diff --git a/core/core.services.yml b/core/core.services.yml
index 680c23b80840d1bf8647d6acd3e4446b47c56076..de408ad9f30e4659d03201cafabc2a5fa8988ed5 100644
--- a/core/core.services.yml
+++ b/core/core.services.yml
@@ -575,7 +575,7 @@ services:
       - { name: event_subscriber }
   entity.definition_update_manager:
     class: Drupal\Core\Entity\EntityDefinitionUpdateManager
-    arguments: ['@entity.manager']
+    arguments: ['@entity.manager', '@entity.last_installed_schema.repository']
   entity.last_installed_schema.repository:
     class: Drupal\Core\Entity\EntityLastInstalledSchemaRepository
     arguments: ['@keyvalue']
diff --git a/core/lib/Drupal/Core/Entity/EntityDefinitionUpdateManager.php b/core/lib/Drupal/Core/Entity/EntityDefinitionUpdateManager.php
index d9a4d843f517e8accc3aa89fbefdfaacf57e5e76..c5d0abd5028ae456ad6b1c417781afe66e604493 100644
--- a/core/lib/Drupal/Core/Entity/EntityDefinitionUpdateManager.php
+++ b/core/lib/Drupal/Core/Entity/EntityDefinitionUpdateManager.php
@@ -21,14 +21,29 @@ class EntityDefinitionUpdateManager implements EntityDefinitionUpdateManagerInte
    */
   protected $entityManager;
 
+  /**
+   * The last installed schema repository.
+   *
+   * @var \Drupal\Core\Entity\EntityLastInstalledSchemaRepositoryInterface
+   */
+  protected $entityLastInstalledSchemaRepository;
+
   /**
    * Constructs a new EntityDefinitionUpdateManager.
    *
    * @param \Drupal\Core\Entity\EntityManagerInterface $entity_manager
    *   The entity manager.
+   * @param \Drupal\Core\Entity\EntityLastInstalledSchemaRepositoryInterface $entity_last_installed_schema_repository
+   *   The last installed schema repository service.
    */
-  public function __construct(EntityManagerInterface $entity_manager) {
+  public function __construct(EntityManagerInterface $entity_manager, EntityLastInstalledSchemaRepositoryInterface $entity_last_installed_schema_repository = NULL) {
     $this->entityManager = $entity_manager;
+
+    if (!isset($entity_last_installed_schema_repository)) {
+      @trigger_error('The $entity_last_installed_schema_repository parameter was added in Drupal 8.6.x and will be required in 9.0.0. See https://www.drupal.org/node/2973262.', E_USER_DEPRECATED);
+      $entity_last_installed_schema_repository = \Drupal::service('entity.last_installed_schema.repository');
+    }
+    $this->entityLastInstalledSchemaRepository = $entity_last_installed_schema_repository;
   }
 
   /**
@@ -63,7 +78,7 @@ public function getChangeSummary() {
       // Process field storage definition changes.
       if (!empty($change_list['field_storage_definitions'])) {
         $storage_definitions = $this->entityManager->getFieldStorageDefinitions($entity_type_id);
-        $original_storage_definitions = $this->entityManager->getLastInstalledFieldStorageDefinitions($entity_type_id);
+        $original_storage_definitions = $this->entityLastInstalledSchemaRepository->getLastInstalledFieldStorageDefinitions($entity_type_id);
 
         foreach ($change_list['field_storage_definitions'] as $field_name => $change) {
           switch ($change) {
@@ -108,7 +123,7 @@ public function applyUpdates() {
       // Process field storage definition changes.
       if (!empty($change_list['field_storage_definitions'])) {
         $storage_definitions = $this->entityManager->getFieldStorageDefinitions($entity_type_id);
-        $original_storage_definitions = $this->entityManager->getLastInstalledFieldStorageDefinitions($entity_type_id);
+        $original_storage_definitions = $this->entityLastInstalledSchemaRepository->getLastInstalledFieldStorageDefinitions($entity_type_id);
 
