Live Queries

Our todo list app can have multiple users using it at the same time. However, changes made by one user are not seen by others unless they manually refresh their browser.

Let's add realtime multiplayer capabilities to this app.

Realtime updated todo list

Let's update our component like follows (make sure you add and remove some lines as indicated)

/src/routes/+page.svelte

<script lang="ts">
	import { repo } from "remult";
	import { Task } from "../shared/Task";
	import { onDestroy } from "svelte";

	let tasks = $state<Task[]>([]);
	let unSub: (() => void) | null = null;

	$effect(() => {
		// repo(Task)
		//   .find()
		//   .then((t) => (tasks = t));
		unSub = repo(Task) 
			.liveQuery() 
			.subscribe((info) => { 
				tasks = info.applyChanges(tasks); 
			}); 
	});

	onDestroy(() => {
		unSub && unSub();
	});

	let newTaskTitle = $state("");
	const addTask = async (event: Event) => {
		event.preventDefault();
		try {
			const newTask = await repo(Task).insert({ title: newTaskTitle });
			// tasks = [...tasks, newTask];
			newTaskTitle = "";
		} catch (error) {
			alert((error as { message: string }).message);
		}
	};

	const setCompleted = async (task: Task, completed: boolean) => {
		try {
			await repo(Task).save({ ...task, completed });
		} catch (error) {
			alert((error as { message: string }).message);
		}
	};

	const saveTask = async (e: Event, task: Task) => {
		e.preventDefault();
		try {
			await repo(Task).save({ ...task });
		} catch (error) {
			alert((error as { message: string }).message);
		}
	};

	const deleteTask = async (e: Event, task: Task) => {
		try {
			await repo(Task).delete(task);
			// tasks = tasks.filter((c) => c.id !== task.id);
		} catch (error) {
			alert((error as { message: string }).message);
		}
	};
</script>

<div>
	<h1>todos</h1>
	<main>
		<form onsubmit={addTask}>
			<input bind:value={newTaskTitle} placeholder="What needs to be done?" />
			<button>Add</button>
		</form>

		{#each tasks as task}
			<div>
				<input
					type="checkbox"
					checked={task.completed}
					oninput={(e) => setCompleted(task, e.currentTarget.checked)}
				/>
				<!-- <span>{task.title}</span> -->
				<input name="title" bind:value={task.title} />
				<button onclick={(e) => saveTask(e, task)}>Save</button>
				<button onclick={(e) => deleteTask(e, task)}>Delete</button>
			</div>
		{/each}
	</main>
</div>

Let's review the change:

• Instead of calling the repository's find method we now call the liveQuery method to define the query, and then call its subscribe method to establish a subscription which will update the Tasks state in realtime.

• The subscribe method accepts a callback with an info object that has 3 members:

  • items - an up to date list of items representing the current result - it's useful for readonly use cases.
  • applyChanges - a method that receives an array and applies the changes to it - we send that method to the setTasks state function, to apply the changes to the existing tasks state.
  • changes - a detailed list of changes that were received

• The subscribe method returns an unsubscribe function, we return it to the onDestroy hook so that it'll be called when the component unmounts.

• As all relevant CRUD operations (made by all users) will immediately update the component's state, we removed the manual adding/removing of new Tasks to the component's state.

Open the todo app in two (or more) browser windows/tabs, make some changes in one window and notice how the others are updated in realtime.

Under the hood

The default implementation of live-queries uses HTTP Server-Sent Events (SSE) to push realtime updates to clients, and stores live-query information in-memory.

For scalable production / serverless environments, live-query updates can be pushed using integration with third-party realtime providers, such as Ably, and live-query information can be stored to any database supported by Remult.