Creating a Metricset

A metricset is the part of a Metricbeat module that fetches and structures the data from the remote service. Each module can have multiple metricsets. In this guide, you learn how to create your own metricset.

When creating a metricset for the first time, it generally helps to look at the implementation of existing metricsets for inspiration.

To create a new metricset:

After running the mage commands, you’ll find the metricset, along with its generated files, under module/{module}/{metricset}. This directory contains the following files:

Let’s look at the files in more detail next.

The first file is {metricset}.go. It contains the logic on how to fetch data from the service and convert it for sending to the output.

The generated file looks like this:

https://github.com/elastic/beats/blob/main/metricbeat/scripts/module/metricset/metricset.go.tmpl

package {metricset}

import (
    "github.com/elastic/elastic-agent-libs/mapstr"
	"github.com/elastic/beats/v7/libbeat/common/cfgwarn"
	"github.com/elastic/beats/v7/metricbeat/mb"
)

// init registers the MetricSet with the central registry as soon as the program
// starts. The New function will be called later to instantiate an instance of
// the MetricSet for each host is defined in the module's configuration. After the
// MetricSet has been created then Fetch will begin to be called periodically.
func init() {
	mb.Registry.MustAddMetricSet("{module}", "{metricset}", New)
}

// MetricSet holds any configuration or state information. It must implement
// the mb.MetricSet interface. And this is best achieved by embedding
// mb.BaseMetricSet because it implements all of the required mb.MetricSet
// interface methods except for Fetch.
type MetricSet struct {
	mb.BaseMetricSet
	counter int
}

// New creates a new instance of the MetricSet. New is responsible for unpacking
// any MetricSet specific configuration options if there are any.
func New(base mb.BaseMetricSet) (mb.MetricSet, error) {
	cfgwarn.Beta("The {module} {metricset} metricset is beta.")

	config := struct{}{}
	if err := base.Module().UnpackConfig(&config); err != nil {
		return nil, err
	}

	return &MetricSet{
		BaseMetricSet: base,
		counter:       1,
	}, nil
}

// Fetch method implements the data gathering and data conversion to the right
// format. It publishes the event which is then forwarded to the output. In case
// of an error set the Error field of mb.Event or simply call report.Error().
func (m *MetricSet) Fetch(report mb.ReporterV2) error {
	report.Event(mb.Event{
		MetricSetFields: mapstr.M{
			"counter": m.counter,
		},
	})
	m.counter++

	return nil
}

The package clause and import declaration are part of the base structure of each Go file. You should only modify this part of the file if your implementation requires more imports.

The init method registers the metricset with the central registry. In Go the init() function is called before the execution of all other code. This means the module will be automatically registered with the global registry.

The New method, which is passed to MustAddMetricSet, will be called after the setup of the module and before starting to fetch data. You normally don’t need to change this part of the file.

func init() {
	mb.Registry.MustAddMetricSet("{module}", "{metricset}", New)
}

The MetricSet type defines all fields of the metricset. As a minimum it must be composed of the mb.BaseMetricSet fields, but can be extended with additional entries. These variables can be used to persist data or configuration between multiple fetch calls.

You can add more fields to the MetricSet type, as you can see in the following example where the username and password string fields are added:

type MetricSet struct {
	mb.BaseMetricSet
	username    string
	password    string
}

The New function creates a new instance of the MetricSet. The setup process of the MetricSet is also part of New. This method will be called before Fetch is called the first time.

The New function also sets up the configuration by processing additional configuration entries, if needed.

func New(base mb.BaseMetricSet) (mb.MetricSet, error) {

	config := struct{}{}

	if err := base.Module().UnpackConfig(&config); err != nil {
		return nil, err
	}

	return &MetricSet{
		BaseMetricSet: base,
	}, nil
}

The Fetch method is the central part of the metricset. Fetch is called every time new data is retrieved. If more than one host is defined, Fetch is called once for each host. The frequency of calling Fetch is based on the period defined in the configuration file.

