Lua

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Lua Filter allows you to modify the incoming records using custom Lua Scripts.

Due to the necessity to have a flexible filtering mechanism, now is possible to extend Fluent Bit capabilities writing simple filters using Lua programming language. A Lua based filter takes two steps:

  • Configure the Filter in the main configuration

  • Prepare a Lua script that will be used by the Filter

Content:

Configuration Parameters

The plugin supports the following configuration parameters:

Key

Description

Script

Path to the Lua script that will be used.

Call

Lua function name that will be triggered to do filtering. It's assumed that the function is declared inside the Script defined above.

Type_int_key

If the key is matched, that field will be converted to integer.

Getting Started

In order to test the filter, you can run the plugin from the command line or through the configuration file. The following examples uses the dummy input plugin for data ingestion, invoke Lua filter using the test.lua script and calls the cb_print() function which only print the same information to the standard output:

Command Line

From the command line you can use the following options:

$ fluent-bit -i dummy -F lua -p script=test.lua -p call=cb_print -m '*' -o null

Configuration File

In your main configuration file append the following Input, Filter & Output sections:

[INPUT]
Name dummy
[FILTER]
Name lua
Match *
script test.lua
call cb_print
[OUTPUT]
Name null
Match *

Lua Script Filter API

The life cycle of a filter have the following steps:

  • Upon Tag matching by filter_lua, it may process or bypass the record.

  • If filterlua accepts the record, it will invoke the function defined in the _call property which basically is the name of a function defined in the Lua script.

  • Invoke Lua function passing each record in JSON format.

  • Upon return, validate return value and take some action (described above)

Callback Prototype

The Lua script can have one or multiple callbacks that can be used by filter_lua, it prototype is as follows:

function cb_print(tag, timestamp, record)
return code, timestamp, record
end

Function Arguments

name

description

tag

Name of the tag associated with the incoming record.

timestamp

Unix timestamp with nanoseconds associated with the incoming record. The original format is a double (seconds.nanoseconds)

record

Lua table with the record content

Return Values

Each callback must return three values:

name

data type

description

code

integer

The code return value represents the result and further action that may follows. If code equals -1, means that filterlua must drop the record. If _code equals 0 the record will not be modified, otherwise if code equals 1, means the original timestamp or record have been modified so it must be replaced by the returned values from timestamp (second return value) and record (third return value).

timestamp

double

If code equals 1, the original record timestamp will be replaced with this new value.

record

table

if code equals 1, the original record information will be replaced with this new value. Note that the format of this value must be a valid Lua table.

Code Examples

For functional examples of this interface, please refer to the code samples provided in the source code of the project located here:

https://github.com/fluent/fluent-bit/tree/master/scripts

Number Type

In Lua, Fluent Bit treats number as double. It means an integer field (e.g. IDs, log levels) will be converted double. To avoid type conversion, Type_int_key property is available.