diff -u b/modules/redirect_404/src/EventSubscriber/Redirect404Subscriber.php b/modules/redirect_404/src/EventSubscriber/Redirect404Subscriber.php
--- b/modules/redirect_404/src/EventSubscriber/Redirect404Subscriber.php
+++ b/modules/redirect_404/src/EventSubscriber/Redirect404Subscriber.php
@@ -4,7 +4,7 @@
 
 use Drupal\Core\Language\LanguageManagerInterface;
 use Drupal\Core\Path\CurrentPathStack;
-use Drupal\redirect_404\RedirectNotFoundStorage;
+use Drupal\redirect_404\RedirectNotFoundStorageInterface;
 use Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent;
 use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;
 use Symfony\Component\HttpKernel\KernelEvents;
@@ -32,7 +32,7 @@
   /**
    * The redirect storage.
    *
-   * @var \Drupal\redirect_404\RedirectNotFoundStorage
+   * @var \Drupal\redirect_404\RedirectNotFoundStorageInterface
    */
   protected $redirectStorage;
 
@@ -43,10 +43,10 @@
    *   The current path.
    * @param \Drupal\Core\Language\LanguageManagerInterface $language_manager
    *   The language manager.
-   * @param \Drupal\redirect_404\RedirectNotFoundStorage $redirect_storage
+   * @param \Drupal\redirect_404\RedirectNotFoundStorageInterface $redirect_storage
    *   A redirect storage.
    */
-  public function __construct(CurrentPathStack $current_path, LanguageManagerInterface $language_manager, RedirectNotFoundStorage $redirect_storage) {
+  public function __construct(CurrentPathStack $current_path, LanguageManagerInterface $language_manager, RedirectNotFoundStorageInterface $redirect_storage) {
     $this->currentPath = $current_path;
     $this->languageManager = $language_manager;
     $this->redirectStorage = $redirect_storage;
diff -u b/modules/redirect_404/src/RedirectNotFoundStorage.php b/modules/redirect_404/src/RedirectNotFoundStorage.php
--- b/modules/redirect_404/src/RedirectNotFoundStorage.php
+++ b/modules/redirect_404/src/RedirectNotFoundStorage.php
@@ -7,7 +7,7 @@
 /**
  * Controller class for redirect not found storage.
  */
-class RedirectNotFoundStorage {
+class RedirectNotFoundStorage implements RedirectNotFoundStorageInterface {
 
   /**
    * Active database connection.
@@ -27,12 +27,7 @@
   }
 
   /**
-   * Merges a record to the redirect_404 table.
-   *
-   * @param string $path
-   *   The path of the current request.
-   * @param string $langcode
-   *   The ID of the language code.
+   * {@inheritdoc}
    */
   public function logRequest($path, $langcode) {
     $this->database->merge('redirect_404')
@@ -48,12 +43,7 @@
   }
 
   /**
-   * Marks a record as resolved in the redirect_404 table.
-   *
-   * @param string $path
-   *   The path of the current request.
-   * @param string $langcode
-   *   The ID of the language code.
+   * {@inheritdoc}
    */
   public function resolveLogRequest($path, $langcode) {
     $this->database->update('redirect_404')
@@ -64,20 +54,7 @@
   }
 
   /**
-   * Returns the 404 request data.
-   *
-   * @param array $header
-   *   An array containing arrays of the redirect_404 fields data.
-   * @param string $search
-   *   The search text.
-   *
-   * @return array
-   *   A list of objects with the properties:
-   *   - path
-   *   - count
-   *   - timestamp
-   *   - langcode
-   *   - resolved
+   * {@inheritdoc}
    */
   public function listRequests(array $header = [], $search = NULL) {
     $query = $this->database
only in patch2:
unchanged:
--- /dev/null
+++ b/modules/redirect_404/src/RedirectNotFoundStorageInterface.php
@@ -0,0 +1,48 @@
+<?php
+
+namespace Drupal\redirect_404;
+
+/**
+ * Defines a common interface for redirect 404 classes.
+ */
+interface RedirectNotFoundStorageInterface {
+
+  /**
+   * Merges a record to the redirect_404 table.
+   *
+   * @param string $path
+   *   The path of the current request.
+   * @param string $langcode
+   *   The ID of the language code.
+   */
+  public function logRequest($path, $langcode);
+
+  /**
+   * Marks a record as resolved in the redirect_404 table.
+   *
+   * @param string $path
+   *   The path of the current request.
+   * @param string $langcode
+   *   The ID of the language code.
+   */
+  public function resolveLogRequest($path, $langcode);
+
+  /**
+   * Returns the 404 request data.
+   *
+   * @param array $header
+   *   An array containing arrays of the redirect_404 fields data.
+   * @param string $search
+   *   The search text.
+   *
+   * @return array
+   *   A list of objects with the properties:
+   *   - path
+   *   - count
+   *   - timestamp
+   *   - langcode
+   *   - resolved
+   */
+  public function listRequests(array $header = [], $search = NULL);
+
+}