Fetch must publish the event using the mb.ReporterV2.Event method. If an error happens, Fetch can return an error, or if Event is being called in a loop, published using the mb.ReporterV2.Error method. This means that Metricbeat always sends an event, even on failure. You must make sure that the error message helps to identify the actual error.

The following example shows a metricset Fetch method with a counter that is incremented for each Fetch call:

func (m *MetricSet) Fetch(report mb.ReporterV2) error {

	report.Event(mb.Event{
		MetricSetFields: common.MapStr{
			"counter": m.counter,
		}
	})
	m.counter++

	return nil
}

The JSON output derived from the reported event will be identical to the naming and structure you use in common.MapStr. For more details about MapStr and its functions, see the MapStr API docs.

Event can be called multiple times inside of the Fetch method for metricsets that might expose multiple events. Event returns a bool that indicates if the metricset is already closed and no further events can be processed, in which case Fetch should return immediately. If there is an error while processing one of many events, it can be published using the mb.ReporterV2.Error method, as opposed to returning an error value.

In Metricbeat we aim to normalize the metric names from all metricsets to respect a common set of conventions. This makes it easy for users to find and interpret metrics. To simplify parsing, converting, renaming, and restructuring of the object read from the monitored system to the Metricbeat format, we have created the schema package that allows you to declaratively define transformations.

For example, assuming this input object:

input := map[string]interface{}{
	"testString":     "hello",
	"testInt":        "42",
	"testBool":       "true",
	"testFloat":      "42.1",
	"testObjString":  "hello, object",
}

And the requirement to transform it into this one:

common.MapStr{
	"test_string": "hello",
	"test_int":    int64(42),
	"test_bool":   true,
	"test_float":  42.1,
	"test_obj": common.MapStr{
		"test_obj_string": "hello, object",
	},
}

You can use the schema package to transform the data, and optionally mark some fields in a schema as required or not. For example:

import (
	s "github.com/elastic/beats/libbeat/common/schema"
	c "github.com/elastic/beats/libbeat/common/schema/mapstrstr"
)

var (
	schema = s.Schema{
		"test_string": c.Str("testString", s.Required), 
		"test_int":    c.Int("testInt"), 
		"test_bool":   c.Bool("testBool", s.Optional), 
		"test_float":  c.Float("testFloat"),
		"test_obj": s.Object{
			"test_obj_string": c.Str("testObjString", s.IgnoreAllErrors), 
		},
	}
)

func eventMapping(input map[string]interface{}) common.MapStr {
	return schema.Apply(input) 
}

In the above example, note that it is possible to create the schema object once and apply it to all events. You can also use ApplyTo to add additional data to an existing MapStr object:

var (
	schema = s.Schema{
		"test_string": c.Str("testString"),
		"test_int":    c.Int("testInt"),
		"test_bool":   c.Bool("testBool"),
		"test_float":  c.Float("testFloat"),
		"test_obj": s.Object{
			"test_obj_string": c.Str("testObjString"),
		},
	}

	additionalSchema = s.Schema{
		"second_string": c.Str("secondString"),
		"second_int": c.Int("secondInt"),
	}
)

	data, err := schema.Apply(input)
	if err != nil {
		return err
	}

	if m.parseMoreData{
		_, err := additionalSchema.ApplyTo(data, input)
		if len(err) > 0 { 
			return err.Err()
		}
	}

The configuration file for a metricset is handled by the module. If there are multiple metricsets in one module, make sure you add all metricsets to the configuration. For example:

metricbeat:
  modules:
    - module: {module-name}
      metricsets: ["{metricset1}", "{metricset2}"]

For more details about the Metricbeat configuration file, see the topic about Modules in the Metricbeat documentation.

This topic provides basic steps for creating a metricset. For more details about metricsets and how to extend your metricset further, see Metricset Details.