Implement Vulcain directly in API Platform Core

[dunglas]

The API Platform distribution has been including the Vulcain Gateway server for some time now.
Since API Platform 3.2, the API Platform distribution also uses FrankenPHP, which allows the HTTP status code "103 Early Hints" to be used directly from PHP.

Although the Vulcain Gateway server now supports the sending of 103 Early Hints in addition to HTTP/2 Server Push, this is almost useless do that in the gateway server: the Gateway server must retrieve the full JSON response from the upstream PHP server before it can calculate which resources to send. If the response is already in the cache of a reverse proxy, sending 103 is useless (the final response containing Preload hints added by the gateway server will be sent immediately). Otherwise, the proxy will have to wait for the JSON to be generated by the PHP server before calculating the 103 links, so we're in the same situation.

What would be better - in case the response is not already in the reverse proxy cache - would be for the PHP server to send the 103 response as soon as possible: ideally, in Platform API providers, when the data has been retrieved from the persistence layer, but before serialization generation (which is known to be a tedious task).

In addition to preloading, it would be interesting to also implement the filtering part of Vulcain (building on the existing "property" filter). This would not be very useful for responses that can be cached (it's better to store the complete response in the cache, and let the proxy do the filtering), but it will prevent some useless work when the response isn't cacheable.