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f4 Plugin Architecture (F4-RPC)

f4 uses an Out-of-Process RPC architecture based on stdin/stdout and the MessagePack binary protocol.

Why Out-of-Process?

In earlier versions, f4 experimented with embedded WASM (wazero) and Lua (gopher-lua) interpreters to keep plugins strictly in-process while maintaining safety. This approach was abandoned due to several critical flaws:

  1. Binary Bloat: Embedding multiple interpreters bloated the f4 core binary.
  2. Platform Access Limitations: WASM (WASI) severely restricts access to native OS capabilities (raw network sockets, complex file system operations), making it nearly impossible to write powerful plugins like process manager or Widows registry editor without building massive, fragile bridging layers.
  3. Language Lock-in: Forcing developers to write in a specific dialect of Lua constrained ecosystem growth.

The F4-RPC Solution

By moving to a separate-process model communicating over standard pipes:

  1. Zero Core Bloat: The f4 binary remains extremely lean.
  2. Language Agnostic: You can write a plugin in Go, Python, Rust, C++, Node.js, or LuaJIT. As long as the process can read/write MessagePack to stdin/stdout, it works.
  3. Unrestricted Power: Plugins run as native OS processes and can access the network, native libraries, and the full filesystem directly.
  4. Binary Efficiency: We chose MessagePack over JSON-RPC to minimize serialization overhead and reduce data transfer latency for large ReadDir chunks.

How it works

  1. f4 scans the plugins/ directory for executable files.
  2. It launches each executable via os/exec, hooking into its stdin, stdout, and stderr.
  3. stderr is immediately piped to f4's internal debug.log for easy debugging.
  4. f4 sends a Plugin.Init request over stdin.
  5. The plugin replies with its capabilities (e.g., registered Virtual File Systems).
  6. When a user interacts with the plugin's VFS, f4 transparently routes ReadDir, Stat, Open, etc., as RPC calls.

Building a Plugin (Go SDK)

For Go developers, an SDK is provided in sdk/f4plugin. It handles all the multiplexing and binary protocol details, exposing a clean, synchronous interface. Check plugins/dummy/main.go for a reference implementation.