The Lua filter allows you to modify the incoming records (even split one record into multiple records) using custom Lua scripts.

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

  1. Configure the Filter in the main configuration

  2. Prepare a Lua script that will be used by the Filter

Configuration Parameters

The plugin supports the following configuration parameters:



Path to the Lua script that will be used. This can be a relative path against the main configuration file.


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


If these keys are matched, the fields are converted to integer. If more than one key, delimit by space. Note that starting from Fluent Bit v1.6 integer data types are preserved and not converted to double as in previous versions.


If these keys are matched, the fields are handled as array. If more than one key, delimit by space. It is useful the array can be empty.


If enabled, Lua script will be executed in protected mode. It prevents Fluent Bit from crashing when invalid Lua script is executed or the triggered Lua function throws exceptions. Default is true.


By default when the Lua script is invoked, the record timestamp is passed as a floating number which might lead to precision loss when it is converted back. If you desire timestamp precision, enabling this option will pass the timestamp as a Lua table with keys sec for seconds since epoch and nsec for nanoseconds.


Inline LUA code instead of loading from a path via script.


If enabled, null will be converted to flb_null in Lua. It is useful to prevent removing key/value since nil is a special value to remove key value from map in Lua. Default is false.

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 use the dummy input plugin for data ingestion, invoke Lua filter using the test.lua script and call the cb_print() function which only prints 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:

    Name    dummy

    Name    lua
    Match   *
    script  test.lua
    call    cb_print

    Name    null
    Match   *

Lua Script Filter API

The life cycle of a filter have the following steps:

  1. Upon Tag matching by this filter, it may process or bypass the record.

  2. If tag matched, it will accept the record and invoke the function defined in the call property which basically is the name of a function defined in the Lua script.

  3. Invoke Lua function and pass each record in JSON format.

  4. Upon return, validate return value and continue the pipeline.

Callback Prototype

The Lua script can have one or multiple callbacks that can be used by this filter. The function prototype is as follows:

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

Function Arguments



Name of the tag associated with the incoming record.


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


Lua table with the record content

Return Values

Each callback must return three values:

namedata typedescription



The code return value represents the result and further action that may follows. If code equals -1, means that the record will be dropped. If code equals 0, the record will not be modified, otherwise if code equals 1, means the original timestamp and record have been modified so it must be replaced by the returned values from timestamp (second return value) and record (third return value). If code equals 2, means the original timestamp is not modified and the record has been modified so it must be replaced by the returned values from record (third return value). The code 2 is supported from v1.4.3.



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



If code equals 1, the original record information will be replaced with this new value. Note that the record value must be a valid Lua table. This value can be an array of tables (i.e., array of objects in JSON format), and in that case the input record is effectively split into multiple records. (see below for more details)

Code Examples

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

Inline configuration

The Fluent Bit smoke tests include examples to verify during CI.

    flush:           1
    daemon:          off
    log_level:       info

        - name:    random
          tag:     test
          samples: 10

        - name:  lua
          match: "*"
          call:  append_tag
          code:  |
              function append_tag(tag, timestamp, record)
                 new_record = record
                 new_record["tag"] = tag
                 return 1, timestamp, new_record

        - name:  stdout
          match: "*"

In classic mode:

	flush 1
	daemon off
	log_level debug

	Name random
	Tag test
	Samples 10

	Name Lua
	Match *
	call append_tag
	code function append_tag(tag, timestamp, record) new_record = record new_record["tag"] = tag return 1, timestamp, new_record end

	Name stdout
	Match *

Environment variable processing

As an example that combines a bit of LUA processing with the Kubernetes filter that demonstrates using environment variables with LUA regex and substitutions.

Kubernetes pods generally have various environment variables set by the infrastructure automatically which may contain useful information.

In this example, we want to extract part of the Kubernetes cluster API name.

The environment variable is set like so: KUBERNETES_SERVICE_HOST:

We want to extract the sandboxbsh name and add it to our record as a special key.

