How do I enable the ETag header?

Question

I'd like my JSON:API responses to return the e-tag header so that consuming remote sites can determine (e.g. with a HEAD request) if they need to ditch their cached responses and re-fetch. There is no last-modified or e-tag headers being sent at all - for any Drupal page for that matter. How can I enable this? Thanks!

I do see X-Drupal-Dynamic-Cache and X-Drupal-Cache headers, but this isn't enough to go on.

Running Drupal 8.6.10 and JSON:API 8.x-2.4.

Answer 1

I changed the "Browser and proxy cache maximum age" setting in Configuration > Development > Performance and it appeared to do the trick. It was previously set to "no caching".

Answer 2

1. Create a new module (etag)
2. Add etag.services.yml

   services:
      etag.response_subscriber:
        class: Drupal\etag\EventSubscriber\ResponseSubscriber
        tags:
          - { name: event_subscriber }

3. Create ResponseSubscriber class inside modules/custom/etag/src/EventSubscriber/ResponseSubscriber.php

<?php

namespace Drupal\etag\EventSubscriber;

use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpKernel\KernelEvents;
use Symfony\Component\HttpKernel\Event\ResponseEvent;

class ResponseSubscriber implements EventSubscriberInterface {

  public static function getSubscribedEvents(): array {
    return [
      KernelEvents::RESPONSE => 'onResponse',
    ];
  }

  /**
   * This method is called whenever the KernelEvents::RESPONSE event is dispatched.
   *
   * @param ResponseEvent $event
   */
  public function onResponse(ResponseEvent $event): void {
    $response = $event->getResponse();
    $etag_value = '"' . md5($response->getContent()) . '"';

    $response->headers->set('ETag', $etag_value);

  }
}