# Lua
Lua Filter allows you to modify the incoming records using custom [Lua](https://www.lua.org/) 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
## 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 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. |
| protected\_mode | If enabled, Lua script will be executed in protected mode. It prevents to crash when invalid Lua script is executed. Default is true. |
| time\_as\_table | By default when the Lua script is invoked, the record timestamp is passed as a Floating number which might lead to loss precision when the data 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. |
## 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](https://github.com/fluent/fluent-bit-docs/tree/2a0c790c69100939636c7ed50bebe2fa06a3a57f/pipeline/input/dummy.md) input plugin for data ingestion, invoke Lua filter using the [test.lua](https://github.com/fluent/fluent-bit/blob/master/scripts/test.lua) script and calls the [cb\_print()](https://github.com/fluent/fluent-bit/blob/master/scripts/test.lua#L29) function which only print the same information to the standard output:
### Command Line
From the command line you can use the following options:
```bash
$ 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:
```python
[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 filter\_lua 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:
```lua
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 filter\_lua must drop the record. 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. |
| 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:
### 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.
### Protected Mode
Fluent Bit supports protected mode to prevent crash when executes invalid Lua script. See also [Error Handling in Application Code](https://www.lua.org/pil/24.3.1.html).