drupal_8_services.php

<?php

  // Get the request object.
  $request = \Drupal::request();
  // Query - get all parameter. 
  $q = $request->query->all();
  // Check if a query parameter exists.
  if ($request->query->has('commerce_product_v_id')) {
    // Get the query parameter.
    $pvid = $request->query->get('commerce_product_v_id');
  }



// Drupal messenger (from commerce_stock)
\Drupal::messenger()->addMessage(t('Updated the stock.')); 
\Drupal::messenger()->addError(t('The maximum quantity for %name that can be ordered is %qty.', [
          '%name' => $name,
          '%qty' => $stock_level,
        ]));
      }

Working with key/value pairs
reading from the $request uses the symfony ParameterBag a a container for key/value pairs