Plugin Development
Overview
With fluentd, you can install and create custom plugins.
To install or create a custom plugin, the file name need to be <TYPE>_<NAME>.rb
. Note that TYPE
is a prefix indicating the type of plugin, and could be:
in
input
out
output
filter
filter
parser
parser
formatter
formatter
storage
storage
buf
buffer
sd
service discovery
Installing Custom Plugins
To install a plugin, put the Ruby script in /etc/fluent/plugin
directory. (The file name needs to have the TYPE
prefix.) Alternatively, you can create a Ruby Gem package that includes:
For example, an email output plugin would have the path:
The packaged gem can be distributed and installed using RubyGems. For further information, see the list of Fluentd plugins for third-party plugins.
Writing Plugins
To create a plugin as a Ruby script (to put it in /etc/fluent/plugin/
directory), create a <TYPE>_<NAME>.rb
file in any editor/IDE of your choice.
Example:
See the respective plugin development article, "How to write XXX plugin", for API details.
A single Ruby script is easy to write but hard to test, manage, version. publish, etc.
If you want to publish a plugin under version control, you should use bundle gem
to create the plugin source tree and then git init
it. It requires the bundler
gem in your Ruby environment.
Example:
It generates source code directory tree under lib
, fluent-plugin-my_awesome.gemspec
file, README.md
and other relevant files.
Fluentd plugin projects use a bit different code tree under lib
i.e.:
Generating Plugin Project Skeleton
To make things easier, fluent-plugin-generate
is provided to generate the project skeleton for writing a Fluentd plugin as a Gem package.
For example, to generate the project skeleton for an input plugin http2
, the command would be:
If you want to generate a project skeleton without LICENSE, use --no-license
option. For more details, see fluent-plugin-generate --help
.
Debugging Plugins
Run fluentd
with the -vv
command-line option to show debugging messages:
The stdout
and copy
output plugins are useful for debugging. The stdout
output plugin dumps matched events to the standard output (console). It can be configured like this:
The copy output plugin copies the matched events to the multiple output plugins. You can use it in conjunction with the stdout plugin:
You can use stdout filter instead of copy and stdout combination. The result is the same as above but more simpler.
Writing Tests for Plugins
Fluentd provides unit test frameworks for plugins:
It is strongly recommended to use test-unit
as the unit test library. Fluentd's test drivers assume that the test code uses it. Add test-unit
as the development dependency in your gemspec
, add Rake
task to run tests in your Rakefile
and write test code in test/plugin/test_in_my_awesome.rb
.
gemspec
Rakefile
Run bundle
to execute tests:
See Writing Plugin Test Code for more details on writing tests.
Writing Documents for Plugins
Following is a snippet of README.md
showing the project skeleton generated with fluent-plugin-generate
.
For example:
You should write the plugin description and configurations.
You can generate configuration using fluent-plugin-config-format
command.
Example (input dummy):
For more details, see fluent-plugin-config-format --help
.
Note
The following slides can help understand how Fluentd works before diving into writing custom plugins.
These slides are from Naotoshi Seo's RubyKaigi 2014 talk.
The slides are based on Fluentd v0.12. There are many differences between v0.12 and v1 API, but it may help build an understanding of overall Fluentd's design.
Fluentd Version and Plugin API
Fluentd now has two active versions, v1 and v0.12. v1 is the current stable with the brand-new Plugin API. v0.12 is the old stable and it has the old Plugin API.
The important point is v1 supports v1 and v0.12 APIs. It means the plugin for v0.12 works with v1.
It is recommended to use the new v1 plugin API for writing new plugins.
Send a patch or fork?
If you have a problem with any existing plugins or a new feature idea, sending a patch is better. If the plugin author is non-active, try to become its new plugin maintainer first. Forking a plugin and release its alternative version, e.g. fluent-plugin-xxx-alt
is considered the last option.
Be careful not breaking Fluentd's core functionality
Any fluentd plugin can unknowingly break fluentd completely (and possibly break other plugins) by requiring some incompatible modules.
One typical example is reloading yajl/json_gem
from plugin. (yajl/json_gem
compatibility layer is problematic, not for yajl
itself) It breaks a functionality to parse Fluentd's configuration completely.
Fluentd's plug-in mechanism has a merit to extend functionality, but plugin developer must be careful a possibility of breaking it.
Further Reading
Slides: Dive into Fluentd Plugin (outdated)
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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