Hook a Persistence Layer with a Processor

// src/App/ApiResource.php
namespace App\ApiResource;
use ApiPlatform\Metadata\ApiResource;
use App\State\BookProcessor;
use App\State\BookProvider;

#[ApiResource(processor: BookProcessor::class, provider: BookProvider::class)]
class Book
{
    public function __construct(public string $id, public string $title)
    {
    }
}

// src/App/State.php
namespace App\State;
use ApiPlatform\Metadata\Operation;
use ApiPlatform\State\ProcessorInterface;
use ApiPlatform\State\ProviderInterface;
use App\ApiResource\Book;
use Ramsey\Collection\CollectionInterface;

final class BookProcessor implements ProcessorInterface
{
    /**
     * @param Book $data
     */
    public function process($data, Operation $operation, array $uriVariables = [], array $context = []): Book
    {
        $id = $uriVariables['id'] ?? $data->id;
        file_put_contents(sprintf('book-%s.json', $id), json_encode($data));
        return $data;
    }
}
final class BookProvider implements ProviderInterface
{
    public function provide(Operation $operation, array $uriVariables = [], array $context = []): ?Book
    {
        if ($operation instanceof CollectionInterface) {
            throw new \RuntimeException('Not supported.');
        }
        $file = sprintf('book-%s.json', $uriVariables['id']);
        if (!file_exists($file)) {
            return null;
        }
        $data = json_decode(file_get_contents($file));
        return new Book($data->id, $data->title);
    }
}

// src/App/Playground.php
namespace App\Playground;
use Symfony\Component\HttpFoundation\Request;
function request(): Request
{
    return Request::create(uri: '/books', method: 'POST', server: ['CONTENT_TYPE' => 'application/ld+json'], content: json_encode(['id' => '1', 'title' => 'API Platform rocks.']));
}