Fluentd
1.0
1.0
  • Introduction
  • Overview
    • Life of a Fluentd event
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    • fluent-package v5 vs td-agent v4
  • Installation
    • Before Installation
    • Install fluent-package
      • RPM Package (Red Hat Linux)
      • DEB Package (Debian/Ubuntu)
      • .dmg Package (macOS)
      • .msi Installer (Windows)
    • Install calyptia-fluentd
      • RPM Package (Red Hat Linux)
      • DEB Package (Debian/Ubuntu)
      • .dmg Package (macOS)
      • .msi Installer (Windows)
    • Install by Ruby Gem
    • Install from Source
    • Post Installation Guide
    • Obsolete Installation
      • Treasure Agent v4 (EOL) Installation
        • Install by RPM Package v4 (Red Hat Linux)
        • Install by DEB Package v4 (Debian/Ubuntu)
        • Install by .dmg Package v4 (macOS)
        • Install by .msi Installer v4 (Windows)
      • Treasure Agent v3 (EOL) Installation
        • Install by RPM Package v3 (Red Hat Linux)
        • Install by DEB Package v3 (Debian/Ubuntu)
        • Install by .dmg Package v3 (macOS)
        • Install by .msi Installer v3 (Windows)
  • Configuration
    • Config File Syntax
    • Config File Syntax (YAML)
    • Routing Examples
    • Config: Common Parameters
    • Config: Parse Section
    • Config: Buffer Section
    • Config: Format Section
    • Config: Extract Section
    • Config: Inject Section
    • Config: Transport Section
    • Config: Storage Section
    • Config: Service Discovery Section
  • Deployment
    • System Configuration
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  • Container Deployment
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  • Monitoring Fluentd
    • Overview
    • Monitoring by Prometheus
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  • Input Plugins
    • tail
    • forward
    • udp
    • tcp
    • unix
    • http
    • syslog
    • exec
    • sample
    • monitor_agent
    • windows_eventlog
  • Output Plugins
    • file
    • forward
    • http
    • exec
    • exec_filter
    • secondary_file
    • copy
    • relabel
    • roundrobin
    • stdout
    • null
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    • elasticsearch
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    • mongo
    • mongo_replset
    • rewrite_tag_filter
    • webhdfs
    • buffer
  • Filter Plugins
    • record_transformer
    • grep
    • parser
    • geoip
    • stdout
  • Parser Plugins
    • regexp
    • apache2
    • apache_error
    • nginx
    • syslog
    • ltsv
    • csv
    • tsv
    • json
    • msgpack
    • multiline
    • none
  • Formatter Plugins
    • out_file
    • json
    • ltsv
    • csv
    • msgpack
    • hash
    • single_value
    • stdout
    • tsv
  • Buffer Plugins
    • memory
    • file
    • file_single
  • Storage Plugins
    • local
  • Service Discovery Plugins
    • static
    • file
    • srv
  • Metrics Plugins
    • local
  • How-to Guides
    • Stream Analytics with Materialize
    • Send Apache Logs to S3
    • Send Apache Logs to Minio
    • Send Apache Logs to Mongodb
    • Send Syslog Data to Graylog
    • Send Syslog Data to InfluxDB
    • Send Syslog Data to Sematext
    • Data Analytics with Treasure Data
    • Data Collection with Hadoop (HDFS)
    • Simple Stream Processing with Fluentd
    • Stream Processing with Norikra
    • Stream Processing with Kinesis
    • Free Alternative To Splunk
    • Email Alerting like Splunk
    • How to Parse Syslog Messages
    • Cloud Data Logging with Raspberry Pi
  • Language Bindings
    • Java
    • Ruby
    • Python
    • Perl
    • PHP
    • Nodejs
    • Scala
  • Plugin Development
    • How to Write Input Plugin
    • How to Write Base Plugin
    • How to Write Buffer Plugin
    • How to Write Filter Plugin
    • How to Write Formatter Plugin
    • How to Write Output Plugin
    • How to Write Parser Plugin
    • How to Write Storage Plugin
    • How to Write Service Discovery Plugin
    • How to Write Tests for Plugin
    • Configuration Parameter Types
    • Upgrade Plugin from v0.12
  • Plugin Helper API
    • Plugin Helper: Child Process
    • Plugin Helper: Compat Parameters
    • Plugin Helper: Event Emitter
    • Plugin Helper: Event Loop
    • Plugin Helper: Extract
    • Plugin Helper: Formatter
    • Plugin Helper: Inject
    • Plugin Helper: Parser
    • Plugin Helper: Record Accessor
    • Plugin Helper: Server
    • Plugin Helper: Socket
    • Plugin Helper: Storage
    • Plugin Helper: Thread
    • Plugin Helper: Timer
    • Plugin Helper: Http Server
    • Plugin Helper: Service Discovery
  • Troubleshooting Guide
  • Appendix
    • Update from v0.12 to v1
    • td-agent v2 vs v3 vs v4
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On this page
  • How To Use Formatters From Plugins
  • Methods
  • Writing Tests
  • Overview of Tests

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  1. Plugin Development

How to Write Formatter Plugin

PreviousHow to Write Filter PluginNextHow to Write Output Plugin

Last updated 3 years ago

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Fluentd supports for output plugins. The plugin filenames starting with formatter_ are registered as Formatter Plugins.

