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WASM Input Plugins

WebAssembly is binary instruction format for stack based virtual machine.

Fluent Bit currently supports integration of wasm plugins built as wasm/wasi objects for input and filter plugins only. The interface for the WASM filter plugins is currently under development but is functional.

Prerequisites

Building Fluent Bit

There are no additional requirements to execute WASM plugins.

For Build WASM programs

Currently, fluent-bit supports the following WASM toolchains:

  • Rust on wasm32-unknown-unknown.
    • rustc 1.62.1 (e092d0b6b 2022-07-16) or later
  • TinyGo on wasm32-wasi
    • v0.24.0 or later
  • WASI SDK 13 or later.

Getting Started

Compile Fluent Bit with WASM support, e.g:

$ cd build/
$ cmake ..
$ make

Once compiled, we can see new plugins in which handles wasm, e.g:

$ bin/fluent-bit -h
Usage: fluent-bit [OPTION]
Inputs
  # ... other input plugin stuffs
  exec_wasi               Exec WASI Input

Filters
  # ... other filter plugin stuffs
  wasm                    WASM program filter

Build a WASM Input for Input Plugin

Currently, Fluent Bit's WASM input assumes WASI ABI that is also known as wasm32-wasi on Rust target and wasm32-wasi on TinyGo target.

To Install Additional Components

TinyGo and WASI SDK support wasm target by default. When using Rust's wasm32-wasi target, users must install wasm32-wasi via rustup. Then, installing that target components as:

$ rustup target add wasm32-wasi

Requirements of WASM/WASI programs

WASM input plugins execute the function that has wasi main function entrypoint. And Fluent Bit's WASM input plugin communicates via stdout on WASM programs.

WASM programs should handle stdout for ingesting logs into Fluent Bit.

Once built, a WASM/WASI program will be available. Then, that built program can be executed with the following Fluent Bit configuration:

[INPUT]
    Name exec_wasi
    Tag  exec.wasi.local
    WASI_Path /path/to/wasi_built_json.wasm
    # For security reasons, WASM/WASI program cannot access its outer world
    # without accessible permissions.
    # accessible_paths .,/path/to/fluent-bit

[OUTPUT]
    Name  stdout
    Match *

For further example that handles structured logs, fluent-bit has a Rust serde-json example: https://github.com/fluent/fluent-bit/tree/master/examples/wasi_serde_json