# Pushing Related Resources Using HTTP/2

> HTTP/2 allows a server to pre-emptively send (or "push") responses (along with corresponding "promised" requests) to a client in association with a previous client-initiated request. This can be useful when the server knows the client will need to have those responses available in order to fully process the response to the original request.
>
> —[RFC 7540](https://tools.ietf.org/html/rfc7540#section-8.2)

API Platform leverages this capability by pushing relations of a resource to clients.

```php
<?php

namespace App\Entity;

use ApiPlatform\Core\Annotation\ApiProperty;
use ApiPlatform\Core\Annotation\ApiResource;

/**
 * @ApiResource
 */
class Book
{
    /**
     * @var Author
     * @ApiProperty(push=true)
     */
    public $author;
    
    // ...
}
```

By setting the `push` attribute to `true` on a property holding a relation, API Platform will automatically add a valid `Link` HTTP header with the `preload` relation.
According to the [Preload W3C Candidate Recommendation](https://www.w3.org/TR/preload/), web servers and proxy servers can read this header, fetch the related resource and send it to the client using Server Push.
[NGINX](https://www.nginx.com/blog/nginx-1-13-9-http2-server-push/), [Apache](https://httpd.apache.org/docs/current/howto/http2.html#push), [Cloudflare](https://www.cloudflare.com/website-optimization/http2/serverpush/), [Fastly](https://docs.fastly.com/guides/performance-tuning/http2-server-push) and [Akamai](https://blogs.akamai.com/2017/03/http2-server-push-the-what-how-and-why.html) honor this header. 

Using this feature maximises HTTP cache hits for your API resources.
For best performance, this feature should be used in conjunction with [the built-in HTTP cache invalidation system (based on Varnish)](performance.md#enabling-the-built-in-http-cache-invalidation-system).