diff --git a/core/modules/update/lib/Drupal/update/Controller/UpdateController.php b/core/modules/update/lib/Drupal/update/Controller/UpdateController.php
index 6081361..37ab020 100644
--- a/core/modules/update/lib/Drupal/update/Controller/UpdateController.php
+++ b/core/modules/update/lib/Drupal/update/Controller/UpdateController.php
@@ -9,7 +9,7 @@
 
 use Drupal\Core\DependencyInjection\ContainerInjectionInterface;
 use Drupal\Core\Extension\ModuleHandlerInterface;
-use Drupal\update\UpdateManager;
+use Drupal\update\UpdateManagerInterface;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 
 /**
@@ -27,7 +27,7 @@ class UpdateController implements ContainerInjectionInterface {
   /**
    * Update manager service.
    *
-   * @var \Drupal\update\UpdateManager
+   * @var \Drupal\update\UpdateManagerInterface
    */
   protected $updateManager;
 
@@ -37,10 +37,10 @@ class UpdateController implements ContainerInjectionInterface {
    * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
    *   Module Handler Service.
    *
-   * @param \Drupal\update\UpdateManager $update_manager
+   * @param \Drupal\update\UpdateManagerInterface $update_manager
    *   Update Manager Service.
    */
-  public function __construct(ModuleHandlerInterface $module_handler, UpdateManager $update_manager) {
+  public function __construct(ModuleHandlerInterface $module_handler, UpdateManagerInterface $update_manager) {
     $this->moduleHandler = $module_handler;
     $this->updateManager = $update_manager;
   }
diff --git a/core/modules/update/lib/Drupal/update/UpdateManager.php b/core/modules/update/lib/Drupal/update/UpdateManager.php
index f82a55c..fda2433 100644
--- a/core/modules/update/lib/Drupal/update/UpdateManager.php
+++ b/core/modules/update/lib/Drupal/update/UpdateManager.php
@@ -13,9 +13,9 @@
 use Drupal\Core\Utility\ProjectInfo;
 
 /**
- * Manages project update information.
+ * Default implementation of UpdateManagerInterface.
  */
-class UpdateManager {
+class UpdateManager implements UpdateManagerInterface {
 
   /**
    * The update settings
@@ -91,7 +91,7 @@ public function __construct(ConfigFactory $config_factory, ModuleHandlerInterfac
   }
 
   /**
-   * Clears out all the available update data and initiates re-fetching.
+   * {@inheritdoc}
    */
   public function refreshUpdateData() {
 
@@ -117,51 +117,7 @@ public function refreshUpdateData() {
   }
 
   /**
-   * Fetches an array of installed and enabled projects.
-   *
-   * This is only responsible for generating an array of projects (taking into
-   * account projects that include more than one module or theme). Other
-   * information like the specific version and install type (official release,
-   * dev snapshot, etc) is handled later in update_process_project_info() since
-   * that logic is only required when preparing the status report, not for
-   * fetching the available release data.
-   *
-   * This array is fairly expensive to construct, since it involves a lot of disk
-   * I/O, so we store the results. However, since this is not the data about
-   * available updates fetched from the network, it is acceptable to invalidate it
-   * somewhat quickly. If we keep this data for very long, site administrators are
-   * more likely to see incorrect results if they upgrade to a newer version of a
-   * module or theme but do not visit certain pages that automatically clear this
-   * data.
-   *
-   * @return
-   *   An associative array of currently enabled projects keyed by the
-   *   machine-readable project short name. Each project contains:
-   *   - name: The machine-readable project short name.
-   *   - info: An array with values from the main .info.yml file for this project.
-   *     - name: The human-readable name of the project.
-   *     - package: The package that the project is grouped under.
-   *     - version: The version of the project.
-   *     - project: The Drupal.org project name.
-   *     - datestamp: The date stamp of the project's main .info.yml file.
-   *     - _info_file_ctime: The maximum file change time for all of the .info.yml
-   *       files included in this project.
-   *   - datestamp: The date stamp when the project was released, if known.
-   *   - includes: An associative array containing all projects included with this
-   *     project, keyed by the machine-readable short name with the human-readable
-   *     name as value.
-   *   - project_type: The type of project. Allowed values are 'module' and
-   *     'theme'.
-   *   - project_status: This indicates if the project is enabled and will always
-   *     be TRUE, as the function only returns enabled projects.
-   *   - sub_themes: If the project is a theme it contains an associative array of
-   *     all sub-themes.
-   *   - base_themes: If the project is a theme it contains an associative array
-   *     of all base-themes.
-   *
-   * @see update_process_project_info()
-   * @see update_calculate_project_data()
-   * @see \Drupal\update\UpdateManager::projectStorage()
+   * {@inheritdoc}
    */
   public function getProjects() {
     if (empty($this->projects)) {
@@ -188,31 +144,7 @@ public function getProjects() {
   }
 
