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Config File Syntax (YAML)

This article describes the basic concepts of Fluentd configuration file syntax for yaml format.

Introduction: The Lifecycle of a Fluentd Event

Here is a brief overview of the lifecycle of a Fluentd event to help you understand the rest of this page:
The configuration file allows the user to control the input and output behavior of Fluentd by 1) selecting input and output plugins; and, 2) specifying the plugin parameters. The file is required for Fluentd to operate properly.

YAML adoption

Fluentd starts to support YAML configuration format but this is not 1-by-1 correspondence for Fluentd config file syntax.
Normal Fluentd configuration syntax has the following the list of directives:
  1. 1.
    source directives determine the input sources
  2. 2.
    match directives determine the output destinations
  3. 3.
    filter directives determine the event processing pipelines
  4. 4.
    system directives set system-wide configuration
  5. 5.
    label directives group the output and filter for internal routing
  6. 6.
    worker directives limit to the specific workers
  7. 7.
    @include directives include other files
In YAML configuration world, we reconstructed them for YAML format.
In YAML syntax, Fluentd will handle the two top level objects:
  1. 1.
    system The top level object that specifies system settings
  2. 2.
    config Another top level object that defines data pipeline
Under config object, Fluentd will handle the following elements:
  1. 1.
    source elements determine the input sources
  2. 2.
    match elements determine the output destinations
  3. 3.
    filter elements determine the event processing pipelines
  4. 4.
    label elements group the output and filter for internal routing

Special YAML elements

  1. 1.
    !include defines including rules for other files
  2. 2.
    !fluent/s defines Fluentd string format that is equivalent for double quoted string
  3. 3.
    !fluent/json defines Fluentd JSON format that is used for Hash type object
  4. 4.
    $tag defines tag for output plugin
  5. 5.
    $label defines label routes for input plugin
  6. 6.
    $arg defines directive arg's equivalent objects (e.g. <DIRECTIVE arg>)
  7. 7.
    $name defines label directive name equivalent objects (e.g. <label name>)
  8. 8.
    $type specifies @type or type for instantiating plugin type
Let's actually create a configuration file step by step.

1. source: where all the data come from

Fluentd input sources are enabled by selecting and configuring the desired input plugins using source elements. Fluentd standard input plugins include http and forward. The http provides an HTTP endpoint to accept incoming HTTP messages whereas forward provides a TCP endpoint to accept TCP packets. Of course, it can be both at the same time. You may add multiple source configurations as required.
config:
# Receive events from 24224/tcp
# This is used by log forwarding and the fluent-cat command
- source:
$type: forward
port: 24224
# http://<ip>:9880/myapp.access?json={"event":"data"}
- source:
$type: http
port: 9880
Each source element must include a $type object to specify the input plugin to use.

Didn't find your input source? You can write your own plugin!

You can add new input sources by writing your own plugins. For further information regarding Fluentd input sources, please refer to the Input Plugin Overview article.

2. "match": Tell fluentd what to do!

The match element looks for events with matching tags and processes them. The most common use of the match element is to output events to other systems. For this reason, the plugins that correspond to the match element are called output plugins. Fluentd standard output plugins include file and forward. Let's add those to our configuration file.
config:
# Receive events from 24224/tcp
# This is used by log forwarding and the fluent-cat command
- source:
$type: forward
port: 24224
# http://<ip>:9880/myapp.access?json={"event":"data"}
- source:
$type: http
port: 9880
# Match events tagged with "myapp.access" and
# store them to /var/log/fluent/access.%Y-%m-%d
# Of course, you can control how you partition your data
# with the time_slice_format option.
- match:
$tag: myapp.access
$type: file
path: /var/log/fluent/access
Each match element must include a $tag and a $type parameter. Only events with a $tag matching the pattern will be sent to the output destination (in the above example, only the events with the tag myapp.access are matched. See the section below for more advanced usage). The $type parameter specifies the output plugin to use.
Just like input sources, you can add new output destinations by writing custom plugins. For further information regarding Fluentd output destinations, please refer to the Output Plugin Overview article.

3. "filter": Event processing pipeline

The filter element has the same syntax as match but filter could be chained for processing pipeline. Using filters, event flow is like this:
Input -> filter 1 -> ... -> filter N -> Output
Let's add standard record_transformer filter to match example.
config:
# http://this.host:9880/myapp.access?json={"event":"data"}
- source:
$type: http
port: 9880
- filter:
$tag: myapp.access
$type: record_transformer
record:
host_param: !fluent/s "#{Socket.gethostname}"
- match:
$tag: myapp.access
$type: file
path: /var/log/fluent/access
The received event {"event":"data"} goes to record_transformer filter first. The record_transformer filter adds host_param field to the event; and, then the filtered event {"event":"data","host_param":"webserver1"} goes to the file output plugin.
You can also add new filters by writing your own plugins. For further information regarding Fluentd filter destinations, please refer to the Filter Plugin Overview article.

