Currently, Fluent Bit supports two configuration formats:

• Classic mode.

• Yaml. (YAML configuration is production ready since Fluent Bit 2.0.)

CLI flags

Fluent Bit also supports a CLI interface with various flags matching up to the configuration options available.

Fluent Bit might optionally use a configuration file to define how the service will behave.

Before proceeding we need to understand how the configuration schema works.

The schema is defined by three concepts:

• Sections

• Entries: Key/Value

• Indented Configuration Mode

A simple example of a configuration file is as follows:

Sections

A section is defined by a name or title inside brackets. Looking at the example above, a Service section has been set using [SERVICE] definition. Section rules:

• All section content must be indented (4 spaces ideally).

• Multiple sections can exist on the same file.

• A section is expected to have comments and entries, it cannot be empty.

Entries: Key/Value

A section may contain Entries, an entry is defined by a line of text that contains a Key and a Value, using the above example, the [SERVICE] section contains two entries, one is the key Daemon with value off and the other is the key Log_Level with the value debug. Entries rules:

• An entry is defined by a key and a value.

• A key must be indented.

• A key must contain a value which ends in the breakline.

Also commented lines are set prefixing the # character, those lines are not processed but they must be indented too.

Indented Configuration Mode

Fluent Bit configuration files are based in a strict Indented Mode, that means that each configuration file must follow the same pattern of alignment from left to right when writing text. By default an indentation level of four spaces from left to right is suggested. Example:

As you can see there are two sections with multiple entries and comments, note also that empty lines are allowed and they do not need to be indented.

YAML configuration feature was introduced since FLuent Bit version 1.9 as experimental, and it is production ready since Fluent Bit 2.0.

One of the ways to configure Fluent Bit is using a main configuration file. Fluent Bit allows to use one configuration file which works at a global scope and uses the Format and Schema defined previously.

The main configuration file supports four types of sections:

• Service

• Input

• Filter

• Output

In addition, it's also possible to split the main configuration file in multiple files using the feature to include external files:

• Include File

Service

The Service section defines global properties of the service, the keys available as of this version are described in the following table:

The following is an example of a SERVICE section:

For scheduler and retry details, please check there:

Input

An INPUT section defines a source (related to an input plugin), here we will describe the base configuration for each INPUT section. Note that each input plugin may add it own configuration keys:

The Name is mandatory and it let Fluent Bit know which input plugin should be loaded. The Tag is mandatory for all plugins except for the input forward plugin (as it provides dynamic tags).

Example

The following is an example of an INPUT section:

Filter

A FILTER section defines a filter (related to an filter plugin), here we will describe the base configuration for each FILTER section. Note that each filter plugin may add it own configuration keys:

The Name is mandatory and it let Fluent Bit know which filter plugin should be loaded. The Match or Match_Regex is mandatory for all plugins. If both are specified, Match_Regex takes precedence.

Example

The following is an example of an FILTER section:

Output

The OUTPUT section specify a destination that certain records should follow after a Tag match. Currently, Fluent Bit can route up to 256 OUTPUT plugins. The configuration support the following keys:

Example

The following is an example of an OUTPUT section:

Example: collecting CPU metrics

The following configuration file example demonstrates how to collect CPU metrics and flush the results every five seconds to the standard output:

Visualize

You can also visualize Fluent Bit INPUT, FILTER, and OUTPUT configuration via

Include File

To avoid complicated long configuration files is better to split specific parts in different files and call them (include) from one main file.

Starting from Fluent Bit 0.12 the new configuration command @INCLUDE has been added and can be used in the following way:

The configuration reader will try to open the path somefile.conf, if not found, it will assume it's a relative path based on the path of the base configuration file, e.g:

• Main configuration file path: /tmp/main.conf

• Included file: somefile.conf

• Fluent Bit will try to open somefile.conf, if it fails it will try /tmp/somefile.conf.

The @INCLUDE command only works at top-left level of the configuration line, it cannot be used inside sections.

Wildcard character (*) is supported to include multiple files, e.g:

Note files matching the wildcard character are included unsorted. If plugins ordering between files need to be preserved, the files should be included explicitly.

Certain configuration directives in Fluent Bit refer to unit sizes such as when defining the size of a buffer or specific limits, we can find these in plugins like , or in generic properties like .

Starting from v0.11.10, all unit sizes have been standardized across the core and plugins, the following table describes the options that can be used and what they mean:

Fluent Bit supports the usage of environment variables in any value associated to a key when using a configuration file.

The variables are case sensitive and can be used in the following format:

When Fluent Bit starts, the configuration reader will detect any request for ${MY_VARIABLE} and will try to resolve its value.

When Fluent Bit is running under systemd (using the official packages), environment variables can be set in the following files:

• /etc/default/fluent-bit

Fluent Bit works internally with structured records and it can be composed of an unlimited number of keys and values. Values can be anything like a number, string, array, or a map.

Having a way to select a specific part of the record is critical for certain core functionalities or plugins, this feature is called Record Accessor.

consider Record Accessor a simple grammar to specify record content and other miscellaneous values.