   /**
-   * Retrieves update storage data or empties it.
-   *
-   * Two very expensive arrays computed by this module are the list of all
-   * installed modules and themes (and .info.yml data, project associations, etc), and
-   * the current status of the site relative to the currently available releases.
-   * These two arrays are stored and used whenever possible. The data is cleared
-   * whenever the administrator visits the status report, available updates
-   * report, or the module or theme administration pages, since we should always
-   * recompute the most current values on any of those pages.
-   *
-   * Note: while both of these arrays are expensive to compute (in terms of disk
-   * I/O and some fairly heavy CPU processing), neither of these is the actual
-   * data about available updates that we have to fetch over the network from
-   * updates.drupal.org. That information is stored in the
-   * 'update_available_releases' collection -- it needs to persist longer than 1
-   * hour and never get invalidated just by visiting a page on the site.
-   *
-   * @param $key
-   *   The key of data to return. Valid options are 'update_project_data' and
-   *   'update_project_projects'.
-   *
-   * @return
-   *   The stored value of the $projects array generated by
-   *   update_calculate_project_data() or update_get_projects(), or an empty array
-   *   when the storage is cleared.
+   * {@inheritdoc}
    */
   public function projectStorage($key) {
     $projects = array();
@@ -240,10 +172,7 @@ public function projectStorage($key) {
   }
 
   /**
-   * Batch callback: Processes a step in batch for fetching available update data.
-   *
-   * @param $context
-   *   Reference to an array used for Batch API storage.
+   * {@inheritdoc}
    */
   public function fetchDataBatch(&$context) {
     if (empty($context['sandbox']['max'])) {
diff --git a/core/modules/update/lib/Drupal/update/UpdateManagerInterface.php b/core/modules/update/lib/Drupal/update/UpdateManagerInterface.php
new file mode 100644
index 0000000..8c1ab56
--- /dev/null
+++ b/core/modules/update/lib/Drupal/update/UpdateManagerInterface.php
@@ -0,0 +1,107 @@
+<?php
+/**
+ * @file
+ * Contains  \Drupal\update\UpdateManagerInterface.
+ */
+
+namespace Drupal\update;
+
+/**
+ * Manages project update information.
+ */
+interface UpdateManagerInterface {
+
+  /**
+   * Fetches an array of installed and enabled projects.
+   *
+   * This is only responsible for generating an array of projects (taking into
+   * account projects that include more than one module or theme). Other
+   * information like the specific version and install type (official release,
+   * dev snapshot, etc) is handled later in update_process_project_info() since
+   * that logic is only required when preparing the status report, not for
+   * fetching the available release data.
+   *
+   * This array is fairly expensive to construct, since it involves a lot of
+   * disk I/O, so we store the results. However, since this is not the data
+   * about available updates fetched from the network, it is acceptable to
+   * invalidate it somewhat quickly. If we keep this data for very long, site
+   * administrators are more likely to see incorrect results if they upgrade to
+   * a newer version of a module or theme but do not visit certain pages that
+   * automatically clear this data.
+   *
+   * @return array
+   *   An associative array of currently enabled projects keyed by the
+   *   machine-readable project short name. Each project contains:
+   *   - name: The machine-readable project short name.
+   *   - info: An array with values from the main .info.yml file for this
+   *     project.
+   *     - name: The human-readable name of the project.
+   *     - package: The package that the project is grouped under.
+   *     - version: The version of the project.
+   *     - project: The Drupal.org project name.
+   *     - datestamp: The date stamp of the project's main .info.yml file.
+   *     - _info_file_ctime: The maximum file change time for all of the
+   *       .info.yml
+   *       files included in this project.
+   *   - datestamp: The date stamp when the project was released, if known.
+   *   - includes: An associative array containing all projects included with
+   *     this project, keyed by the machine-readable short name with the
+   *     human-readable name as value.
+   *   - project_type: The type of project. Allowed values are 'module' and
+   *     'theme'.
+   *   - project_status: This indicates if the project is enabled and will
+   *     always be TRUE, as the function only returns enabled projects.
+   *   - sub_themes: If the project is a theme it contains an associative array
+   *     of all sub-themes.
+   *   - base_themes: If the project is a theme it contains an associative array
+   *     of all base-themes.
+   *
+   * @see update_process_project_info()
+   * @see update_calculate_project_data()
+   * @see \Drupal\update\UpdateManager::projectStorage()
+   */
+  public function getProjects();
+
+  /**
+   * Processes a step in batch for fetching available update data.
+   *
+   * @param array $context
+   *   Reference to an array used for Batch API storage.
+   */
+  public function fetchDataBatch(&$context);
+
+  /**
+   * Clears out all the available update data and initiates re-fetching.
+   */
+  public function refreshUpdateData();
+
+  /**
+   * Retrieves update storage data or empties it.
+   *
+   * Two very expensive arrays computed by this module are the list of all
+   * installed modules and themes (and .info.yml data, project associations,
+   * etc), and the current status of the site relative to the currently
+   * available releases. These two arrays are stored and used whenever possible.
+   * The data is cleared whenever the administrator visits the status report,
+   * available updates report, or the module or theme administration pages,
+   * since we should always recompute the most current values on any of those
+   * pages.
+   *
+   * Note: while both of these arrays are expensive to compute (in terms of disk
+   * I/O and some fairly heavy CPU processing), neither of these is the actual
+   * data about available updates that we have to fetch over the network from
+   * updates.drupal.org. That information is stored in the
+   * 'update_available_releases' collection -- it needs to persist longer than 1
+   * hour and never get invalidated just by visiting a page on the site.
+   *
+   * @param string $key
+   *   The key of data to return. Valid options are 'update_project_data' and
+   *   'update_project_projects'.
+   *
+   * @return array
+   *   The stored value of the $projects array generated by
+   *   update_calculate_project_data() or update_get_projects(), or an empty
+   *   array when the storage is cleared.
+   */
+  public function projectStorage($key);
+}
diff --git a/core/modules/update/lib/Drupal/update/UpdateProcessor.php b/core/modules/update/lib/Drupal/update/UpdateProcessor.php
index f9a5018..9f90126 100644
--- a/core/modules/update/lib/Drupal/update/UpdateProcessor.php
+++ b/core/modules/update/lib/Drupal/update/UpdateProcessor.php
@@ -113,21 +113,7 @@ public function __construct(ConfigFactory $config_factory, QueueFactory $queue_f
   }
 