         foreach ($change_list['field_storage_definitions'] as $field_name => $change) {
           $storage_definition = isset($storage_definitions[$field_name]) ? $storage_definitions[$field_name] : NULL;
@@ -123,10 +138,17 @@ public function applyUpdates() {
    * {@inheritdoc}
    */
   public function getEntityType($entity_type_id) {
-    $entity_type = $this->entityManager->getLastInstalledDefinition($entity_type_id);
+    $entity_type = $this->entityLastInstalledSchemaRepository->getLastInstalledDefinition($entity_type_id);
     return $entity_type ? clone $entity_type : NULL;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function getEntityTypes() {
+    return $this->entityLastInstalledSchemaRepository->getLastInstalledDefinitions();
+  }
+
   /**
    * {@inheritdoc}
    */
@@ -173,7 +195,7 @@ public function installFieldStorageDefinition($name, $entity_type_id, $provider,
    * {@inheritdoc}
    */
   public function getFieldStorageDefinition($name, $entity_type_id) {
-    $storage_definitions = $this->entityManager->getLastInstalledFieldStorageDefinitions($entity_type_id);
+    $storage_definitions = $this->entityLastInstalledSchemaRepository->getLastInstalledFieldStorageDefinitions($entity_type_id);
     return isset($storage_definitions[$name]) ? clone $storage_definitions[$name] : NULL;
   }
 
@@ -211,7 +233,7 @@ protected function doEntityUpdate($op, $entity_type_id) {
         break;
 
       case static::DEFINITION_UPDATED:
-        $original = $this->entityManager->getLastInstalledDefinition($entity_type_id);
+        $original = $this->entityLastInstalledSchemaRepository->getLastInstalledDefinition($entity_type_id);
         $this->entityManager->onEntityTypeUpdate($entity_type, $original);
         break;
     }
@@ -262,7 +284,7 @@ protected function getChangeList() {
     $change_list = [];
 
     foreach ($this->entityManager->getDefinitions() as $entity_type_id => $entity_type) {
-      $original = $this->entityManager->getLastInstalledDefinition($entity_type_id);
+      $original = $this->entityLastInstalledSchemaRepository->getLastInstalledDefinition($entity_type_id);
 
       // @todo Support non-storage-schema-changing definition updates too:
       //   https://www.drupal.org/node/2336895.
@@ -277,7 +299,7 @@ protected function getChangeList() {
         if ($this->entityManager->getStorage($entity_type_id) instanceof DynamicallyFieldableEntityStorageInterface) {
           $field_changes = [];
           $storage_definitions = $this->entityManager->getFieldStorageDefinitions($entity_type_id);
-          $original_storage_definitions = $this->entityManager->getLastInstalledFieldStorageDefinitions($entity_type_id);
+          $original_storage_definitions = $this->entityLastInstalledSchemaRepository->getLastInstalledFieldStorageDefinitions($entity_type_id);
 
           // Detect created field storage definitions.
           foreach (array_diff_key($storage_definitions, $original_storage_definitions) as $field_name => $storage_definition) {
diff --git a/core/lib/Drupal/Core/Entity/EntityDefinitionUpdateManagerInterface.php b/core/lib/Drupal/Core/Entity/EntityDefinitionUpdateManagerInterface.php
index 7754170dbb4410f39455ec2bd086735b35bf1b02..a219f3c046ceba8a229555990f390f657f1fa1ad 100644
--- a/core/lib/Drupal/Core/Entity/EntityDefinitionUpdateManagerInterface.php
+++ b/core/lib/Drupal/Core/Entity/EntityDefinitionUpdateManagerInterface.php
@@ -108,6 +108,18 @@ public function applyUpdates();
    */
   public function getEntityType($entity_type_id);
 
+  /**
+   * Returns all the entity type definitions, ready to be manipulated.
+   *
+   * When needing to apply updates to existing entity type definitions, this
+   * method should always be used to retrieve all the definitions ready to be
+   * manipulated.
+   *
+   * @return \Drupal\Core\Entity\EntityTypeInterface[]
+   *   The last installed entity type definitions, keyed by the entity type ID.
+   */
+  public function getEntityTypes();
+
   /**
    * Installs a new entity type definition.
    *
diff --git a/core/lib/Drupal/Core/Entity/EntityLastInstalledSchemaRepository.php b/core/lib/Drupal/Core/Entity/EntityLastInstalledSchemaRepository.php
index 61300125c9948380f37491974bf0cc91005f8629..70e552c742ff4ead98756d5d936c25d8fa0d9b7e 100644
--- a/core/lib/Drupal/Core/Entity/EntityLastInstalledSchemaRepository.php
+++ b/core/lib/Drupal/Core/Entity/EntityLastInstalledSchemaRepository.php
@@ -34,6 +34,28 @@ public function getLastInstalledDefinition($entity_type_id) {
     return $this->keyValueFactory->get('entity.definitions.installed')->get($entity_type_id . '.entity_type');
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function getLastInstalledDefinitions() {
+    $all_definitions = $this->keyValueFactory->get('entity.definitions.installed')->getAll();
+
+    // Filter out field storage definitions.
+    $filtered_keys = array_filter(array_keys($all_definitions), function ($key) {
+        return substr($key, -12) === '.entity_type';
+    });
+    $entity_type_definitions = array_intersect_key($all_definitions, array_flip($filtered_keys));
+
+    // Ensure that the returned array is keyed by the entity type ID.
+    $keys = array_keys($entity_type_definitions);
+    $keys = array_map(function ($key) {
+      $parts = explode('.', $key);
+      return $parts[0];
+    }, $keys);
+
+    return array_combine($keys, $entity_type_definitions);
+  }
+
   /**
    * {@inheritdoc}
    */
diff --git a/core/lib/Drupal/Core/Entity/EntityLastInstalledSchemaRepositoryInterface.php b/core/lib/Drupal/Core/Entity/EntityLastInstalledSchemaRepositoryInterface.php
index 53f02a00063f30d7f969559f5137cfc712288ff7..af910bb69004350caaffb54dc432f4d6d1bc8520 100644
--- a/core/lib/Drupal/Core/Entity/EntityLastInstalledSchemaRepositoryInterface.php
+++ b/core/lib/Drupal/Core/Entity/EntityLastInstalledSchemaRepositoryInterface.php
@@ -41,6 +41,34 @@ interface EntityLastInstalledSchemaRepositoryInterface {
    */
   public function getLastInstalledDefinition($entity_type_id);
 
+  /**
+   * Gets the entity type definitions in their most recently installed state.
+   *
+   * During the application lifetime, entity type definitions can change. For
+   * example, updated code can be deployed. The
+   * \Drupal\Core\Entity\EntityTypeManagerInterface::getDefinitions() method
+   * will always return the definitions as determined by the current codebase.
+   * This method returns the definitions from the last time that a
+   * \Drupal\Core\Entity\EntityTypeListener event was completed. In other words,
+   * the definitions that the entity type's handlers have incorporated into the
+   * application state. For example, if the entity type's storage handler is
+   * SQL-based, the definition for which database tables were created.
+   *
+   * Application management code can check if
+   * \Drupal\Core\Entity\EntityTypeManagerInterface::getDefinitions() differs
+   * from getLastInstalledDefinitions() and decide whether to:
+   * - Invoke the appropriate \Drupal\Core\Entity\EntityTypeListenerInterface
+   *   event so that handlers react to the new definitions.
+   * - Raise a warning that the application state is incompatible with the
+   *   codebase.
+   * - Perform some other action.
+   *
+   * @return \Drupal\Core\Entity\EntityTypeInterface[]
+   *   An array containing the installed definition for all entity types, keyed
+   *   by the entity type ID.
+   */
+  public function getLastInstalledDefinitions();
+
   /**
    * Stores the entity type definition in the application state.
    *
diff --git a/core/tests/Drupal/KernelTests/Core/Entity/EntityDefinitionUpdateTest.php b/core/tests/Drupal/KernelTests/Core/Entity/EntityDefinitionUpdateTest.php
index 01044f724a1d314060ed3f2efa3b86244676b396..1a743eceebed491d2e95ad1e5c5d71f9389fc550 100644
--- a/core/tests/Drupal/KernelTests/Core/Entity/EntityDefinitionUpdateTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Entity/EntityDefinitionUpdateTest.php
@@ -20,6 +20,8 @@
 /**
  * Tests EntityDefinitionUpdateManager functionality.
  *
+ * @coversDefaultClass \Drupal\Core\Entity\EntityDefinitionUpdateManager
+ *
  * @group Entity
  */
 class EntityDefinitionUpdateTest extends EntityKernelTestBase {
@@ -1180,4 +1182,18 @@ public function testInitialValueFromFieldErrorHandling() {
     }
   }
 
+  /**
+   * @covers ::getEntityTypes
+   */
+  public function testGetEntityTypes() {
+    $entity_type_definitions = $this->entityDefinitionUpdateManager->getEntityTypes();
+
+    // Ensure that we have at least one entity type to check below.
+    $this->assertGreaterThanOrEqual(1, count($entity_type_definitions));
+
+    foreach ($entity_type_definitions as $entity_type_id => $entity_type) {
+      $this->assertEquals($this->entityDefinitionUpdateManager->getEntityType($entity_type_id), $entity_type);
+    }
+  }
+
 }