See for more details on the common APIs of all the plugins.

Following is an example of a custom formatter (formatter_my_csv.rb) that outputs events in CSV format. It takes a required parameter called csv_fields and outputs the fields. It assumes that the values of the fields are valid CSV fields.

require 'fluent/plugin/formatter'

module Fluent::Plugin
  class MyCSVFormatter < Formatter
    # Register MyCSVFormatter as 'my_csv'.
    Fluent::Plugin.register_formatter('my_csv', self)

    config_param :csv_fields, :array, value_type: :string

    # This method does further processing. Configuration parameters can be
    # accessed either via `conf` hash or member variables.
    def configure(conf)
      super
    end

    # This is the method that formats the data output.
    def format(tag, time, record)
      values = []

      # Look up each required field and collect them from the record
      @csv_fields.each do |field|
        v = record[field]
        unless v
          log.error "#{field} is missing."
        end
        values << v.to_s
      end

      # Output by joining the fields with a comma
      values.join(',')
    end
  end
end

Save this as formatter_my_csv.rb in a loadable plugin path.

With out_file output plugin:

# ...

<match test>
  @type file
  path /path/to/output/file
  <format>
    @type my_csv
    csv_fields k1,k2
  </format>
</match>

For a matched record e.g. {"k1": 100, "k2": 200}, the output CSV file would look like this:

100,200

How To Use Formatters From Plugins

Formatter plugins are designed to be used from other plugins, like Input, Filter and Output. Formatter plugin helper helps achieve this:

# in class definition
helpers :formatter

# in #configure
@formatter = formatter_create(type: 'json')

# in #filter, #format or ...
es.each do |time, record|
  row = @formatter.format(@tag, time, record)
  # ...
end

Methods

The formatter plugins implement filter method to format the input Hash record as a String object.

#format(tag, time, record)

It receives an event represented by tag, time and record; and, after formatting returns a String object.

Formatter plugins must implement this method.

Writing Tests

Fluentd formatter plugin has one or more points to be tested. Others (parsing configurations, controlling buffers, retries, flushes and many others) are controlled by the Fluentd core.

Fluentd also provides test driver for plugins. You can write tests for your own plugins very easily:

# test/plugin/test_formatter_your_own.rb

require 'test/unit'
require 'fluent/test/driver/formatter'

# your own plugin
require 'fluent/plugin/formatter_your_own'

class FormatterYourOwnTest < Test::Unit::TestCase
  def setup
    # common setup
  end

  CONFIG = %[
    fields a,b,c
  ]

  def create_driver(conf = CONF)
    Fluent::Test::Driver::Formatter.new(Fluent::Plugin::YourOwnFormatter).configure(conf)
  end

  sub_test_case 'configured with invalid configurations' do
    test 'empty' do
      assert_raise(Fluent::ConfigError) do
        create_driver('')
      end
    end
    # ...
  end

  sub_test_case 'plugin will format record' do
    test 'record has a field' do
      d = create_driver(CONFIG)
      tag = 'test'
      time = event_time
      record = { 'message' => 'This is message' }
      formatted = d.instance.format(tag, time, record)
      expected = '...'
      assert_equal(expected, formatted)
    end
  end
end

Overview of Tests

Testing for formatter plugins is mainly for:

  • Validation of configuration parameters (i.e. #configure)

  • Validation of the formatted records

To make testing easy, the plugin test driver provides a logger and the functionality to override the system, parser and other configurations.

The lifecycle of the plugin and its test driver is:

  1. Instantiate plugin driver which then instantiates the plugin

  2. Configure plugin

  3. Run test code

  4. Assert results of tests by data provided by the driver

For:

  • configuration tests, repeat steps # 1-2

  • full feature tests, repeat step # 1-4

See for details.

For more details, see .

If this article is incorrect or outdated, or omits critical information, please . is an open-source project under . All components are available under the Apache 2 License.

pluggable, customizable formats
Plugin Base Class API
Formatter Plugin Helper API
Testing API for Plugins
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