   /**
-   * Adds a task to the queue for fetching release history data for a project.
-   *
-   * We only create a new fetch task if there's no task already in the queue for
-   * this particular project (based on 'update_fetch_task' key-value collection).
-   *
-   * @param array $project
-   *   Associative array of information about a project as created by
-   *   update_get_projects(), including keys such as 'name' (short name), and the
-   *   'info' array with data from a .info.yml file for the project.
-   *
-   * @see \Drupal\update\UpdateManager::getProjects()
-   * @see update_get_available()
-   * @see \Drupal\update\UpdateManager::refreshUpdateData()
-   * @see \Drupal\update\UpdateProcessor::fetchData()
-   * @see \Drupal\update\UpdateProcessor::processFetchTask()
+   * {@inheritdoc}
    */
   public function createFetchTask($project) {
     if (empty($this->fetchTasks)) {
@@ -141,7 +127,7 @@ public function createFetchTask($project) {
   }
 
   /**
-   * Attempts to drain the queue of tasks for release history data to fetch.
+   * {@inheritdoc}
    */
   public function fetchData() {
     $end = time() + $this->updateSettings->get('fetch.timeout');
@@ -152,16 +138,7 @@ public function fetchData() {
   }
 
   /**
-   * Processes a task to fetch available update data for a single project.
-   *
-   * Once the release history XML data is downloaded, it is parsed and saved in
-   * an entry just for that project.
-   *
-   * @param array $project
-   *   Associative array of information about the project to fetch data for.
-   *
-   * @return bool
-   *   TRUE if we fetched parsable XML, otherwise FALSE.
+   * {@inheritdoc}
    */
   public function processFetchTask($project) {
     global $base_url;
@@ -272,37 +249,21 @@ protected function parseXml($raw_xml) {
   }
 
   /**
-   * Retrieves the number of items in the update fetch queue.
-   *
-   * @return int
-   *   An integer estimate of the number of items in the queue.
-   *
-   * @see \Drupal\Core\Queue\QueueInterface::numberOfItems()
+   * {@inheritdoc}
    */
   public function numberOfQueueItems() {
     return $this->fetchQueue->numberOfItems();
   }
 
   /**
-   * Claims an item in the update fetch queue for processing.
-   *
-   * @return
-   *   On success we return an item object. If the queue is unable to claim an
-   *   item it returns false.
-   *
-   * @see \Drupal\Core\Queue\QueueInterface::claimItem()
+   * {@inheritdoc}
    */
   public function claimQueueItem() {
     return $this->fetchQueue->claimItem();
   }
 
   /**
-   * Deletes a finished item from the update fetch queue.
-   *
-   * @param $item
-   *   The item returned by \Drupal\Core\Queue\QueueInterface::claimItem().
-   *
-   * @see \Drupal\Core\Queue\QueueInterface::deleteItem()
+   * {@inheritdoc}
    */
   public function deleteQueueItem($item) {
     return $this->fetchQueue->deleteItem($item);