          Name                lua
          Alias               filter-iots-lua
          Match               iots_thread.*
          Script              filters.lua
          Call                set_landscape_deployment

  filters.lua: |
    -- Use a Lua function to create some additional entries based
    -- on substrings from the kubernetes properties.
    function set_landscape_deployment(tag, timestamp, record)
        local landscape = os.getenv("KUBERNETES_SERVICE_HOST")
        if landscape then
            -- Strip the landscape name from this field, KUBERNETES_SERVICE_HOST
            -- Should be of this format
            -- Take off the leading "api."
            --print("landscape1:" .. landscape)
            landscape = landscape:gsub("^[^.]+.", "")
            --print("landscape2:" .. landscape)
            -- Take off everything including and after the - in the cluster name
            -- sandboxbsh
            landscape = landscape:gsub("-.*$", "")
            -- print("landscape3:" .. landscape)
            record["iot_landscape"] = landscape
        -- 2 - replace existing record with this update
        return 2, timestamp, record

Number Type

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

Protected Mode

Fluent Bit supports protected mode to prevent crash when executes invalid Lua script. See also Error Handling in Application Code.

Record Split

The Lua callback function can return an array of tables (i.e., array of records) in its third record return value. With this feature, the Lua filter can split one input record into multiple records according to custom logic.

For example:

Lua script

function cb_split(tag, timestamp, record)
    if record["x"] ~= nil then
        return 2, timestamp, record["x"]
        return 2, timestamp, record


    Name    stdin

    Name    lua
    Match   *
    script  test.lua
    call    cb_split

    Name    stdout
    Match   *


{"x": [ {"a1":"aa", "z1":"zz"}, {"b1":"bb", "x1":"xx"}, {"c1":"cc"} ]}
{"x": [ {"a2":"aa", "z2":"zz"}, {"b2":"bb", "x2":"xx"}, {"c2":"cc"} ]}
{"a3":"aa", "z3":"zz", "b3":"bb", "x3":"xx", "c3":"cc"}


[0] stdin.0: [1538435928.310583591, {"a1"=>"aa", "z1"=>"zz"}]
[1] stdin.0: [1538435928.310583591, {"x1"=>"xx", "b1"=>"bb"}]
[2] stdin.0: [1538435928.310583591, {"c1"=>"cc"}]
[3] stdin.0: [1538435928.310588359, {"z2"=>"zz", "a2"=>"aa"}]
[4] stdin.0: [1538435928.310588359, {"b2"=>"bb", "x2"=>"xx"}]
[5] stdin.0: [1538435928.310588359, {"c2"=>"cc"}]
[6] stdin.0: [1538435928.310589790, {"z3"=>"zz", "x3"=>"xx", "c3"=>"cc", "a3"=>"aa", "b3"=>"bb"}]

See also Fluent Bit: PR 811.

Response code filtering

In this example, we want to filter istio logs to exclude lines with response codes between 1 and 399. Istio is configured to write the logs in json format.

Lua script

Script response_code_filter.lua

function cb_response_code_filter(tag, timestamp, record)
  response_code = record["response_code"]
  if (response_code == nil or response_code == '') then
    return 0,0,0
  elseif (response_code ~= 0 and response_code < 400) then
    return -1,0,0
    return 0,0,0


Configuration to get istio logs and apply response code filter to them.

        Name                tail
        Path                /var/log/containers/*_istio-proxy-*.log
        multiline.parser    docker, cri
        Tag                 istio.*
        Mem_Buf_Limit       64MB
        Skip_Long_Lines     Off

        Name                lua
        Match               istio.*
        Script              response_code_filter.lua
        call                cb_response_code_filter

        Name                stdout
        Match               *


    "log": {
        "response_code": 200,
        "bytes_sent": 111328341,
        "authority": "randomservice.randomservice",
        "duration": 14493,
        "request_id": "2e9d38f8-36a9-40a6-bdb2-47c8eb7d399d",
        "upstream_local_address": "",
        "downstream_local_address": "",
        "upstream_cluster": "outbound|80||randomservice.svc.cluster.local",
        "x_forwarded_for": null,
        "route_name": "default",
        "upstream_host": "",
        "user_agent": "RandomUserAgent",
        "response_code_details": "via_upstream",
        "downstream_remote_address": "",
        "bytes_received": 1148,
        "path": "/?parameter=random",
        "response_flags": "-",
        "start_time": "2022-07-28T11:16:51.663Z",
        "upstream_transport_failure_reason": null,
        "method": "POST",
        "connection_termination_details": null,
        "protocol": "HTTP/1.1",
        "requested_server_name": null,
        "upstream_service_time": "6161"
    "stream": "stdout",
    "time": "2022-07-28T11:17:06.704109897Z"


In the output only the messages with response code 0 or greater than 399 are shown.

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