What version of Ruby does fluentd support?

Fluentd v0.12 works on 1.9.3 or later. Since v1.x, 2.1 or later.

Known Issue

I use Fluentd with Ruby 2.0 but Fluentd seems deadlocked. Why?

The rubygems of Ruby 2.0-p353 has a deadlock problem (#9224). If you use Ruby 2.0-p353, upgrading Ruby to latest patch level or Ruby 2.1 resolve this problem.

Please make sure that you are using either RHEL 7.1 and ruby- package or alternatively you can choose more recent Ruby provided by Red Hat Software Collections (https://access.redhat.com/documentation/en/red-hat-software-collections/)

Unfortunately, Ruby 2.0 package of Ubuntu 14.04 use Ruby 2.0-p353. So don't use Fluentd with Ruby 2.0 package on this environment. You can install specified Ruby version using rbenv.

Using td-agent is another way to avoid this problem because td-agent includes own Ruby.


I have millisecond timestamp log but fluentd drops subsecond. Why?

In v0.12 or earlier, fluentd's event time is second unit. It means fluentd converts millisecond/nanosecond timestamp into second timestamp.

Since v1.x, event tims has nanosecond resolution, so fluentd can handle millisecond/nanosecond timestamp properly.

Visit v1.x document

I have a weird timestamp value, what happened?

The timestamps of Fluentd and its logger libraries depend on your system's clock. It's highly recommended that you set up NTP on your nodes so that your clocks remain synced with the correct clocks.

I installed td-agent and want to add custom plugins. How do I do it?

td-agent has own Ruby so you should install gems into td-agent's Ruby, not system Ruby.


Please use td-agent-gem as shown below.

$ /usr/sbin/td-agent-gem install <plugin name>

For example, issue the following command if you are adding fluent-plugin-twitter.

$ /usr/sbin/td-agent-gem install fluent-plugin-twitter

Now you might be wondering, "Why do I need to specify the full path?" The reason is that td-agent does not modify any host environment variable, including PATH. If you want to make all td-agent/fluentd related programs available without writing "/usr/lib/..." every time, you can add

export PATH=$PATH:/opt/td-agent/embedded/bin/

to your ~/.bash_profile.

If you would like to find out more about plugin management, please take a look at the Plugin Management article.

I installed the plugin and it updates fluentd from v0.12 to v1.x. Why?

You installed v1.0 based plugin. See Plugin Management.

How can I match (send) an event to multiple outputs?

You can use the copy output plugin to send the same event to multiple output destinations.

How can I use environment variables to configure parameters dynamically?

Use "#{ENV['YOUR_ENV_VARIABLE']}". For example,

some_field "#{ENV['FOO_HOME']}"

Note that it must be double quotes and not single quotes

Fluentd raises an error for host:port. Why?

There are several reasons:

  • If you get Address already in use error, other process has already

    used host:port. Check port conflict between processes / plugins.

  • If you get Permission denied error, you try to use well-known

    port. Search well-known port for how to use well-known port. Use

    capabilities or something

If you get other errors, google it.

I got "no patterns matched" in the log, why?

This means you emit the event but no <match> directive for emitted event. For example, if you emit the event with foo.bar tag, you need to define <match> for foo.bar tag like <match foo.**>.

See also: Life of a Fluentd event or Config File

File buffer doesn't work properly, why?

file buffer has limitations. Check buf_file article.

I got enconding error inside plugin. How to fix it?

You may hit "\xC3" from ASCII-8BIT to UTF-8" like UndefinedConversionError in the plugin. This error happens when string encoding is set to ASCII-8BIT but actual content is UTF-8. Fluentd and almost plugins treat the logs as a ASCII-8BIT by default but some libraries assume the log encoding is UTF-8. This is why this error happens.

There are several approaches to avoid this problem.

Plugin Development

How do I develop a custom plugin?

Please refer to the Plugin Development Guide.


How can I parse <my complex text log>?

If you are willing to write Regexp, fluentd-ui's in_tail editor or Fluentular is a great tool to verify your Regexps.

If you do NOT want to write any Regexp, look at the Grok parser.

If this article is incorrect or outdated, or omits critical information, please let us know. Fluentd is a open source project under Cloud Native Computing Foundation (CNCF). All components are available under the Apache 2 License.

Last updated