create-metricset.asciidoc

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. If you want to create your own Beat that uses Metricbeat as a library, see [creating-beat-from-metricbeat].

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:

1. Run the following command inside the metricbeat beat directory:

   make create-metricset

   You need Python to run this command, then, you’ll be prompted to enter a module and metricset name. Remember that a module represents the service you want to retrieve metrics from (like Redis) and a metricset is a specific set of grouped metrics (like info on Redis). Only use characters [a-z] and, if required, underscores (_). No other characters are allowed.

   When you run make create-metricset, it creates all the basic files for your metricset, along with the required module files if the module does not already exist. See [creating-metricbeat-module] for more details about the module files.

   Note	We use {metricset}, {module}, and {beat} in this guide as placeholders. You need to replace these with the actual names of your metricset, module, and beat.

   The metricset that you created is already a functioning metricset and can be compiled.

2. Compile your new metricset by running the following command:

   make collect
   make

   The first command, make collect, updates all generated files with the most recent files, data, and meta information from the metricset. The second command, make, compiles your source code and provides you with a binary called metricbeat in the same folder. You can run the binary in debug mode with the following command:

   ./metricbeat -e -d "*"

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

• {metricset}.go

• _meta/docs.asciidoc

• _meta/data.json

• _meta/fields.yml

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

{metricset}.go File

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/master/metricbeat/scripts/module/metricset/metricset.go.tmpl

link:../../metricbeat/scripts/module/metricset/metricset.go.tmpl[role=include]

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.

Initialisation

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 AddMetricSet, 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() {
	if err := mb.Registry.AddMetricSet("{module}", "{metricset}", New); err != nil {
		panic(err)
	}
}

Definition

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
}

Creation

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
}

Fetching

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.

Multi Fetching

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.

Parsing and Normalizing Fields

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), (1)
		"test_int":    c.Int("testInt"), (2)
		"test_bool":   c.Bool("testBool", s.Optional), (3)
		"test_float":  c.Float("testFloat"),
		"test_obj": s.Object{
			"test_obj_string": c.Str("testObjString", s.IgnoreAllErrors), (4)
		},
	}
)

func eventMapping(input map[string]interface{}) common.MapStr {
	return schema.Apply(input) (5)
}

1. Marks a field as required.

2. If a field has no schema option set, it is equivalent to Required.

3. Marks the field as optional.

4. Ignore any value conversion error

5. By default, Apply will fail and return an error if any required field is missing. Using the optional second argument, you can specify how Apply handles different fields of the schema. The possible values are:

   • AllRequired is the default behavior. Returns an error if any required field is missing, including fields that are required because no schema option is set.

   • FailOnRequired will fail if a field explicitly marked as required is missing.

   • NotFoundKeys(cb func([]string)) takes a callback function that will be called with a list of missing keys, allowing for finer-grained error handling.

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 { (1)
			return err.Err()
		}
	}

1. ApplyTo returns a raw MultiError object, making it suitable for finer-grained error handling.

Configuration File

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}"]

Note	Make sure that you run make collect after updating the config file so that your changes are also applied to the global configuration file and the docs.

For more details about the Metricbeat configuration file, see the topic about {metricbeat-ref}/configuration-metricbeat.html[Modules] in the Metricbeat documentation.

What to Do Next

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