Library API

Fluent Bit library it's written in C language and can be used from any C or C++ application. Before to digging into the specification is recommended to understand the workflow involved in the runtime.

Workflow

Fluent Bit runs as a service, meaning that the API exposed for developers provide interfaces to create and manage a context, specify inputs/outputs, set configuration parameters and set routing paths for the event/records. A typical usage of the library involves:

  • Create library instance/context and set properties.
  • Enable input plugin(s) and set properties.
  • Enable output plugin(s) and set properties.
  • Start the library runtime.
  • Optionally ingest records manually.
  • Stop the library runtime.
  • Destroy library instance/context.

Data Types

Starting from Fluent Bit v0.9, there is only one data type exposed by the library, by convention prefixed with flb_.

Type Description
flb_ctx_t Main library context. It aims to reference the context returned by flb_create();

API Reference

Library Context Creation

As described earlier, the first step to use the library is to create a context of it, for the purpose the function flb_create() is used.

Prototype

flb_ctx_t *flb_create();

Return Value

On success, flb_create() returns the library context; on error, it returns NULL.

Usage

flb_ctx_t *ctx;

ctx = flb_create();
if (!ctx) {
    return NULL;
}

Set Service Properties

Using the flb_service_set() function is possible to set context properties.

Prototype

int flb_service_set(flb_ctx_t *ctx, ...);

Return Value

On success it returns 0; on error it returns a negative number.

Usage

The flb_service_set() allows to set one or more properties in a key/value string mode, e.g:

int ret;

ret = flb_service_set(ctx, "Flush", "1", NULL);

The above example specified the values for the properties Flush , note that the value is always a string (char *) and once there is no more parameters a NULL argument must be added at the end of the list.

Enable Input Plugin Instance

When built, Fluent Bit library contains a certain number of built-in input plugins. In order to enable an input plugin, the function flb_input() is used to create an instance of it.

For plugins, an instance means a context of the plugin enabled. You can create multiples instances of the same plugin.

Prototype

int flb_input(flb_ctx_t *ctx, char *name, void *data);

The argument ctx represents the library context created by flb_create(), then name is the name of the input plugin that is required to enable.

The third argument data can be used to pass a custom reference to the plugin instance, this is mostly used by custom or third party plugins, for generic plugins passing NULL is OK.

Return Value

On success, flb_input() returns an integer value >= zero (similar to a file descriptor); on error, it returns a negative number.

Usage

int in_ffd;

in_ffd = flb_input(ctx, "cpu", NULL);

Set Input Plugin Properties

A plugin instance created through flb_input(), may provide some configuration properties. Using the flb_input_set() function is possible to set these properties.

Prototype

int flb_input_set(flb_ctx_t *ctx, int in_ffd, ...);

Return Value

On success it returns 0; on error it returns a negative number.

Usage

The flb_input_set() allows to set one or more properties in a key/value string mode, e.g:

int ret;

ret = flb_input_set(ctx, in_ffd,
                    "tag", "my_records",
                    "ssl", "false",
                    NULL);

The argument ctx represents the library context created by flb_create(). The above example specified the values for the properties tag and ssl, note that the value is always a string (char *) and once there is no more parameters a NULL argument must be added at the end of the list.

The properties allowed per input plugin are specified on each specific plugin documentation.

Enable Output Plugin Instance

When built, Fluent Bit library contains a certain number of built-in output plugins. In order to enable an output plugin, the function flb_output() is used to create an instance of it.

For plugins, an instance means a context of the plugin enabled. You can create multiples instances of the same plugin.

Prototype

int flb_output(flb_ctx_t *ctx, char *name, void *data);

The argument ctx represents the library context created by flb_create(), then name is the name of the output plugin that is required to enable.

The third argument data can be used to pass a custom reference to the plugin instance, this is mostly used by custom or third party plugins, for generic plugins passing NULL is OK.

Return Value

On success, flb_output() returns the output plugin instance; on error, it returns a negative number.

Usage

int out_ffd;

out_ffd = flb_output(ctx, "stdout", NULL);

Set Output Plugin Properties

A plugin instance created through flb_output(), may provide some configuration properties. Using the flb_output_set() function is possible to set these properties.

Prototype

int flb_output_set(flb_ctx_t *ctx, int out_ffd, ...);

Return Value

On success it returns an integer value >= zero (similar to a file descriptor); on error it returns a negative number.

Usage

The flb_output_set() allows to set one or more properties in a key/value string mode, e.g:

int ret;

ret = flb_output_set(ctx, out_ffd,
                     "tag", "my_records",
                     "ssl", "false",
                     NULL);

The argument ctx represents the library context created by flb_create(). The above example specified the values for the properties tag and ssl, note that the value is always a string (char *) and once there is no more parameters a NULL argument must be added at the end of the list.

The properties allowed per output plugin are specified on each specific plugin documentation.

Start Fluent Bit Engine

Once the library context have been created and the input/output plugin instances are set, the next step is to start the engine. When started, the engine runs inside a new thread (POSIX thread) without blocking the caller application. To start the engine the function flb_start() is used.

Prototype

int flb_start(flb_ctx_t *ctx);

Return Value

On success it returns 0; on error it returns a negative number.

Usage

This simple call only needs as argument ctx which is the reference to the context created at the beginning with flb_create():

int ret;

ret = flb_start(ctx);

Stop Fluent Bit Engine

To stop a running Fluent Bit engine, we provide the call flb_stop() for that purpose.

Prototype

int flb_stop(flb_ctx_t *ctx);

The argument ctx is a reference to the context created at the beginnning with flb_create() and previously started with flb_start().

When the call is invoked, the engine will wait a maximum of five seconds to flush buffers and release the resources in use. A stopped context can be re-started any time but without any data on it.

Return Value

On success it returns 0; on error it returns a negative number.

Usage

int ret;

ret = flb_stop(ctx);

Destroy Library Context

A library context must be destroyed after is not longer necessary, note that a previous flb_stop() call is mandatory. When destroyed all resources associated are released.

Prototype

void flb_destroy(flb_ctx_t *ctx);

The argument ctx is a reference to the context created at the beginnning with flb_create().

Return Value

No return value.

Usage

flb_destroy(ctx);

Ingest Data Manually

There are some cases where the caller application may want to ingest data into Fluent Bit, for this purpose exists the function flb_lib_push().

Prototype

int flb_lib_push(flb_ctx_t *ctx, int in_ffd, void *data, size_t len);

The first argument is the context created previously through flb_create(). in_ffd is the numeric reference of the input plugin (for this case it should be an input of plugin lib type), data is a reference to the message to be ingested and len the number of bytes to take from it.

Return Value

On success, it returns the number of bytes written; on error it returns -1.

Usage

For more details and an example about how to use this function properly please refer to the next section Ingest Records Manually.

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