4. Set system-wide configuration: the system element

System-wide configurations are set by system element. Most of them are also available via command line options. For example, the following configurations are available:
  • log_level
  • suppress_repeated_stacktrace
  • emit_error_log_interval
  • suppress_config_dump
  • without_source
  • process_name (Only available in system element. No fluentd option)
Example:
system:
# equal to -qq option
log_level: error
# equal to --without-source option
without_source:
# ...
See also System Configuration article for more detail.

process_name

If this parameter is set, fluentd supervisor and worker process names are changed.
system:
process_name: fluentd1
With this configuration, ps command shows the following result:
% ps aux | grep fluentd1
foo 45673 0.4 0.2 2523252 38620 s001 S+ 7:04AM 0:00.44 worker:fluentd1
foo 45647 0.0 0.1 2481260 23700 s001 S+ 7:04AM 0:00.40 supervisor:fluentd1
This feature requires Ruby 2.1 or later.

5. Group filter and output: the "label" element

The label element groups filter and output for internal routing. The label reduces the complexity of tag handling.
The label element's parameter is a builtin plugin parameter so $name parameter is needed.
Here is a configuration example:
config:
- source:
$type: forward
- source:
$type: tail
$label: '@SYSTEM'
- filter:
$tag: access.**
$type: record_transformer
record:
# ...
- match:
$tag: '**'
$type: elasticsearch
# ...
- label:
$name: '@SYSTEM'
config:
- filter:
$tag: var.log.middleware.**
$type: grep
# ...
- match:
$tag: '**'
$type: s3
# ...
Note that $label parameter value should be quoted and label element should contain config element within its elements.
In this configuration, forward events are routed to record_transformer filter / elasticsearch output and in_tail events are routed to grep filter / s3 output inside @SYSTEM label.
The $label parameter is useful for event flow separation without the $tag route rules.

@ERROR label

The @ERROR label is a builtin label used for error record emitted by plugin's emit_error_event API.
If $label with $name: '@ERROR' is set, the events are routed to this label when the related errors are emitted e.g. the buffer is full or the record is invalid.

@ROOT label

The @ROOT label is a builtin label used for getting root router by plugin's event_emitter_router API.
This label is introduced since v1.14.0 to assign a label back to the default route. For example, timed-out event records are handled by the concat filter can be sent to the default route.

6. Limit to specific workers: the worker element

When setting up multiple workers, you can use the worker element to limit plugins to run on specific workers.
This is useful for input and output plugins that do not support multiple workers.
You can use the $arg N or $arg N-M to specify workers. The number is a zero-based worker index.
See Multi Process Workers article for details about multiple workers.
Here is a configuration example:
system:
workers: 4
config:
- source:
$type: sample
tag: test.allworkers
sample: "{\"message\": \"Run with all workers.\"}"
- worker:
$arg: 0
config:
- source:
$type: sample
tag: test.oneworker
sample: "{\"message\": \"Run with only worker-0.\"}"
- worker:
$arg: 0-1
config:
- source:
$type: sample
tag: test.someworkers
sample: "{\"message\": \"Run with worker-0 and worker-1.\"}"
- filter:
$type: record_transformer
$tag: test.**
record:
worker_id: !fluent/s "#{worker_id}"
- match:
$type: stdout
$tag: test.**
The outputs of this config are as follows:
... test.allworkers: {"message":"Run with all workers.","worker_id":"0"}
... test.allworkers: {"message":"Run with all workers.","worker_id":"1"}
... test.allworkers: {"message":"Run with all workers.","worker_id":"2"}
... test.allworkers: {"message":"Run with all workers.","worker_id":"3"}
... test.oneworker: {"message":"Run with only worker-0.","worker_id":"0"}
... test.someworkers: {"message":"Run with worker-0 and worker-1.","worker_id":"0"}
... test.someworkers: {"message":"Run with worker-0 and worker-1.","worker_id":"1"}

7. Reuse your config: the !include YAML tag

The element in separate configuration files can be imported using the !include element:
config:
# Include config files in the ./config.d directory
- !include config.d/*.conf
The !include YAML tag supports regular file path, glob pattern, and http URL conventions:
config:
# absolute path
- !include /path/to/config.conf
# if using a relative path, the YAML tag will use
# the dirname of this config file to expand the path
- !include extra.conf
# glob match pattern
- !include config.d/*.conf
# http
- !include http://example.com/fluent.conf
Note that for the glob pattern, files are expanded in alphabetical order. If there are a.conf and b.conf then fluentd parses a.conf first. But, you should not write the configuration that depends on this order. It is so error-prone, therefore, use multiple separate !include YAML tags for safety.
config:
# If you have a.conf, b.conf, ..., z.conf and a.conf / z.conf are important
# This is bad
- !include *.conf
# This is good
- !include a.conf
- !include config.d/*.conf
- !include z.conf

Share the Same Parameters

The !include YAML tag can be used under sections to share the same parameters:
# config file
config:
- match:
$type: forward
# ...
buffer:
$type: file
path: /path/to/buffer/forward
<<: !include /path/to/out_buf_params.yaml
- match:
$type: elasticsearch
# ...
buffer:
$type: file
path: /path/to/buffer/es
<<: !include /path/to/out_buf_params.yaml
# /path/to/out_buf_params.yaml
flush_interval: 5s
total_limit_size: 100m
chunk_limit_size: 1m
Note that, in the middle of element case of !include YAML tag usage, users must use <<: syntax to include other YAML objects successfully.

Note on Match Order

Fluentd tries to match tags in the order that they appear in the config file. So, if you have the following configuration:
config:
# '**' matches all tags. Bad :(
- match:
$tag: '**'
$type: blackhole_plugin
- match:
$tag: myapp.access
$type: file
path: /var/log/fluent/access
then myapp.access is never matched. Wider match patterns should be defined after tight match patterns.
config:
- match:
$tag: myapp.access
$type: file
path: /var/log/fluent/access
# Capture all unmatched tags. Good :)
- match:
$tag: '**'
$type: blackhole_plugin
Of course, if you use two same patterns, the second match is never matched. If you want to send events to multiple outputs, consider out_copy plugin.
The common pitfall is when you put a filter element after match element. It will never work since events never go through the filter for the reason explained above.
config:
# You should NOT put this <filter> block after the <match> block below.
# If you do, Fluentd will just emit events without applying the filter.
- filter:
$tag: myapp.access
$type: record_transformer
...
- match:
$tag: myapp.access
$type: file
path: /var/log/fluent/access

Embedding Ruby Expressions

When using YAML format syntax in Fluentd configuration, you can use !fluent/s "#{...}" to embed arbitrary Ruby code into match patterns. Here is an example:
config:
- match:
$tag: fluent/s "app.#{ENV['FLUENTD_TAG']}"
$type: stdout
If you set the environment variable FLUENTD_TAG to dev, this evaluates to app.dev.

Supported Data Types for Values

Common Plugin Parameters

Check Configuration File

The configuration file can be validated without starting the plugins using the --dry-run option:
$ fluentd --dry-run -c fluent.yaml

Format Tips

This section describes some useful features for the configuration file.

Multiline support for " quoted string, array and hash values

You can write multiline values for " quoted string, array and hash values.
str_param: "foo # Converts to "foo bar". As NL interpretation will be required for continuos newlines.
bar"
str_param: "foo # Converts to "foo\nbar".
bar"
array_param:
- "a"
- "b"
hash_param: !fluent/json {
"k": "v",
"k1": 10
}
Fluentd assumes [ or { is a start of array / hash. So, if you want to set [ or { started but non-JSON parameter, please use ' or ".
Example # 1: mail plugin
config:
- match:
$tag: '**'
$type: mail
subject: "[CRITICAL] foo's alert system"
Example # 2: map plugin
config:
- match:
$tag:tag
$type: map
map: '[["code." + tag, time, { "code" => record["code"].to_i}], ["time." + tag, time, { "time" => record["time"].to_i}]]'
multi: true
This restriction will be removed with the configuration parser improvement.

Embedded Ruby Code

You can evaluate the Ruby code with !fluent/s #{} in " quoted string. This is useful for setting machine information e.g. hostname.
host_param: !fluent/s "#{Socket.gethostname}" # host_param is actual hostname like `webserver1`.
env_param: !fluent/s "foo-#{ENV['FOO_BAR']}" # NOTE that foo-"#{ENV["FOO_BAR"]}" doesn't work.
In YAML config format, hostname and worker_id shortcuts are also available:
host_param: !fluent/s "#{hostname}" # This is same with Socket.gethostname
$id: !fluent/s "out_foo#{worker_id}" # This is same with ENV["SERVERENGINE_WORKER_ID"]
The worker_id shortcut is useful under multiple workers. For example, for a separate plugin id, add worker_id to store the path in s3 to avoid file conflict.
Helper methods use_nil and use_default are available:
some_param: !fluent/s "#{ENV['FOOBAR'] || use_nil}" # Replace with nil if ENV["FOOBAR"] isn't set
some_param: !fluent/s "#{ENV['FOOBAR'] || use_default}" # Replace with the default value if ENV["FOOBAR"] isn't set
Note that these methods not only replace the embedded Ruby code but the entire string with nil or a default value.
some_path: !fluent/s "#{use_nil}/some/path" # some_path is nil, not "/some/path"

In double-quoted string literal, \ is the escape character

The backslash \ is interpreted as an escape character. You need \ for setting ", , , , \ or several characters in double-quoted string literal.
str_param: "foo\nbar" # \n is interpreted as actual LF character
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.