Format

A record accessor rule starts with the character $. Using the structured content above as an example the following table describes how to access a record:

The following table describe some accessing rules and the expected returned value:

If the accessor key does not exist in the record like the last example $labels['undefined'] , the operation is simply omitted, no exception will occur.

Usage Example

The feature is enabled on a per plugin basis, not all plugins enable this feature. As an example consider a configuration that aims to filter records using that only matches where labels have a color blue:

The file content to process in test.log is the following:

Running Fluent Bit with the configuration above the output will be:

Limitations of record_accessor templating

The Fluent Bit record_accessor library has a limitation in the characters that can separate template variables- only dots and commas (. and ,) can come after a template variable. This is because the templating library must parse the template and determine the end of a variable.

The following would be invalid templates because the two template variables are not separated by commas or dots:

• $TaskID-$ECSContainerName

• $TaskID/$ECSContainerName

• $TaskID_$ECSContainerName

However, the following are valid:

• $TaskID.$ECSContainerName

• $TaskID.ecs_resource.$ECSContainerName

• $TaskID.fooo.$ECSContainerName

And the following are valid since they only contain one template variable with nothing after it:

• fooo$TaskID

• fooo____$TaskID

• fooo/bar$TaskID

It's common that Fluent Bit output plugins aims to connect to external services to deliver the logs over the network, this is the case of HTTP, Elasticsearch and Forward within others. Being able to connect to one node (host) is normal and enough for more of the use cases, but there are other scenarios where balancing across different nodes is required. The Upstream feature provides such capability.

An Upstream defines a set of nodes that will be targeted by an output plugin, by the nature of the implementation an output plugin must support the Upstream feature. The following plugin(s) have Upstream support:

• Forward

The current balancing mode implemented is round-robin.

Configuration

To define an Upstream it's required to create an specific configuration file that contains an UPSTREAM and one or multiple NODE sections. The following table describe the properties associated to each section. Note that all of them are mandatory:

Nodes and specific plugin configuration

A Node might contain additional configuration keys required by the plugin, on that way we provide enough flexibility for the output plugin, a common use case is Forward output where if TLS is enabled, it requires a shared key (more details in the example below).

Nodes and TLS (Transport Layer Security)

In addition to the properties defined in the table above, the network operations against a defined node can optionally be done through the use of TLS for further encryption and certificates use.

The TLS options available are described in the section and can be added to the any Node section.

Configuration File Example

The following example defines an Upstream called forward-balancing which aims to be used by Forward output plugin, it register three Nodes:

• node-1: connects to 127.0.0.1:43000

• node-2: connects to 127.0.0.1:44000

• node-3: connects to 127.0.0.1:45000 using TLS without verification. It also defines a specific configuration option required by Forward output called shared_key.

Note that every Upstream definition must exists on it own configuration file in the file system. Adding multiple Upstreams in the same file or different files is not allowed.

Configuration files must be flexible enough for any deployment need, but they must keep a clean and readable format.

Fluent Bit Commands extends a configuration file with specific built-in features. The list of commands available as of Fluent Bit 0.12 series are:

@INCLUDE Command

Configuring a logging pipeline might lead to an extensive configuration file. In order to maintain a human-readable configuration, it's suggested to split the configuration in multiple files.

The @INCLUDE command allows the configuration reader to include an external configuration file, e.g:

The above example defines the main service configuration file and also include two files to continue the configuration:

inputs.conf

outputs.conf

Note that despites the order of inclusion, Fluent Bit will ALWAYS respect the following order:

• Service

• Inputs

• Filters

• Outputs

@SET Command

Fluent Bit supports , one way to expose this variables to Fluent Bit is through setting a Shell environment variable, the other is through the @SET command.

The @SET command can only be used at root level of each line, meaning it cannot be used inside a section, e.g:

In an ideal world, applications might log their messages within a single line, but in reality applications generate multiple log messages that sometimes belong to the same context. But when is time to process such information it gets really complex. Consider application stack traces which always have multiple log lines.

Starting from Fluent Bit v1.8, we have implemented a unified Multiline core functionality to solve all the user corner cases. In this section, you will learn about the features and configuration options available.

Concepts

The Multiline parser engine exposes two ways to configure and use the functionality:

• Built-in multiline parser

• Configurable multiline parser

Built-in Multiline Parsers

Without any extra configuration, Fluent Bit exposes certain pre-configured parsers (built-in) to solve specific multiline parser cases, e.g:

Configurable Multiline Parsers

Besides the built-in parsers listed above, through the configuration files is possible to define your own Multiline parsers with their own rules.

A multiline parser is defined in a parsers configuration file by using a [MULTILINE_PARSER] section definition. The Multiline parser must have a unique name and a type plus other configured properties associated with each type.

To understand which Multiline parser type is required for your use case you have to know beforehand what are the conditions in the content that determines the beginning of a multiline message and the continuation of subsequent lines. We provide a regex based configuration that supports states to handle from the most simple to difficult cases.

