WASM Filter 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.


Building Fluent Bit

There are no additional requirements to execute WASM plugins.

Building flb-wamrc (optional)

flb-wamrc is just flb- prefixed AOT (Ahead Of Time) compiler that is provided from wasm-micro-runtime.

For flb-wamrc support, users have to install llvm infrastructure and some additional libraries (libmlir, libPolly, libedit, and libpfm), e.g:

# apt install -y llvm libmlir-14-dev libclang-common-14-dev libedit-dev libpfm4-dev

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

As described in general options in source installation, WASM support is enabled by default. Compile Fluent Bit with WASM support, e.g:

$ cd build/
$ cmake .. [-DFLB_WAMRC=On]
$ make

To support AOT compiled WASM execution as filter plugins, users have to built Fluent Bit with -DFLB_WAMRC=On.

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

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

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

Build a WASM Filter for Filter Plugin

Currently, Fluent Bit's WASM filter assumes C ABI that is also known as wasm32-unknown-unknown 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-unknown-unknown target, users must install wasm32-unknown-unknown via rustup. Then, installing that target components as:

$ rustup target add wasm32-unknown-unknown

Requirements of WASM programs

WASM filter plugins execute the function that has the following signagure.

For C:

// We can use an arbitrary function name for filter operations w/ WASM.
char* c_filter(char*, int, uint32_t, uint32_t, char*, int);

For Go(TinyGo):

//export go_filter
// And this function should be placed in the main package.
func go_filter(tag *uint8, tag_len uint, time_sec uint, time_nsec uint, record *uint8, record_len uint) *uint8

For Rust:

// #[no_mangle] attribute is needed for preventing mangles and align C ABI.
// Also we can use an arbitrary function name for filter operations w/ WASM.
pub extern “C” fn rust_filter(tag: *const c_char, tag_len: u32, time_sec: u32, time_nsec: u32, record: *const c_char, record_len: u32)

Note that //export XXX on TinyGo and #[no_mangle] attributes on Rust are required. This is because TinyGo and Rust will mangle their function names if they are not specified.

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

    Name dummy
    Tag dummy.local

    Name wasm
    Match dummy.*
    WASM_Path /path/to/built_filter.wasm
    Function_Name super_awesome_filter
    accessible_paths .,/path/to/fluent-bit

    Name  stdout
    Match *

For example, one of the examples of Rust WASM filter should generate its filtered logs as follows:

[0] dummy.local: [1666270588.271704000, {"lang"=>"Rust", "message"=>"dummy", "original"=>"{"message":"dummy"}", "tag"=>"dummy.local", "time"=>"2022-10-20T12:56:28.271704000 +0000"}]
[0] dummy.local: [1666270589.270348000, {"lang"=>"Rust", "message"=>"dummy", "original"=>"{"message":"dummy"}", "tag"=>"dummy.local", "time"=>"2022-10-20T12:56:29.270348000 +0000"}]
[0] dummy.local: [1666270590.271107000, {"lang"=>"Rust", "message"=>"dummy", "original"=>"{"message":"dummy"}", "tag"=>"dummy.local", "time"=>"2022-10-20T12:56:30.271107000 +0000"}]

Another example of a Rust WASM filter is the flb_filter_iis filter. This filter takes the Internet Information Services (IIS) w3c logs (with some custom modifications) and transforms the raw string into a standard Fluent Bit JSON structured record.

    Name             dummy
    Dummy            {"log": "2023-08-11 19:56:44 W3SVC1 WIN-PC1 ::1 GET / - 80 ::1 Mozilla/5.0+(Windows+NT+10.0;+Win64;+x64)+AppleWebKit/537.36+(KHTML,+like+Gecko)+Chrome/ - - localhost 304 142 756 1078 -"}
    Tag              iis.*

    Name             wasm
    match            iis.*
    WASM_Path        /plugins/flb_filter_iis_wasm.wasm
    Function_Name    flb_filter_log_iis_w3c_custom
    accessible_paths .

    name             stdout
    match            iis.*

The incoming raw strings from an IIS log are composed of the following fields:

date time s-sitename s-computername s-ip cs-method cs-uri-stem cs-uri-query s-port c-ip cs(User-Agent) cs(Cookie) cs(Referer) cs-host sc-status sc-bytes cs-bytes time-taken c-authorization-header

The output after the filter logic will be:

[0] iis.*: [[1692131925.559486675, {}], {"c_authorization_header"=>"-", "c_ip"=>"::1", "cs_bytes"=>756, "cs_cookie"=>"-", "cs_host"=>"localhost", "cs_method"=>"GET", "cs_referer"=>"-", "cs_uri_query"=>"-", "cs_uri_stem"=>"/", "cs_user_agent"=>"Mozilla/5.0+(Windows+NT+10.0;+Win64;+x64)+AppleWebKit/537.36+(KHTML,+like+Gecko)+Chrome/", "date"=>"2023-08-11 19:56:44", "s_computername"=>"WIN-PC1", "s_ip"=>"::1", "s_port"=>"80", "s_sitename"=>"W3SVC1", "sc_bytes"=>142, "sc_status"=>"304", "source"=>"LogEntryIIS", "tag"=>"iis.*", "time"=>"2023-08-15T20:38:45.559486675 +0000", "time_taken"=>1078}]

This filter approach provides us with several powerful advantages inherent to programming languages. For instance, it:

  • Can be extended by adding type conversion to fields such as sc_bytes, cs_bytes, time_taken. This is particularly useful when we need to validate our data results.

  • Allows for the use of conditions to apply more descriptive filters, for example, "get only all logs that contain status codes above 4xx or 5xx".

  • Can be used to define a allow/deny list using a data structure array or a file to store predefined IP addresses.

  • Makes it possible to call an external resource such as an API or database to enhance our data.

  • Allows all methods to be thoroughly tested and shared as a binary bundle or library. These examples can be applied in our demo and can serve as an ideal starting point to create more complex logic, depending on our requirements.

Optimize execution of WASM programs

To optimize WASM program execution, there is the option of using flb-wamrc. flb-wamrc will reduce runtime footprint and to be best performance for filtering operations. This tool will be built when -DFLB_WAMRC=On cmake option is specified and llvm infrastructure is installed on the building box.

$ flb-wamrc -o /path/to/built_wasm.aot /path/to/built_wasm.wasm

For further optimizations to the specific CPU such as Cortex-A57 series, e.g:

$ flb-wamrc --size-level=3 --target=aarch64v8 --cpu=cortex-a57 -o /path/to/built_wasm.aot /path/to/built_wasm.wasm

Then, when AOT (Ahead Of Time) compiling is succeeded:

Create AoT compiler with:
  target:        aarch64v8
  target cpu:    cortex-a57
  cpu features:
  opt level:     3
  size level:    3
  output format: AoT file
Compile success, file /path/to/built_wasm.aot was generated.

Note that AOT compiling should generate CPU architecture-dependent objects. If users want to use AOT compiled object on the different archtecture, it must align the target and target cpu for actual environments.

Further Concrete Examples

  • C Filter

    • https://github.com/fluent/fluent-bit/tree/master/examples/filter_wasm_c

  • Rust Filter

    • https://github.com/fluent/fluent-bit/tree/master/examples/filter_rust

  • TinyGo Filter

    • https://github.com/fluent/fluent-bit/tree/master/examples/filter_wasm_go

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