How can I get the node ID from a path alias?
I'm not sure if there's a direct function, but one route is to use the path alias manager service to lookup the internal path then regex it for a valid node id to perform an object load on: 

$path = \Drupal::service('path.alias_manager')-&gt;getPathByAlias('/this-is-the-alias');
if(preg_match('/node\/(\d+)/', $path, $matches)) {
  $node = \Drupal\node\Entity\Node::load($matches[1]);
}