svelte-adapter-uws

A SvelteKit adapter powered by uWebSockets.js - the fastest HTTP/WebSocket server for Node.js, written in C++ and exposed through V8.

Most users don’t need to read this page. svelte-realtime handles the adapter for you. This is for power users who want deeper control over WebSocket hooks, plugins, TLS, or deployment.

What you get

• HTTP & HTTPS - native TLS via uWebSockets.js SSLApp, no reverse proxy needed
• WebSocket & WSS - built-in pub/sub with a reactive Svelte client store
• In-memory static file cache - assets loaded once at startup, served from RAM with precompressed brotli/gzip variants
• Backpressure handling - streaming responses that won’t blow up memory
• Graceful shutdown - waits for in-flight requests before exiting
• Health check endpoint - /healthz out of the box
• Zero-config WebSocket - just set websocket: true and go

Quick start

Installation

npm install svelte-adapter-uws
npm install uNetworking/uWebSockets.js#v20.60.0

Note: uWebSockets.js is a native C++ addon installed directly from GitHub. Docker requires node:22-trixie-slim or another glibc >= 2.38 image.

For WebSocket support during development, also install ws:

npm install -D ws

HTTP

// svelte.config.js
import adapter from 'svelte-adapter-uws';

export default {
  kit: {
    adapter: adapter()
  }
};

npm run build
node build

WebSocket

Three steps: enable the adapter, add the Vite plugin, use the client store.

svelte.config.js

import adapter from 'svelte-adapter-uws';

export default {
  kit: {
    adapter: adapter({ websocket: true })
  }
};

vite.config.js (required for dev mode and production builds)

import { sveltekit } from '@sveltejs/kit/vite';
import uws from 'svelte-adapter-uws/vite';

export default {
  plugins: [sveltekit(), uws()]
};

src/routes/+page.svelte

<script>
  import { on, status } from 'svelte-adapter-uws/client';
  const notifications = on('notifications');
</script>

{#if $status === 'open'}
  <span>Connected</span>
{/if}

{#if $notifications}
  <p>{$notifications.event}: {JSON.stringify($notifications.data)}</p>
{/if}

HTTPS / WSS

No reverse proxy needed. uWebSockets.js handles TLS natively:

npm run build
SSL_CERT=/path/to/cert.pem SSL_KEY=/path/to/key.pem node build

The client store automatically uses wss:// when the page is served over HTTPS.

Development, preview & production