Gamend - LibGodot Integration

Previously I had added integration with Godot from Gamend:

• Host (Elixir) -> Godot (WebSocket)

The Problem

This was nice as a proof of concept, but in real life, it was slow:

The Simple Function is a HTTP call with Authentication, while the Complex Function one does also database access or cache access.

In Godot it is so slow because the communication layer is WebSocket, and whenever Godot calls back into Host, it does so with HTTP. For a simple case, it’s just the WebSocket overhead:

Gamend --WS-> Godot --WS-> Gamend

For a more complex case, it is both the WebSocket and the HTTP layer from Gamend:

Gamend
| (Send input)
WS
v
Godot -HTTP-> Gamend (eg. create KV)
      -HTTP-> Gamend (eg. get KV)
| (Receive result)
WS
v
Gamend

The delay added from both WS (WebSocket) and HTTP (calling back into the server for Admin SDK) is too big to make it feasible.

LibGodot

LibGodot is an upcoming feature of Godot where one can now build the engine as a shared library (or static), and then start the engine from within the host application.

Until that ships, I cloned locally migeran/libgodot (which uses Godot 4.3 with some changes) and after some time of building both godot and godot-cpp, I built the cpp-sample and ran it:

int main(int argc, char **argv) {
    LibGodot libgodot;

    godot::GodotInstance *instance = libgodot.create_godot_instance(argc, argv);

    instance->start();
    while (!instance->iteration()) {}
    libgodot.destroy_godot_instance(instance);
}

How it works? It starts godot and calls iteration as to move the engine process function forward.

LibGodot in Gamend

I extended this functionality and sample to have a function for sending data to Godot, and one where Godot can send data back to Elixir (Gamend).

About the main thread

Since Elixir runs it’s application inside BEAM and that doesn’t offer access to the main thread (and Godot really wants to run on the main thread), I ended up starting a thread and running there Godot. That means Godot won’t be able to load menus (and possibly other stuff) that require main thread. This is fine for my use cases, as I run it headless anyway.

Data Bus

As an initial API, I made a Data Bus for sending and receiving data from and to Elixir from Godot.

extends MeshInstance3D

var bus

func _ready() -> void:
	bus = Engine.get_singleton("ElixirBus")
	if bus:
		bus.connect("new_message", Callable(self, "_on_new_message"))
		bus.send_event("status", "godot_ready")

func _on_new_message(_queued: int) -> void:
  for msg in bus.drain():
    bus.send_event("request_handled", msg)
    print("Elixir message: ", msg)
  
func _process(delta):
  print("A")
  print(rotation)
	rotate_x(delta)

Conclusion

And it works:

And the time is much better than before, in order of 0.1 - 0.5 ms, closer to what Elixir was obtaining: