Asset Preloading and Resource Hints with HTTP/2 and WebLink

Symfony provides native support (via the WebLink component) for managing Link HTTP headers, which are the key to improve the application performance when using HTTP/2 and preloading capabilities of modern web browsers.

Link headers are used in HTTP/2 Server Push and W3C's Resource Hints to push resources (e.g. CSS and JavaScript files) to clients before they even know that they need them. WebLink also enables other optimizations that work with HTTP 1.x:

Asking the browser to fetch or to render another web page in the background;
Making early DNS lookups, TCP handshakes or TLS negotiations.

Something important to consider is that all these HTTP/2 features require a secure HTTPS connection, even when working on your local machine. The main web servers (Apache, nginx, Caddy, etc.) support this, but you can also use the Docker installer and runtime for Symfony created by Kévin Dunglas, from the Symfony community.

Installation

In applications using :ref:`Symfony Flex <symfony-flex>`, run the following command to install the WebLink feature before using it:

$ composer require symfony/web-link

Preloading Assets

Imagine that your application includes a web page like this:

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>My Application</title>
    <link rel="stylesheet" href="/app.css">
</head>
<body>
    <main role="main" class="container">
        <!-- ... -->
    </main>
</body>
</html>

In a traditional HTTP workflow, when this page is loaded, browsers make one request for the HTML document and another for the linked CSS file. However, with HTTP/2, your application can send the CSS file's contents to the browser before it requests them.

To achieve this, update your template to use the preload() Twig function provided by WebLink. Note that the "as" attribute is required, as browsers use it to prioritize resources correctly and comply with the content security policy:

<head>
    <!-- ... -->
    {# note that you must add two <link> tags per asset:
       one to link to it and the other one to tell the browser to preload it #}
    <link rel="preload" href="{{ preload('/app.css', {as: 'style'}) }}" as="style">
    <link rel="stylesheet" href="/app.css">
</head>

If you reload the page, the perceived performance will improve because the server responded with both the HTML page and the CSS file when the browser only requested the HTML page.

Tip

When using the :doc:`AssetMapper component </frontend/asset_mapper>` to link to assets (e.g. importmap('app')), there's no need to add the <link rel="preload"> tag. The importmap() Twig function automatically adds the Link HTTP header for you when the WebLink component is available.

Note

You can preload an asset by wrapping it with the preload() function:

<head>
    <!-- ... -->
    <link rel="preload" href="{{ preload(asset('build/app.css')) }}" as="style">
    <!-- ... -->
</head>

Additionally, according to the Priority Hints specification, you can signal the priority of the resource to download using the importance attribute:

<head>
    <!-- ... -->
    <link rel="preload" href="{{ preload('/app.css', {as: 'style', importance: 'low'}) }}" as="style">
    <!-- ... -->
</head>

How does it work?

The WebLink component manages the Link HTTP headers added to the response. When using the preload() function in the previous example, the following header was added to the response: Link </app.css>; rel="preload"; as="style" According to the Preload specification, when an HTTP/2 server detects that the original (HTTP 1.x) response contains this HTTP header, it will automatically trigger a push for the related file in the same HTTP/2 connection.

Popular proxy services and CDNs including Cloudflare, Fastly and Akamai also leverage this feature. It means that you can push resources to clients and improve performance of your applications in production right now.

If you want to prevent the push but let the browser preload the resource by issuing an early separate HTTP request, use the nopush option:

<head>
    <!-- ... -->
    <link rel="preload" href="{{ preload('/app.css', {as: 'style', nopush: true}) }}" as="style">
    <!-- ... -->
</head>

Resource Hints

Resource Hints are used by applications to help browsers when deciding which resources should be downloaded, preprocessed or connected to first.

The WebLink component provides the following Twig functions to send those hints:

dns_prefetch(): "indicates an origin (e.g. https://foo.cloudfront.net) that will be used to fetch required resources, and that the user agent should resolve as early as possible".
preconnect(): "indicates an origin (e.g. https://www.google-analytics.com) that will be used to fetch required resources. Initiating an early connection, which includes the DNS lookup, TCP handshake, and optional TLS negotiation, allows the user agent to mask the high latency costs of establishing a connection".
prefetch(): "identifies a resource that might be required by the next navigation, and that the user agent should fetch, such that the user agent can deliver a faster response once the resource is requested in the future".
prerender(): "identifies a resource that might be required by the next navigation, and that the user agent should fetch and execute, such that the user agent can deliver a faster response once the resource is requested later".

The component also supports sending HTTP links not related to performance and any link implementing the PSR-13 standard. For instance, any link defined in the HTML specification:

<head>
    <!-- ... -->
    <link rel="alternate" href="{{ link('/index.jsonld', 'alternate') }}">
    <link rel="preload" href="{{ preload('/app.css', {as: 'style', nopush: true}) }}" as="style">
    <!-- ... -->
</head>

The previous snippet will result in this HTTP header being sent to the client: Link: </index.jsonld>; rel="alternate",</app.css>; rel="preload"; nopush

You can also add links to the HTTP response directly from controllers and services:

// src/Controller/BlogController.php
namespace App\Controller;

use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\WebLink\GenericLinkProvider;
use Symfony\Component\WebLink\Link;

class BlogController extends AbstractController
{
    public function index(Request $request): Response
    {
        // using the addLink() shortcut provided by AbstractController
        $this->addLink($request, (new Link('preload', '/app.css'))->withAttribute('as', 'style'));

        // alternative if you don't want to use the addLink() shortcut
        $linkProvider = $request->attributes->get('_links', new GenericLinkProvider());
        $request->attributes->set('_links', $linkProvider->withLink(
            (new Link('preload', '/app.css'))->withAttribute('as', 'style')
        ));

        return $this->render('...');
    }
}

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