NewDesc(
			"clustermanager_oom_crashes_total",
			"Number of OOM crashes.",
			[]string{"host"},
			prometheus.Labels{"zone": zone},
		),
		RAMUsageDesc: prometheus.NewDesc(
			"clustermanager_ram_usage_bytes",
			"RAM usage as reported to the cluster manager.",
			[]string{"host"},
			prometheus.Labels{"zone": zone},
		),
	}
}

func ExampleCollector() {
	workerDB := NewClusterManager("db")
	workerCA := NewClusterManager("ca")

	// Since we are dealing with custom Collector implementations, it might
	// be a good idea to try it out with a pedantic registry.
	reg := prometheus.NewPedanticRegistry()
	reg.MustRegister(workerDB)
	reg.MustRegister(workerCA)
}