golang-output-plugins.md

Golang Output Plugins

Fluent Bit currently supports integration of Golang plugins built as shared objects for output plugins only. The interface for the Golang plugins is currently under development but is functional.

Getting Started

Compile Fluent Bit with Golang support, e.g:

$ cd build/
$ cmake -DFLB_DEBUG=On -DFLB_PROXY_GO=On ../
$ make

Once compiled, we can see a new option in the binary -e which stands for external plugin, e.g:

$ bin/fluent-bit -h
Usage: fluent-bit [OPTION]

Available Options
  -c  --config=FILE    specify an optional configuration file
  -d, --daemon        run Fluent Bit in background mode
  -f, --flush=SECONDS    flush timeout in seconds (default: 1)
  -i, --input=INPUT    set an input
  -m, --match=MATCH    set plugin match, same as '-p match=abc'
  -o, --output=OUTPUT    set an output
  -p, --prop="A=B"    set plugin configuration property
  -e, --plugin=FILE    load an external plugin (shared lib)
  ...

Build a Go Plugin

The fluent-bit-go package is available to assist developers in creating Go plugins.

https://github.com/fluent/fluent-bit-go

At a minimum, a Go plugin looks like this:

package main

import "github.com/fluent/fluent-bit-go/output"

//export FLBPluginRegister
func FLBPluginRegister(def unsafe.Pointer) int {
    // Gets called only once when the plugin.so is loaded
    return output.FLBPluginRegister(def, "gstdout", "Stdout GO!")
}

//export FLBPluginInit
func FLBPluginInit(plugin unsafe.Pointer) int {
    // Gets called only once for each instance you have configured.
    return output.FLB_OK
}

//export FLBPluginFlushCtx
func FLBPluginFlushCtx(ctx, data unsafe.Pointer, length C.int, tag *C.char) int {
    // Gets called with a batch of records to be written to an instance.
    return output.FLB_OK
}

//export FLBPluginExit
func FLBPluginExit() int {
    return output.FLB_OK
}

func main() {
}

the code above is a template to write an output plugin, it's really important to keep the package name as main and add an explicit main() function. This is a requirement as the code will be build as a shared library.

To build the code above, use the following line:

$ go build -buildmode=c-shared -o out_gstdout.so out_gstdout.go

Once built, a shared library called out\_gstdout.so will be available. It's really important to double check the final .so file is what we expect. Doing a ldd over the library we should see something similar to this:

$ ldd out_gstdout.so
    linux-vdso.so.1 =>  (0x00007fff561dd000)
    libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007fc4aeef0000)
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fc4aeb27000)
    /lib64/ld-linux-x86-64.so.2 (0x000055751a4fd000)

Run Fluent Bit with the new plugin

$ bin/fluent-bit -e /path/to/out_gstdout.so -i cpu -o gstdout

Configuration file

Fluent Bit can load / run Golang plugin using two configuration file.

• Plugins configuration file
• Main configuration file

Plugins configuration file

Key	Description	Default Value
Path	A path for a Golang plugin.

Example

[PLUGINS]
    Path /path/to/out_gstdout.so

Main configuration file

The keys for Golang plugin available as of this version are described in the following table:

Key	Description	Default Value
Plugins_file	Path for a plugins configuration file. A plugins configuration file allows to define paths for external plugins, for an example see here.

Example

The following is an example of a main configuration file.

[SERVICE]
    plugins_file /path/to/plugins.conf

[INPUT]
    Name dummy

[OUTPUT]
    Name gstdout

Config key constraint

Some config keys are reserved by Fluent Bit and must not be used by a custom plugin, they are: alias,host,ipv6,listen,log_level,log_suppress_interval,match,match_regex,mem_buf_limit,port,retry_limit,routable,storage.pause_on_chunks_overlimit, storage.total_limit_size, storage.type, tag,threaded,tls,tls.ca_file, tls.ca_path, tls.crt_file, tls.debug, tls.key_file, tls.key_passwd, tls.verify, tls.vhost, workers

Run using a configuration file

We can load a main configuration file using -c option. Note: No need to specify a plugins configuration file from command line.

fluent-bit -c fluent-bit.conf