How does RPC work under the hood? | Godot Engine | Page 1

heavy meteor Jul 7, 2025, 2:43 AM

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I'm an experienced developer who has created a multiplayer game before with ENet, in my own engine.

I want to know more about how @rpc works under the hood.

how does the system identify objects across client-server boundary? Is each node given a unique id, shared across client-server?
If so, how are these id's set?
Is there a way to access these ids?

heavy meteor Jul 7, 2025, 3:29 AM

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yo!! :))

finite axle Jul 7, 2025, 3:35 AM

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I checked the source, it's done by NodePath. First it checks if every peer has been sent an RPC with that path before, if not then it sends a message like

[path_hash][node_path][rpc_data]

Else it just sends

[path_hash][rpc_data]

There's no way to get this info, the hashtable is maintained automatically.

heavy meteor Jul 7, 2025, 3:35 AM

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thank you so much

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Interesting. So there's an internal map

{
  [node_hash] -> node_path
}

that client and server both keep

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and on the first packet sent, the path/hash is registered. That's so amazing

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I built a similar system for my game with ids, but it was all hand-made (and probably buggy 🤣)

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thanks <3

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Do you have any tips for diving into the source code? I suspect that I will have questions like this in the future for other things; I'd like to be able to just dive through the godot source

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Oh wait im silly

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its the hash -> node-path, other way around

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woops

finite axle Jul 7, 2025, 3:50 AM

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The nodes communicate with "servers" using a handle called an RID. When node properties are changed, they may call like RenderingServer.set_transform(self.rid, self.transform) sort of thing. It's normal for these servers to act like a black box with most actions being sending data into them. Keeps the API surface small.

The nodes usually are found in /scene and the server in /server. Sometimes a server is implemented there too, if not it can be found in /modules or /drivers.

So my search went like
MultiplayerServer -> /servers/multiplayer_server -> /modules/multiplayer/ -> ... -> https://github.com/godotengine/godot/blob/53be3b78d1634406f1fb29e3802c608a5f5104a1/modules/multiplayer/scene_rpc_interface.cpp#L306

#How does RPC work under the hood?