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How to Write Tests for Plugin
This article explains how to write Fluentd plugin test code using test-unit.
You can write test code with any other testing framework such as RSpec, minitest, etc.

Basics of Plugin Testing

Fluentd provides useful Test Drivers according to plugin type. We can write maintainable test code for plugins using them. We can write helper.rb for output plugin as follows:
$LOAD_PATH.unshift(File.expand_path("../../", __FILE__))
require "test-unit"
require "fluent/test"
require "fluent/test/driver/output"
require "fluent/test/helpers"
Test::Unit::TestCase.include(Fluent::Test::Helpers)
Test::Unit::TestCase.extend(Fluent::Test::Helpers)
Note that Fluentd provides useful Test Drivers for input, output, filter, parser and formatter.
The recommended fluentd plugin project structure is:
.
├── Gemfile
├── LICENSE
├── README.md
├── Rakefile
├── fluent-plugin-<your_fluentd_plugin_name>.gemspec
├── lib
│ └── fluent
│ └── plugin
│ └── <plugin_type>_<your_fluentd_plugin_name>.rb
└── test
├── helper.rb
└── plugin
└── test_<plugin_type>_<your_fluentd_plugin_name>.rb

Plugin Test Driver Overview

There are some useful Test Drivers for plugin testing. We can write test code for plugins as following:
# Load the module that defines common initialization method (Required)
require 'fluent/test'
# Load the module that defines helper methods for testing (Required)
require 'fluent/test/helpers'
# Load the test driver (Required)
require 'fluent/test/driver/output'
# Load the plugin (Required)
require 'fluent/plugin/out_file'
class FileOutputTest < Test::Unit::TestCase
include Fluent::Test::Helpers
def setup
Fluent::Test.setup # Setup test for Fluentd (Required)
# Setup test for plugin (Optional)
# ...
end
def teardown
# Terminate test for plugin (Optional)
end
def create_driver(conf = CONFIG)
Fluent::Test::Driver::Output.new(Fluent::Plugin::FileOutput).configure(conf)
end
# Configuration related test group
sub_test_case 'configuration' do
test 'basic configuration' do
d = create_driver(basic_configuration)
assert_equal 'something', d.instance.parameter_name
end
end
# Another test group goes here
sub_test_case 'path' do
test 'normal' do
d = create_driver('...')
d.run(default_tag: 'test') do
d.feed(event_time, record)
end
events = d.events
assert_equal(1, events.size)
end
end
end

Testing Utility Methods

You can get a plugin instance by calling Test Driver instance's #instance method. If utility methods are private, use __send__.
# ...
class FileOutputTest < Test::Unit::TestCase
# ...
# Group by utility method
sub_test_case '#compression_suffix' do
test 'returns empty string for nil (no compression method specified)' do
d = create_driver
assert_equal('', d.instance.compression_suffix(nil))
end
end
end

Test Driver Base API

The methods in this section are available for all Test Driver.

initialize(klass, opts: {}, &block)

Initializes Test Driver instance.
  • klass: A class of Fluentd plugin
  • opts: Overwrite system config. This parameter is useful for testing multi
    workers.
  • block: Customize plugin behavior. We can overwrite plugin code in this
    block.
Example:
def create_driver(conf={})
d = Fluent::Test::Driver::Output.new(Fluent::Plugin::MyOutput) do
attr_accessor :exceptions
# Almost same as to reopen plugin class
def prefer_buffered_processing
false
end
def process(tag, es)
# Drop events
end
end
d.configure(conf)
end

configure(conf, syntax: :v1)

Configures plugin instance managed by the Test Driver.
  • conf: Fluent::Config::Element or String
  • syntax: { :v1, :v0, :ruby } :v0 is obsolete.
Example:
def create_driver(conf={})
Fluent::Test::Driver::Output.new(Fluent::Plugin::MyOutput).configure(conf)
end
def test_process
conf = %[
path /path/to/something
host localhost
port 24229
]
d = create_driver(conf)
end

end_if(&block)

Registers the conditions to stop the running Test Driver gracefully.
All registered conditions must be true before Test Driver stops.

break_if(&block)

Registers the conditions to stop the running Test Driver.
Test Driver should stop running if some of the breaking conditions are true.

broken?

Returns true when some of the breaking conditions are true. Otherwise false.

run(timeout: nil, start: true, shutdown: true, &block)

Runs the Test Driver. This Test Driver will stop running immediately after evaluating the block if given.
Otherwise, you must register the conditions to stop the running Test Driver.
This method may be overridden in subclasses.
  • timeout: timeout (seconds)
  • start: if true, start the Test Driver. Otherwise, invoke instance_start
    method to start it.
  • shutdown: if true, shut down the running Test Driver.
Example:
# Run Test Driver and feed an event (output)
d = create_driver
d.run do
d.feed(time, record)
end
# Emit multiple events (output)
d = create_driver
d.run(default_tag: 'test', expect_emits: 1, timeout: 10, start: true, shutdown: false) { d.feed(time, { "k1" => 1 })}
d.run(default_tag: 'test', expect_emits: 1, timeout: 10, start: false, shutdown: false) { d.feed(time, { "k1" => 2 })}
d.run(default_tag: 'test', expect_emits: 1, timeout: 10, start: true, shutdown: true ) { d.feed(time, { "k1" => 3 })}

stop?

Returns true when all the stopping conditions are true. Otherwise false.

logs

Returns logs managed by this Test Driver.

instance

Returns the plugin instance managed by this Test Driver.

Test Driver Base Owner API

  • filter
  • output
  • multi_output

run(expect_emits: nil, expect_records: nil, timeout: nil, start: true, shutdown: true, &block)

Run Test Driver. This Test Driver will stop running immediately after evaluating block if given.
Otherwise, you must register conditions to stop running Test Driver.
This method may be overridden in subclasses.
  • expect_emits: set the number of expected emits
  • expect_records: set the number of expected records
  • timeout: timeout (seconds)
  • start: if true, start the Test Driver. Otherwise, invoke the
    instance_start method to start it.
  • shutdown: if true, shut down the running Test Driver.
Example:
# Run Test Driver and feed an event (owner plugin)
d = create_driver
d.run do
d.feed(time, record)
end
# Emit multiple events (owner plugin)
d = create_driver
d.run(default_tag: 'test', expect_emits: 1, timeout: 10, start: true, shutdown: false) { d.feed(time, { "k1" => 1 })}
d.run(default_tag: 'test', expect_emits: 1, timeout: 10, start: false, shutdown: false) { d.feed(time, { "k1" => 2 })}
d.run(default_tag: 'test', expect_emits: 1, timeout: 10, start: true, shutdown: true ) { d.feed(time, { "k1" => 3 })}

events(tag: nil)

Returns the events filtered by the given tag.
  • tag: filter by this tag. If omitted, it returns all the events.

event_streams(tag: nil)

Returns the event streams filtered by the given tag.
  • tag: filter by this tag. If omitted, it returns all the event streams.

emit_count

Returns the number of invoking router.emit.
If you want to delay the stopping of the Test Driver until a certain number of records are emitted, you can use d.run(expected_records: n) instead.
Example:
d.run do
d.feed('test', record)
end
assert_equal(1, d.emit_count)

record_count

Returns the number of records.
If you want to delay the stopping of the Test Driver until a certain number of records are emitted, you can use d.run(expected_records: n) instead.

error_events(tag: nil)

Returns error events filtered by the given tag.
  • tag: filter by this tag. If omitted, it returns all error events.

Test Driver Base owned API

configure(conf, syntax: :v1)

Configures plugin instance managed by this Test Driver.
  • conf: Fluent::Config::Element, String, or Hash
  • syntax: Supported: { :v1, :v0, :ruby } :v0 is obsolete.

Test Driver Event Feeder API

  • filter
  • output
  • multi_output

run(default_tag: nil, **kwargs, &block)

Runs EventFeeder.
  • default_tag: the default tag of the event
Example:
d.run(default_tag: 'test') do
d.feed(event_time, { 'message' => 'Hello, Fluentd!' })
end

feed(tag, time, record)

Feeds an event to plugin instance.
  • tag: the tag of the event
  • time: event timestamp
  • record: event record
Example:
d.run do
d.feed('test', event_time, { 'message' => 'Hello, Fluentd!' })
end

feed(tag, array_event_stream)

