Development helpers for mackerel agent plugin
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go-mackerel-plugin Build Status

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

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, float, integer, percentage, bytes, bytes/sec, iops 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.
  • Stacked: If Stacked is true, the line is stacked.
  • Scale: Each value is multiplied by Scale.

Example of graph definition.

var graphdef = map[string]mackerelplugin.Graphs{
	"memcached.connections": {
		Label: "Memcached Connections",
		Unit:  "integer",
		Metrics: []mackerelplugin.Metrics{
			{Key: "curr_connections", Label: "Connections", Diff: false},


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


You can find an example implementation in _example/ directory.

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},


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 {