Development helpers for mackerel agent plugin
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itchyny Merge pull request #31 from mackerelio/go-1.9
Upgrade Go version to 1.9
Latest commit 212a27a Feb 9, 2018

go-mackerel-plugin-helper Build Status

This package provides helper methods to create mackerel agent plugin easily.

Recommend to use go-mackerel-plugin

We recommend to use go-mackerel-plugin instead of go-mackerel-plugin-helper to create mackerel agent plugin. It is because you can create a plugin more simply by go-mackerel-plugin than by go-mackerel-plugin-helper.

How to use

Graph Definition

A plugin can specify Graphs and Metrics. Graphs represents one graph and includes some Metricss which represent each line.

Graphs includes followings:

  • Label: Label for the graph
  • Unit: Unit for lines, integer, float can be specified.
  • Metrics: Array of Metrics which represents each line.

Metics includes followings:

  • Name: Key of the line
  • Label: Label of the line
  • Diff: If Diff is true, differential is used as value.
  • Type: 'float64', 'uint64' or 'uint32' can be specified. Default is float64
  • Stacked: If Stacked is true, the line is stacked.
  • Scale: Each value is multiplied by Scale.
var graphdef = map[string](mackerelplugin.Graphs){
	"memcached.cmd": {
		Label: "Memcached Command",
		Unit:  "integer",
		Metrics: [](mackerelplugin.Metrics){
			{Name: "cmd_get", Label: "Get", Diff: true, Type: "uint64"},
			{Name: "cmd_set", Label: "Set", Diff: true, Type: "uint64"},
			{Name: "cmd_flush", Label: "Flush", Diff: true, Type: "uint64"},
			{Name: "cmd_touch", Label: "Touch", Diff: true, Type: "uint64"},

Calculate Differential of Counter

Many status values of popular middle-wares are provided as counter. But current Mackerel API can accept only absolute values, so differential values must be calculated beside agent plugins.

Diff of Metrics is a flag whether values must be treated as counter or not. If this flag is set, this package calculate differential values automatically with current values and previous values, which are saved to a temporally file.

Adjust Scale Value

Some status values such as jstat memory usage are provided as scaled values. For example, OGC value are provided KB scale.

Scale of Metrics is a multiplier for adjustment of the scale values.

var graphdef = map[string](mackerelplugin.Graphs){
    "jvm.old_space": {
        Label: "JVM Old Space memory",
        Unit:  "float",
        Metrics: [](mackerelplugin.Metrics){
            {Name: "OGCMX", Label: "Old max", Diff: false, Scale: 1024},
            {Name: "OGC", Label: "Old current", Diff: false, Scale: 1024},
            {Name: "OU", Label: "Old used", Diff: false, Scale: 1024},

Deal with counter overflow

If Type of metrics is uint64 or uint32 and Diff is true, the helper check counter overflow. When differential value is negative, overflow or counter reset may be occurred. If the differential value is ten-times above last value, the helper judge this is counter reset, not counter overflow, then the helper set value is unknown. If not, the helper recognizes counter overflow occurred.


MackerelPlugin interface has Tempfile field. The Tempfile is used to calculate differences in metrics with Diff: true. If this field is omitted, the filename of the temporaty file is automatically generated from plugin filename.

Default value of Tempfile

mackerel-agent's plugins should place its Tempfile under os.Getenv("MACKEREL_PLUGIN_WORKDIR") unless specified explicitly. Since this helper handles the environmental value, it's recommended not to set default Tempfile path. But if a plugin wants to set default Tempfile filename by itself, use MackerelPlugin.SetTempfileByBasename(), which sets Tempfile path considering the environmental value.

  helper.Tempfile = *optTempfile
  if optTempfile == nil {


A plugin must implement this interface and the main method.

type PluginWithPrefix interface {
	FetchMetrics() (map[string]interface{}, error)
	GraphDefinition() map[string]Graphs
	MetricKeyPrefix() string
func main() {
	optHost := flag.String("host", "localhost", "Hostname")
	optPort := flag.String("port", "11211", "Port")
	optTempfile := flag.String("tempfile", "", "Temp file name")
    optMetricKeyPrefix := flag.String("metric-key-prefix", "memcached", "Metric Key Prefix")

	var memcached MemcachedPlugin

	memcached.Target = fmt.Sprintf("%s:%s", *optHost, *optPort)
    memcached.prefix = *optMetricKeyPrefix
	helper := mackerelplugin.NewMackerelPlugin(memcached)
	helper.Tempfile = *optTempfile


old Plugin interface

Plugin interface is old one. PluginWithPrefix interface is recommended now.