Feeds an array of event stream to plugin instance.
  • tag: the tag of the event
  • array_event_stream: array of [time, record]
    • time: event timestamp
    • record: event record
Example:
d.run do
d.feed('test', [
[event_time, { 'message' => 'Hello, user1!' }],
[event_time, { 'message' => 'Hello, user2!' }]
])
end

feed(tag, es)

Feeds an event stream to plugin instance.
  • tag: the tag of the event
  • es: event stream object
Example:
es = Fluent::OneEventStream.new(event_time, { 'message' => 'Hello, Fluentd!' })
d.run do
d.feed('test', es)
end

feed(record)

Feeds an event with default tag to plugin instance.
  • record: event record
Example:
d.run(default_tag: 'test') do
d.feed({ 'message' => 'Hello, Fluentd!' })
# Same as above ^
d.feed(event_time, { 'message' => 'Hello, Fluentd!' })
end

feed(time, record)

Feeds an event with default tag to plugin instance.
  • time: event timestamp
  • record: event record
Example:
d.run(default_tag: 'test') do
d.feed(event_time, { 'message' => 'Hello, Fluentd!' })
end

feed(array_event_stream)

Feeds an array of event stream with default tag to plugin instance.
  • array_event_stream: array of [time, record]
    • time: event timestamp
    • record: event record
Example:
d.run(default_tag: 'test') do
d.feed([
[event_time, { 'message' => 'Hello, user1!' }],
[event_time, { 'message' => 'Hello, user2!' }]
])
end

feed(es)

Feeds an event stream with default tag to plugin instance.
  • es: event stream object
Example:
es = Fluent::OneEventStream.new(event_time, { 'message' => 'Hello, Fluentd!' })
d.run(default_tag: 'test') do
d.feed(es)
end

Test Driver Filter API

filtered_records

Collects the filtered records.
d = create_driver(config)
d.run do
d.feed('filter.test', event_time, { 'foo' => 'bar', 'message' => msg })
end
d.filtered_records

Test Driver Output API

run(flush: true, wait_flush_completion: true, force_flush_retry: false, **kwargs, &block)

  • flush: flush forcibly
  • wait_flush_completion: if true, waiting for flush to complete
  • force_flush_retry: if true, retrying flush forcibly
Run Test Driver. This Test Driver will be stop running immediately after evaluating the block if given.
Otherwise, you must register conditions to stop running Test Driver.
Example:
d = create_driver(config)
d.run do
d.feed('filter.test', event_time, { 'foo' => 'bar', 'message' => msg })
end

formatted

Returns the formatted records.
Example:
d = create_driver(config)
d.run do
d.feed('filter.test', event_time, { 'foo' => 'bar', 'message' => msg })
end
d.formatted

flush

Flushes forcibly.
Example:
d = create_driver(config)
d.run do
d.feed('filter.test', event_time, { 'foo' => 'bar', 'message' => msg })
end
d.flush

Test Helpers

assert_equal_event_time(expected, actual, message = nil)

Asserts the EventTime instance.
  • expected: expected EventTime instance
  • actual: actual EventTime instance
  • message: message to display when assertion fails
Example:
parser = create_parser
parser.parse(text) do |time, record|
assert_equal_event_time(event_time('2017-12-27 09:43:50.123456789'), time)
end

config_element(name = 'test', argument = '', params = {}, elements = [])

Create Fluent::Config::Element instance.
  • name: element name such as match, filter, source, buffer, inject,
    format, parse, etc.
  • argument: argument for section defined by config_argument
  • params: parameters for section defined by config_element
  • elements: child elements of this config element
Example:
conf = config_element('match', '**', {
'path' => "#{TMP_DIR}/prohibited/${tag}/file.%Y%m%d.log",
}, [
config_element('buffer', 'time,tag', {
'time_key' => 86400,
'timekey_zone' => '+0000'
})
]
)
d = create_driver(conf)

event_time(str = nil, format: nil)

Creates a Fluent::EventTime instance.
  • str: time represented as String
  • format: parse str as time according to this format. See also
Example:
time = event_time
time = event_time('2016-10-03 23:58:09 UTC')
time = event_time('2016-04-11 16:40:01 +0000')
time = event_time('2016-04-17 11:15:00 -0700')
time = event_time('2011-01-02 13:14:15')
time = event_time('Sep 11 00:00:00', format: '%b %d %H:%M:%S')
time = event_time('28/Feb/2013:12:00:00 +0900', format: '%d/%b/%Y:%H:%M:%S %z')
time = event_time('2017-02-06T13:14:15.003Z', format: '%Y-%m-%dT%H:%M:%S.%L%z')

with_timezone(tz, &block)

Processes the given block with tz. This method overrides ENV['TZ'] while processing its block.
  • tz: timezone. This is set to ENV['TZ'].
Example:
time = with_timezone('UTC+02') do
parser = Fluent::TimeParser.new('%Y-%m-%d %H:%M:%S.%N', true)
parser.parse('2016-09-02 18:42:31.123456789')
end
assert_equal_event_time(time, event_time('2016-09-02 18:42:31.123456789 -02:00', format: '%Y-%m-%d %H:%M:%S.%N %z'))

with_worker_config(root_dir: nil, workers: nil, worker_id: nil, &block)

Processes block with the given parameters. This method overrides the system configuration while processing its block.
This is useful for testing Fluentd's internal behavior related to multi workers.
  • root_dir: root directory
  • workers: the number of workers
  • worker_id: ID of workers
Example:
class Dummy < Fluent::Plugin::Output
end
d = Dummy.new
with_worker_config(workers: 2, worker_id: 1) do
d.configure(conf)
end
# ...

time2str(time, localtime: false, format: nil)

Converts time to String.
This is useful for testing the formatter.
  • time: Fluent::EventTime instance. See also
  • localtime: If true, processes time as localtime. Otherwise UTC.
  • format: See also
formatter = configure_formatter(conf)
formatted = formatter.format(tag, time, record)
assert_equal("#{time2str(time)}\t#{JSON.dump(record)}\n", formatted)

msgpack(type)

  • type: Available types: { :factory, :packer, :unpacker }
Shorthand for:
  • Fluent::MessagePackFactory.factory
  • Fluent::MessagePackFactory.packer
  • Fluent::MessagePackFactory.unpacker
Example:
events = []
factory = msgpack(:factory)
factory.unpacker.feed_each(binary) do |obj|
events << obj
end

capture_stdout(&block)

Captures the standard output while processing the given block.
This is useful for testing Fluentd utility commands.
Example:
captured_string = capture_stdout do
# Print something to STDOUT
puts 'Hello!'
end
assert_equal('Hello!\n', capture_stdout)

Testing Input Plugins

You must test the input plugins' router#emit method. But you do not have to test this method explicitly. Its testing code pattern is encapsulated in the Input Test Driver.
You can write input plugins test like this:
require 'fluent/test'
require 'fluent/test/driver/input'
require 'fluent/test/helpers'
require 'fluent/plugin/input_my'
class MyInputTest < Test::Unit::TestCase
include Fluent::Test::Helpers
setup do
Fluent::Test.setup
end
def create_driver(conf = {})
Fluent::Test::Driver::Input.new(Fluent::Plugin::MyInput).configure(conf)
end
test 'emit' do
d = create_driver(config)
d.run(timeout: 0.5)
d.events.each do |tag, time, record|
assert_equal('input.test', tag)
assert_equal({ 'foo' => 'bar' }, record)
assert(time.is_a?(Fluent::EventTime))
end
end
end

Testing Filter Plugins

You must test filter plugins' #filter method. But you do not have to test this method explicitly. Its testing code pattern is encapsulated in Filter Test Driver.
You can write filter plugins test like this:
require 'fluent/test'
require 'fluent/test/driver/filter'
require 'fluent/test/helpers'
require 'fluent/plugin/filter_my'
class MyInputTest < Test::Unit::TestCase
include Fluent::Test::Helpers
setup do
Fluent::Test.setup
end
def create_driver(conf = {})
Fluent::Test::Driver::Filter.new(Fluent::Plugin::MyFilter).configure(conf)
end
test 'filter' do
d = create_driver(config)
time = event_time
d.run do
d.feed('filter.test', time, { 'foo' => 'bar', 'message' => msg })
end
assert_equal(1, d.filtered_records.size)
end
end

