Using go-mackerel-plugin to create a custom metric plugin

This document explains how to develop a plugin using, a helper library for creating custom metric plugins in Go language (also used in the official plugin).

By following this library’s method of development, you can easily output graph definitions, calculate differences from previously obtained values, and so on. Additionally, this library uses the same method of writing as the official plugin, so if you are considering submitting pull requests to the official plugin repository, we highly recommend using this library.

source code structure

When using go-mackerel-plugin, the plugin source code consists of the following five parts.

  1. The package declaration and import statement
  2. The plugin struct definition
  3. The graph definition output method GraphDefinition defined in struct
  4. The metric acquisition method FetchMetrics defined in struct
  5. The metric key prefix acquisition method MetricKeyPrefix defined in struct
  6. The main() function definition

Let's look at each using mackerel-agent-uptime as an example.

1. The package declaration and import statement

package main

import (

    mp ""

Declare the package as main. Additionally, it’s typical to import go-mackerel-plugin under the alias mp.

2. The plugin struct definition

type UptimePlugin struct {
    Prefix string

This is the definition of the plugin struct. In this struct, the Prefix field is defined. This is used to determine the beginning of the metric namespace when outputting graph definitions. The standard Prefix for the uptime plugin is uptime and metrics are output with the key uptime.seconds, but this field is used when you want to change the uptime to uptime2 for example.

In the uptime plugin, the Prefix field is not particularly useful. However, for middleware plugins for example, if you run the same middleware multiple times on one host and you want to obtain each metric, you’ll need to separate the metric namespaces. Therefore, defining this field is recommended.

In the uptime plugin, only this Prefix field is defined, but if it’s a general middleware plugin, the fields Port and Host will also be required.

Additionally, this plugin struct must satisfy the interface of mp.PluginWithPrefix . The interface definition is as follows.

type PluginWithPrefix interface {
    FetchMetrics() (map[string]float64, error)
    GraphDefinition() map[string]Graphs
    MetricKeyPrefix() string

This is the graph definition output method, the metric acquisition method, and the metric prefix acquisition method.

3. The graph definition output method GraphDefinition defined in struct

func (u UptimePlugin) GraphDefinition() map[string]mp.Graphs {
    labelPrefix := strings.Title(u.MetricKeyPrefix())
    return map[string]mp.Graphs{
        "": {
            Label: labelPrefix,
            Unit:  mp.UnitFloat,
            Metrics: []mp.Metrics{
                {Name: "seconds", Label: "Seconds"},

By defining this GraphDefinition, the graph definition JSON and metrics will be output correctly. With the uptime plugin, only one graph definition is returned with the empty string as the key, but many plugins return multiple graph definitions with keys like "runtime", "memory".

When the plugin outputs the list of graph definitions, it attaches the prefix returned by the MetricKeyPrefix() function before each key of the graph definition. For example, if MetricKeyPrefix() returns uptime, it’s output as "" -> "uptime", "runtime" -> "uptime.runtime".

Label is the graph name displayed in Mackerel, Unit is the unit of graph, and "float", "integer", "percentage", "bytes", "bytes / sec", "iops" are able to be specified similar to the graph definition API.

Metrics specifies multiple Metrics definitions to be rendered within the graph. The fields for mp.Metrics are as follows.

Field Type Explanation
Name string (Required) Name of the metric. It corresponds to the key name of the map to be acquired by FetchMetrics()
Label string Name displayed in Mackerel
Diff bool Whether or not to calculate the difference value in the plugin (Default: false)
Stacked bool Whether or not to display stacked in Mackerel (Default: false)
Scale float64 If specified, the obtained value is multiplied by the value of this Scale before outputting. For example, if you want to correct the value obtained with KB into Byte, 1024 should be specified

4. The metric acquisition method FetchMetrics defined in struct

func (u UptimePlugin) FetchMetrics() (map[string]float64, error) {
    ut, err := uptime.Get()
    if err != nil {
        return nil, fmt.Errorf("Failed to fetch uptime metrics: %s", err)
    return map[string]float64{"seconds": ut}, nil

FetchMetrics() returns values in map[string]float64 format. It returns a map with the uptime value stored with the key as seconds. seconds is the specified Name in the GraphDefinition above.

5. The metric key prefix acquisition method MetricKeyPrefix defined in struct

func (u UptimePlugin) MetricKeyPrefix() string {
    if u.Prefix == "" {
        u.Prefix = "uptime"
    return u.Prefix

Define the method for obtaining the metric prefix. If specified by the plugin user, the specified prefix is returned, if not "uptime" is returned by default.

6. The main() function definition

func main() {
    optPrefix := flag.String("metric-key-prefix", "uptime", "Metric key prefix")
    optTempfile := flag.String("tempfile", "", "Temp file name")

    u := UptimePlugin{
        Prefix: *optPrefix,
    plugin := mp.NewMackerelPlugin(u)
    plugin.Tempfile = *optTempfile

This is the plugin’s main procedure. It parses command line options, creates plugin objects, and runs the plugin.

The Tempfile, specified in plugin, is a file that holds the last values obtained for calculating the difference values. Please note that this should be specified so that overlaps do not occur with other plugins or when there are multiple configurations with varying parameters within the same plugin.

This is just a basic explanation on how to create a custom metric plugin using go-mackerel-plugin. Be sure to try to incorporate your favorite package and create your own original plugin.

If you create a useful plugin that other users might find helpful, follow the document “Creating plugins supported with mkr plugin install” to make the plugin available with mkr plugin install and register it in the plugin registry.

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