Why

One way to achive realtime capabilities in an app is polling.

In Sveltekit, this could be solved from client side with a simple periodic calling of invalidate(...) or invalidateAll()

setInterval(() => {
    invalidate('custom:url');
    //OR
    invalidateAll();
}, 1000);

However, Sveltekit has its own logic about load function invocation, and in certain scenarios calling invalidate causes dependency tracking to kick in, so you could end up rerunning more load functions than you ultimately want.

Invalidating a whole page could cause unnecessary strain on the server by overfetching unchanging properties. Maybe you don't need to invalidate all properties returned by a load function, just a portion of it.

Taking advantage of the independency of +server endpoints, granular polling can be done with them easily.

Usage

Populate your page with a load function

export const load = async () => {
    const dbResult = await db.get('count');
    return { staticProperty: 'foo', dynamicCount: dbResult };
};

Create an endpoint with a GET handler in a +server file. Return an arbitrary, devalue-able object within pollingResponse() with the same keys as in your corresponding load function.

Note: if you are using this endpoint for something else, you could guard this return statement with an beingPolled({url})

import { beingPolled, pollingResponse } from 'sveltekit-polling';

export const GET = async ({ url }) => {
    if (beingPolled(url)) {
        const dbResult = await db.get('count');
        return pollingResponse({ dynamicCount: dbResult });
    }

    return new Response('other usage than polling');
};

Use new Polling(args) in a +page and call polling.begin() to begin polling. Specify your polled keys, and your +server location.

<script>
    import { Polling } from 'sveltekit-polling';
    import { page } from '$app/stores';
    import { onMount } from 'svelte';

    export let data;

    const polling = new Polling({
        page,
        data,
        interval: 1000,
        routeId: '/',
        keys: 'dynamicCount'
    });
    const polledData = polling.polledData;

    onMount(polling.onMount);
</script>

<p>{$polledData.staticProperty}</p><p>{$polledData.dynamicCount}</p>

polledData is a store with all keys from your initial data. Every time data changes (after a form submission for example) or a polling is done, polledData updates.

Note: If you need more control in onMount(), you can control polling with fine grained control:

onMount(() => {
    polling.begin();
    return () => polling.stop();
});

TODO: new Polling() constructor argument docs, type safety across +page and +server and client.