This article explains how to manage Fluentd plugins, including adding third-party plugins.

fluent-gem

The fluent-gem command is used to install Fluentd plugins. This is a wrapper around the gem command.

Example:

fluent-gem install fluent-plugin-grep

Ruby does not guarantee the C extension API compatibility between its major versions. If you update Fluentd's Ruby version, you should reinstall the plugins that depend on C extension.

If Using td-agent, Use /usr/sbin/td-agent-gem

If you are using td-agent, make sure that you use td-agent-gem command to install gems for it. Otherwise, you won't be able to find the installed plugins.

Gem and Native Extension

Some plugins depend on the native extension library. It means that you need to install development packages to build it e.g. gcc, make, autoconf, etc.

Install development packages first, if you see logs like this:

Building native extensions.  This could take a while...
ERROR:  Error installing fluent-plugin-twitter:
        ERROR: Failed to build gem native extension.
​
    /opt/td-agent/embedded/bin/ruby extconf.rb
​
checking for rb_str_scrub()... yes
creating Makefile
​
make "DESTDIR = " clean
sh: 1: make: not found
​
make "DESTDIR = "
sh: 1: make: not found
​
make failed, exit code 127
​
Gem files will remain installed in /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/string-scrub-0.0.3 for inspection.
Results logged to /opt/td-agent/embedded/lib/ruby/gems/2.1.0/extensions/x86_64-linux/2.1.0/string-scrub-0.0.3/gem_make.out

-p option

Fluentd's -p option is used to add an extra plugin directory to the load path. For example, if you put out_foo.rb plugin into /path/to/plugin, you can load it by specifying the -p option like this:

fluentd -p /path/to/plugin

You can specify the -p option more than once.

Add a Plugin via /etc/fluent/plugin

By default, Fluentd adds the /etc/fluent/plugin directory to its load path. Thus, any additional plugins that are placed in /etc/fluent/plugin will be loaded automatically.

If Using td-agent, Use /etc/td-agent/plugin

For td-agent, Fluentd uses the /etc/td-agent/plugin directory instead of /etc/fluent/plugin so put your plugins accordingly.

Plugin Version Management

Fluentd and plugins are evolving so you may hit an unexpected error with the latest version e.g. regression by a new feature, remove a deprecated parameter, change library dependency, etc. To avoid these problems, we recommend fixing fluentd and plugin version on production. If you want to update fluentd or plugins, check the behavior first on your test environment. For example, td-agent fixes fluentd and plugins version in each release.

Fluentd plugins are RubyGems and RubyGems installs the latest version by default. So we don't recommend to execute following commands on production:

• gem install fluentd

• gem install fluent-plugin-elasticsearch

• gem update # This is very dangerous. Update all existing gems

You should specify the target version with -v option:

• gem install fluentd -v 1.2.1

• gem install fluent-plugin-elasticsearch -v 2.10.3

--gemfile option

A Ruby application manages gem dependencies using Gemfile and Bundler. Fluentd's --gemfile option takes the same approach, and is useful for managing plugin versions separated from shared gems.

For example, if you have the following Gemfile at /etc/fluent/Gemfile:

source 'https://rubygems.org'
​
gem 'fluentd', '1.2.1'
gem 'fluent-plugin-elasticsearch', '2.10.3'

You can pass this Gemfile to Fluentd via the --gemfile option:

fluentd --gemfile /etc/fluent/Gemfile

When specifying the --gemfile option, Fluentd will try to install the listed gems using Bundler. Fluentd will only load the listed gems separated from shared gems, and will also prevent unexpected plugin updates.

If you update Fluentd's Ruby version, Bundler will reinstall the listed gems for the new Ruby version. This allows you to avoid the C extension API compatibility problem.

For td-agent

We can manage Fluentd and its plugins based on Gemfile with td-agent.

Use the following drop-in file /etc/systemd/system/td-agent.service.d/override.conf for td-agent 3.1.1:

[Service]
Environment='TD_AGENT_OPTIONS=--gemfile=/etc/td-agent/Gemfile --gem-path=/var/lib/td-agent/vendor/bundle'
ExecStart=
ExecStart=/opt/td-agent/embedded/bin/fluentd --log /var/log/td-agent/td-agent.log --daemon /var/run/td-agent/td-agent.pid $TD_AGENT_OPTIONS

We can also edit this file using the following command:

$ sudo systemctl edit td-agent.service

And, then add /etc/td-agent/Gemfile:

source "https://rubygems.org"
# You can use fixed version of Fluentd and its plugins
gem "fluentd", "1.2.1"
gem "fluent-plugin-elasticsearch", "2.4.0"
gem "fluent-plugin-kafka", "0.6.5"
gem "fluent-plugin-rewrite-tag-filter", "2.0.1"
gem "fluent-plugin-s3", "1.1.0"
gem "fluent-plugin-td", "1.0.0"
gem "fluent-plugin-td-monitoring", "0.2.3"
gem "fluent-plugin-webhdfs", "1.2.2"
# Add plugins you want to use
gem "fluent-plugin-geoip", "1.2.0"

Finally, restart td-agent:

$ sudo systemctl restart td-agent.service

NOTE: With this approach, you will download all the gems listed in Gemfile even if td-agent includes them the first time.

FAQ

fluent-gem list shows multiple plugin versions. Which one is used?

The latest version is used. If command shows the following result:

$ fluent-gem list
...skip...
fluent-plugin-record-modifier (2.0.1, 0.6.0, 0.5.0)

2.0.1 version is used.

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.