Testing Output Plugins

You must test output plugins' #process or #write or #try_write method. But you do not have to test this method explicitly. Its testing code pattern is encapsulated in the Output Test Driver.
You can write output plugins test like this:
require 'fluent/test'
require 'fluent/test/driver/output'
require 'fluent/test/helpers'
require 'fluent/plugin/output_my'
class MyInputTest < Test::Unit::TestCase
include Fluent::Test::Helpers
setup do
Fluent::Test.setup
end
def create_driver(conf = {})
Fluent::Test::Driver::Output.new(Fluent::Plugin::MyOutput).configure(conf)
end
test 'emit' do
d = create_driver(config)
time = event_time
d.run do
d.feed('output.test', time, { 'foo' => 'bar', 'message' => msg })
end
assert_equal(1, d.events.size)
end
end

Testing Parser Plugins

You must test the parser plugins' #parse method.
You can write parser plugins test like this:
require 'fluent/test'
require 'fluent/test/driver/parser'
require 'fluent/test/helpers'
require 'fluent/plugin/parser_my'
class MyParserTest < Test::Unit::TestCase
include Fluent::Test::Helpers
setup do
Fluent::Test.setup
end
def create_driver(conf = {})
Fluent::Test::Driver::Parser.new(Fluent::Plugin::MyParser).configure(conf)
end
def create_parser(conf)
create_driver(conf).instance
end
test 'parse' do
parser = create_parser(conf)
parser.parse(text) do |time, record|
assert_equal(event_time('2017-12-26 11:56:50.1234567879'), time)
assert_equal({ 'message' => 'Hello, Fluentd!' }, record)
end
end
end

Testing Formatter Plugins

You must test the formatter plugins' #format method.
You can write formatter plugins test like this:
require 'fluent/test'
require 'fluent/test/driver/formatter'
require 'fluent/test/helpers'
require 'fluent/plugin/formatter_my'
class MyFormatterTest < Test::Unit::TestCase
include Fluent::Test::Helpers
setup do
Fluent::Test.setup
end
def create_driver(conf = {})
Fluent::Test::Driver::Formatter.new(Fluent::Plugin::MyFormatter).configure(conf)
end
def create_formatter(conf)
create_driver(conf).instance
end
test 'format' do
formatter = create_formatter(conf)
formatted = formatter.format(tag, time, record)
assert_equal('message:awesome\tgreeting:hello', formatted)
end
end

Tests for Logs

Testing logs is easy.
Code example:
# d is a Test Driver instance
assert_equal(1, d.logs.size)
logs = d.logs
assert do
logs.any? { |log| log.include?(expected_log) }
end
assert do
logs.last.match?(/This is last log/)
end
If this article is incorrect or outdated, or omits critical information, please let us know. Fluentd is an open-source project under Cloud Native Computing Foundation (CNCF). All components are available under the Apache 2 License.
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On this page
Basics of Plugin Testing
Plugin Test Driver Overview
Testing Utility Methods
Test Driver Base API
initialize(klass, opts: {}, &block)
configure(conf, syntax: :v1)
end_if(&block)
break_if(&block)
broken?
run(timeout: nil, start: true, shutdown: true, &block)
stop?
logs
instance
Test Driver Base Owner API
run(expect_emits: nil, expect_records: nil, timeout: nil, start: true, shutdown: true, &block)
events(tag: nil)
event_streams(tag: nil)
emit_count
record_count
error_events(tag: nil)
Test Driver Base owned API
configure(conf, syntax: :v1)
Test Driver Event Feeder API
run(default_tag: nil, **kwargs, &block)
feed(tag, time, record)
feed(tag, array_event_stream)
feed(tag, es)
feed(record)
feed(time, record)
feed(array_event_stream)
feed(es)
Test Driver Filter API
filtered_records
Test Driver Output API
run(flush: true, wait_flush_completion: true, force_flush_retry: false, **kwargs, &block)
formatted
flush
Test Helpers
assert_equal_event_time(expected, actual, message = nil)
config_element(name = 'test', argument = '', params = {}, elements = [])
event_time(str = nil, format: nil)
with_timezone(tz, &block)
with_worker_config(root_dir: nil, workers: nil, worker_id: nil, &block)
time2str(time, localtime: false, format: nil)
msgpack(type)
capture_stdout(&block)
Testing Input Plugins
Testing Filter Plugins
Testing Output Plugins
Testing Parser Plugins
Testing Formatter Plugins
Tests for Logs