Potential $effect.server rune?

[peterreeves]

I've had a few projects recently using adapter-node where I'm using either streaming requests or websockets, and when state changes I need to push updates to connected clients. I get all excited to use runes, before remembering that $effect and $effect.root don't run on the server 🥲. I'm currently using old subscriber functions instead, and it's a much worse experience (It really shows how nice runes are by comparison!).

I've been wondering if it would be possible (or even a good idea?) to create an $effect.server rune. I imagine it using it like this:

class MyServerState {
    count = $state(0);
    connectedClients = new Set();
    disposeEffect = null;

    constructor () {
        this.disposeEffect = $effect.server(() => {
            for (const conn of this.connectedClients) {
                conn.sendData(this.count);
            }
        });
    }
    
    dispose = () => {
        this.disposeEffect.?();
        for (const conn of this.connectedClients) {
            conn.disconnect();
        }
        connectedClients.clear();
    }
}

const serverState = new MyServerState();

// Elsewhere ...
serverState.connectedClients.add(clientConnection);
serverState.count += 1;

It would ONLY run on the server, and it returns a dispose function just like $effect.root, so it would not need to be wrapped in a root like $effect does. This probably wouldn't be a good idea for platforms that aren't adapter-node, so I guess in those cases it might get transformed into a no-op?

Still not sure if it's a great idea. $effect is already a footgun so adding another effect rune shouldn't be done carelessly, but it would be really nice to be able to have server-side reactivity with runes 😅

[Ocean-OS]

Currently, there is no reactivity on the server; all $state variables are turned into regular variables and $derived variables are reduced to their expression ($derived(count * 2) -> count * 2), since components on the server can't re-render when state changes occur. Adding an $effect rune to the server wouldn't make too much sense, as it'd be inconsistent if components maintained reactivity in $effects but not the template.
If you need an effect to run (once) on the server, you can use the (deprecated) run function available in the svelte/legacy module.
If you're using SvelteKit and need something to only run on the server, you can use the browser export from $app/environment.

[peterreeves]

Yeah that all makes sense, fair enough. run won't work as I want things to update multiple times.

I've just had the thought that maybe I could try using a third party signal library for my reactive server stuff. That would also provide a nice distinction between browser reactivity (using runes) and server reactivity (using another lib).