Lines and States

Before start configuring your parser you need to know the answer to the following questions:

1. What is the regular expression (regex) that matches the first line of a multiline message ?

2. What are the regular expressions (regex) that match the continuation lines of a multiline message ?

When matching regex, we have to define states, some states define the start of a multiline message while others are states for the continuation of multiline messages. You can have multiple continuation states definitions to solve complex cases.

The first regex that matches the start of a multiline message is called start_state, then other regexes continuation lines can have different state names.

Rules Definition

A rule specifies how to match a multiline pattern and perform the concatenation. A rule is defined by 3 specific components:

1. state name

2. regular expression pattern

3. next state

A rule might be defined as follows (comments added to simplify the definition) :

In the example above, we have defined two rules, each one has its own state name, regex patterns, and the next state name. Every field that composes a rule must be inside double quotes.

The first rule of state name must always be start_state, and the regex pattern must match the first line of a multiline message, also a next state must be set to specify how the possible continuation lines would look like.

Configuration Example

The following example provides a full Fluent Bit configuration file for multiline parsing by using the definition explained above.

Example files content:

By running Fluent Bit with the given configuration file you will obtain:

The lines that did not match a pattern are not considered as part of the multiline message, while the ones that matched the rules were concatenated properly.

Limitations

The multiline parser is a very powerful feature, but it has some limitations that you should be aware of:

• The multiline parser is not affected by the buffer_max_size configuration option, allowing the composed log record to grow beyond this size. Hence, the skip_long_lines option will not be applied to multiline messages.

• It is not possible to get the time key from the body of the multiline message. However, it can be extracted and set as a new key by using a filter.

Get structured data from multiline message

Fluent-bit supports /pat/m option. It allows . matches a new line. It is useful to parse multiline log.

The following example is to get date and message from concatenated log.

Example files content:

By running Fluent Bit with the given configuration file you will obtain:

One of the ways to configure Fluent Bit is using a YAML configuration file that works at a global scope.

The YAML configuration file supports the following sections:

• Env

• Includes

• Service

• Pipeline

  • Inputs

  • Filters

  • Outputs

The YAML configuration file does not support the following sections yet:

• Parsers

Env

The env section allows the definition of configuration variables that will be used later in the configuration file.

Example:

Includes

The includes section allows the files to be merged into the YAML configuration to be identified as a list of filenames. If no path is provided, then the file is assumed to be in a folder relative to the file referencing it.

Example:

Service

The service section defines the global properties of the service. The Service keys available as of this version are described in the following table:

The following is an example of a service section:

For scheduler and retry details, please check there:

Pipeline

A pipeline section will define a complete pipeline configuration, including inputs, filters and outputs subsections.

Each of the subsections for inputs, filters and outputs constitutes an array of maps that has the parameters for each. Most properties are either simple strings or numbers so can be define directly, ie:

This pipeline consists of two inputs; a tail plugin and an http server plugin. Each plugin has its own map in the array of inputs consisting of simple properties. To use more advanced properties that consist of multiple values the property itself can be defined using an array, ie: the record and allowlist_key properties for the record_modifier filter:

In the cases where each value in a list requires two values they must be separated by a space, such as in the record property for the record_modifier filter.

Input

An input section defines a source (related to an input plugin). Here we will describe the base configuration for each input section. Note that each input plugin may add it own configuration keys:

The Name is mandatory and it lets Fluent Bit know which input plugin should be loaded. The Tag is mandatory for all plugins except for the input forward plugin (as it provides dynamic tags).

Example input

The following is an example of an input section for the cpu plugin.

Filter

A filter section defines a filter (related to a filter plugin). Here we will describe the base configuration for each filter section. Note that each filter plugin may add its own configuration keys:

The Name is mandatory and it lets Fluent Bit know which filter plugin should be loaded. The Match or Match_Regex is mandatory for all plugins. If both are specified, Match_Regex takes precedence.

Example filter

The following is an example of a filter section for the grep plugin:

Output

The outputs section specify a destination that certain records should follow after a Tag match. Currently, Fluent Bit can route up to 256 OUTPUT plugins. The configuration supports the following keys:

Example output

The following is an example of an output section:

Example: collecting CPU metrics

The following configuration file example demonstrates how to collect CPU metrics and flush the results every five seconds to the standard output:

Processors

In recent versions of Fluent-Bit, the input and output plugins can run in separate threads. In Fluent-Bit 2.1.2, we have implemented a new interface called "processor" to extend the processing capabilities in input and output plugins directly without routing the data. This interface allows users to apply data transformations and filtering to incoming data records before they are processed further in the pipeline.

This functionality is only exposed in YAML configuration and not in classic configuration mode due to the restriction of nested levels of configuration.

Example: Using processors.

The following configuration file example demonstrates the use of processors to change the log record in the input plugin section by adding a new key "hostname" with the value "monox", and we use lua to append the tag to the log record. Also in the ouput plugin section we added a new key named "output" with the value "new data". All these without the need of routing the logs further in the pipeline.