Drupal 9

Symptoms

You may have error messages like:

You can use IMCE to select a file.

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

Syntax:

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

Example on a controller:

Install via composer

composer global require drush/drush:10.x

Create a Drupal 9 project with composer

On drupal 8 every elements (almost) are an entity, as any entity you can render a node.

NOTE: I don't know this is the best practice, anyway it's working !!!.

Select:
    $con = \Drupal\Core\Database\Database::getConnection();
    $query = $con->select('table_name', 't')
        ->fields('t', ['field_1', 'field_2']);
    $result = $query->execute()->fetchAll();

NULL Condition