Using SSI in Symfony

First, to use SSI, be sure to enable it in your application configuration:

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# config/packages/framework.yaml
framework:
    ssi: { enabled: true }

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<!-- config/packages/framework.xml -->
<?xml version="1.0" encoding="UTF-8" ?>
<container xmlns="http://symfony.com/schema/dic/symfony"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:framework="http://symfony.com/schema/dic/symfony"
    xsi:schemaLocation="http://symfony.com/schema/dic/services
        https://symfony.com/schema/dic/services/services-1.0.xsd
        http://symfony.com/schema/dic/symfony
        https://symfony.com/schema/dic/symfony/symfony-1.0.xsd">

    <framework:config>
        <framework:ssi enabled="true"/>
    </framework:config>
</container>

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// config/packages/framework.php
use Symfony\Config\FrameworkConfig;

return static function (FrameworkConfig $framework): void {
    $framework->ssi()
        ->enabled(true)
    ;
};

Suppose you have a page with private content like a Profile page and you want to cache a static GDPR content block. With SSI, you can add some expiration on this block and keep the page private:

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

use Symfony\Component\HttpKernel\Attribute\Cache;
// ...

class ProfileController extends AbstractController
{
    public function index(): Response
    {
        // by default, responses are private
        return $this->render('profile/index.html.twig');
    }

    #[Cache(smaxage: 600)]
    public function gdpr(): Response
    {
        return $this->render('profile/gdpr.html.twig');
    }
}

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

// ...
class ProfileController extends AbstractController
{
    public function index(): Response
    {
        // by default, responses are private
        return $this->render('profile/index.html.twig');
    }

    public function gdpr(): Response
    {
        $response = $this->render('profile/gdpr.html.twig');

        // sets to public and adds some expiration
        $response->setSharedMaxAge(600);

        return $response;
    }
}

The profile index page has not public caching, but the GDPR block has 10 minutes of expiration. Let's include this block into the main one:

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{# templates/profile/index.html.twig #}

{# you can use a controller reference #}
{{ render_ssi(controller('App\\Controller\\ProfileController::gdpr')) }}

{# ... or a path (in server's SSI configuration is common to use relative paths instead of absolute URLs) #}
{{ render_ssi(path('profile_gdpr')) }}

The render_ssi twig helper will generate something like:

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<!--#include virtual="/_fragment?_hash=abcdef1234&_path=_controller=App\Controller\ProfileController::gdpr" -->

render_ssi ensures that SSI directive is generated only if the request has the header requirement like Surrogate-Capability: device="SSI/1.0" (normally given by the web server). Otherwise it will embed directly the sub-response.