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<h1>PLEASE NOTE: This document applies to the HEAD of the source
tree only. If you are using a released version of Kubernetes, you almost
certainly want the docs that go with that version.</h1>

<strong>Documentation for specific releases can be found at
[releases.k8s.io](http://releases.k8s.io).</strong>

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Instrumenting Kubernetes with a new metric
===================

The following is a step-by-step guide for adding a new metric to the Kubernetes code base.

We use the Prometheus monitoring system's golang client library for instrumenting our code. Once you've picked out a file that you want to add a metric to, you should:

1. Import "github.com/prometheus/client_golang/prometheus".

2. Create a top-level var to define the metric. For this, you have to:
  1. Pick the type of metric. Use a Gauge for things you want to set to a particular value, a Counter for things you want to increment, or a Histogram or Summary for histograms/distributions of values (typically for latency). Histograms are better if you're going to aggregate the values across jobs, while summaries are better if you just want the job to give you a useful summary of the values.
  2. Give the metric a name and description.
  3. Pick whether you want to distinguish different categories of things using labels on the metric. If so, add "Vec" to the name of the type of metric you want and add a slice of the label names to the definition.

   https://github.com/GoogleCloudPlatform/kubernetes/blob/cd3299307d44665564e1a5c77d0daa0286603ff5/pkg/apiserver/apiserver.go#L53
   https://github.com/GoogleCloudPlatform/kubernetes/blob/cd3299307d44665564e1a5c77d0daa0286603ff5/pkg/kubelet/metrics/metrics.go#L31

3. Register the metric so that prometheus will know to export it.

   https://github.com/GoogleCloudPlatform/kubernetes/blob/cd3299307d44665564e1a5c77d0daa0286603ff5/pkg/kubelet/metrics/metrics.go#L74
   https://github.com/GoogleCloudPlatform/kubernetes/blob/cd3299307d44665564e1a5c77d0daa0286603ff5/pkg/apiserver/apiserver.go#L78

4. Use the metric by calling the appropriate method for your metric type (Set, Inc/Add, or Observe, respectively for Gauge, Counter, or Histogram/Summary), first calling WithLabelValues if your metric has any labels

   https://github.com/GoogleCloudPlatform/kubernetes/blob/3ce7fe8310ff081dbbd3d95490193e1d5250d2c9/pkg/kubelet/kubelet.go#L1384
   https://github.com/GoogleCloudPlatform/kubernetes/blob/cd3299307d44665564e1a5c77d0daa0286603ff5/pkg/apiserver/apiserver.go#L87


These are the metric type definitions if you're curious to learn about them or need more information:
https://github.com/prometheus/client_golang/blob/master/prometheus/gauge.go
https://github.com/prometheus/client_golang/blob/master/prometheus/counter.go
https://github.com/prometheus/client_golang/blob/master/prometheus/histogram.go
https://github.com/prometheus/client_golang/blob/master/prometheus/summary.go


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