Method 1 : Form hook_page_attachments

This method allow to store data on drupal session using drupal session table (not php $_SESSION).
Storage is different for each users ans each visitors if anonymous

Install Redis

sudo apt-get update && sudo apt-get upgrade
sudo apt install redis-server
sudo apt install php-redis

Check

sudo systemctl status redis-server
redis-cli ping

When you request a let's encrypt wildcard certificate with virtualmin you may have an error like this:

Syntax:

\Drupal::service('page_cache_kill_switch')->trigger();

Example on a controller: