diff --git a/core/core.services.yml b/core/core.services.yml
index 65e0a40..2ae1da0 100644
--- a/core/core.services.yml
+++ b/core/core.services.yml
@@ -33,16 +33,17 @@ services:
     tags:
       - { name: cache.context}
   cache_tags.invalidator:
+    parent: container.trait
     class: Drupal\Core\Cache\CacheTagsInvalidator
     calls:
       - [setContainer, ['@service_container']]
     tags:
-      - { name: service_collector, call: addInvalidator }
-  cache_tags.checksum:
+      - { name: service_collector, call: addInvalidator, tag: cache_tags_invalidator }
+  cache_tags.invalidator.checksum:
     class: Drupal\Core\Cache\DatabaseCacheTagsChecksum
     arguments: ['@database']
     tags:
-      - { name: cache_tags.invalidator}
+      - { name: cache_tags_invalidator}
   cache.backend.chainedfast:
     class: Drupal\Core\Cache\ChainedFastBackendFactory
     arguments: ['@settings']
@@ -50,13 +51,13 @@ services:
       - [setContainer, ['@service_container']]
   cache.backend.database:
     class: Drupal\Core\Cache\DatabaseBackendFactory
-    arguments: ['@database', '@cache_tags.checksum']
+    arguments: ['@database', '@cache_tags.invalidator.checksum']
   cache.backend.apcu:
     class: Drupal\Core\Cache\ApcuBackendFactory
-    arguments: ['@app.root', '@cache_tags.checksum']
+    arguments: ['@app.root', '@cache_tags.invalidator.checksum']
   cache.backend.php:
     class: Drupal\Core\Cache\PhpBackendFactory
-    arguments: ['@cache_tags.checksum']
+    arguments: ['@cache_tags.invalidator.checksum']
   cache.bootstrap:
     class: Drupal\Core\Cache\CacheBackendInterface
     tags:
diff --git a/core/lib/Drupal/Core/Cache/ApcuBackend.php b/core/lib/Drupal/Core/Cache/ApcuBackend.php
index e406d0f..46a73b6 100644
--- a/core/lib/Drupal/Core/Cache/ApcuBackend.php
+++ b/core/lib/Drupal/Core/Cache/ApcuBackend.php
@@ -10,7 +10,7 @@
 /**
  * Stores cache items in the Alternative PHP Cache User Cache (APCu).
  */
-class ApcuBackend implements CacheBackendInterface, CacheTagsInvalidatorInterface {
+class ApcuBackend implements CacheBackendInterface {
 
   /**
    * The name of the cache bin to use.
@@ -183,11 +183,8 @@ protected function prepareItem($cache, $allow_invalid) {
    * {@inheritdoc}
    */
   public function set($cid, $data, $expire = CacheBackendInterface::CACHE_PERMANENT, array $tags = array()) {
-    if ($tags) {
-      $tags = array_unique($tags);
-      Cache::validateTags($tags);
-    }
-
+    Cache::validateTags($tags);
+    $tags = array_unique($tags);
     $cache = new \stdClass();
     $cache->cid = $cid;
     $cache->created = round(microtime(TRUE), 3);
@@ -278,16 +275,4 @@ public function invalidateAll() {
     }
   }
 
-  /**
-   * {@inheritdoc}
-   */
-  public function invalidateTags(array $tags) {
-    foreach ($tags as $tag) {
-      apc_inc($this->invalidationsTagsPrefix . $tag, 1, $success);
-      if (!$success) {
-        apc_store($this->invalidationsTagsPrefix . $tag, 1);
-      }
-    }
-  }
-
 }
diff --git a/core/lib/Drupal/Core/Cache/CacheBackendInterface.php b/core/lib/Drupal/Core/Cache/CacheBackendInterface.php
index e5aa85d..faf2459 100644
--- a/core/lib/Drupal/Core/Cache/CacheBackendInterface.php
+++ b/core/lib/Drupal/Core/Cache/CacheBackendInterface.php
@@ -178,7 +178,6 @@ public function deleteAll();
    *
    * @see \Drupal\Core\Cache\CacheBackendInterface::delete()
    * @see \Drupal\Core\Cache\CacheBackendInterface::invalidateMultiple()
-   * @see \Drupal\Core\Cache\CacheBackendInterface::invalidateTags()
    * @see \Drupal\Core\Cache\CacheBackendInterface::invalidateAll()
    */
   public function invalidate($cid);
@@ -189,12 +188,11 @@ public function invalidate($cid);
    * Invalid items may be returned in later calls to get(), if the $allow_invalid
    * argument is TRUE.
    *
-   * @param string $cids
+   * @param string[] $cids
    *   An array of cache IDs to invalidate.
    *
    * @see \Drupal\Core\Cache\CacheBackendInterface::deleteMultiple()
    * @see \Drupal\Core\Cache\CacheBackendInterface::invalidate()
-   * @see \Drupal\Core\Cache\CacheBackendInterface::invalidateTags()
    * @see \Drupal\Core\Cache\CacheBackendInterface::invalidateAll()
    */
   public function invalidateMultiple(array $cids);
@@ -205,13 +203,9 @@ public function invalidateMultiple(array $cids);
    * Invalid items may be returned in later calls to get(), if the $allow_invalid
    * argument is TRUE.
    *
-   * @param string $cids
-   *   An array of cache IDs to invalidate.
-   *
    * @see \Drupal\Core\Cache\CacheBackendInterface::deleteAll()
    * @see \Drupal\Core\Cache\CacheBackendInterface::invalidate()
    * @see \Drupal\Core\Cache\CacheBackendInterface::invalidateMultiple()
-   * @see \Drupal\Core\Cache\CacheBackendInterface::invalidateTags()
    */
   public function invalidateAll();
 
diff --git a/core/lib/Drupal/Core/Cache/CacheTagsChecksumInterface.php b/core/lib/Drupal/Core/Cache/CacheTagsChecksumInterface.php
index 265f624..a140112 100644
--- a/core/lib/Drupal/Core/Cache/CacheTagsChecksumInterface.php
+++ b/core/lib/Drupal/Core/Cache/CacheTagsChecksumInterface.php
@@ -10,7 +10,14 @@
 /**
  * Provides checksums for cache tag invalidations.
  *
- * Cache backends can use this to check if any cache tag invalidations
+ * Cache backends can use this to check if any cache tag invalidations happened
+ * for a stored cache item.
+ *
+ * To do so, they can inject the cache_tags.invalidator.checksum service, and
+ * when a cache item is written, store cache tags together with the current
+ * checksum, calculated by getCurrentChecksum(). When a cache item is fetched,
+ * the checksum can be validated with isValid(). The service will return FALSE
+ * if any of those cache tags were invalidated in the meantime.
  *
  * @ingroup cache
  */
diff --git a/core/lib/Drupal/Core/Cache/CacheTagsInvalidatorInterface.php b/core/lib/Drupal/Core/Cache/CacheTagsInvalidatorInterface.php
index 4db0a96..bf63041 100644
--- a/core/lib/Drupal/Core/Cache/CacheTagsInvalidatorInterface.php
+++ b/core/lib/Drupal/Core/Cache/CacheTagsInvalidatorInterface.php
@@ -10,6 +10,10 @@
 /**
  * Defines required methods for classes wanting to handle cache tag changes.
  *
+ * Services that implement this interface must add the cache_tags_invalidator
+ * tag to be notified. Cache backends may implement this interface as well, they
+ * will be notified automatically.
+ *
  * @ingroup cache
  */
 interface CacheTagsInvalidatorInterface {
diff --git a/core/lib/Drupal/Core/Cache/DatabaseBackend.php b/core/lib/Drupal/Core/Cache/DatabaseBackend.php
index 044c910..4610636 100644
--- a/core/lib/Drupal/Core/Cache/DatabaseBackend.php
+++ b/core/lib/Drupal/Core/Cache/DatabaseBackend.php
@@ -150,12 +150,10 @@ protected function prepareItem($cache, $allow_invalid) {
    * Implements Drupal\Core\Cache\CacheBackendInterface::set().
    */
   public function set($cid, $data, $expire = Cache::PERMANENT, array $tags = array()) {
-    if ($tags) {
-      Cache::validateTags($tags);
-      $tags = array_unique($tags);
-      // Sort the cache tags so that they are stored consistently in the database.
-      sort($tags);
-    }
+    Cache::validateTags($tags);
+    $tags = array_unique($tags);
+    // Sort the cache tags so that they are stored consistently in the database.
+    sort($tags);
     $try_again = FALSE;
     try {
       // The bin might not yet exist.
@@ -223,12 +221,10 @@ public function setMultiple(array $items) {
           'tags' => array(),
         );
 
-        if ($item['tags']) {
-          Cache::validateTags($item['tags']);
-          $item['tags'] = array_unique($item['tags']);
-          // Sort the cache tags so that they are stored consistently in the DB.
-          sort($item['tags']);
-        }
+        Cache::validateTags($item['tags']);
+        $item['tags'] = array_unique($item['tags']);
+        // Sort the cache tags so that they are stored consistently in the DB.
+        sort($item['tags']);
 
         $fields = array(
           'cid' => $cid,
diff --git a/core/lib/Drupal/Core/Cache/DatabaseCacheTagsChecksum.php b/core/lib/Drupal/Core/Cache/DatabaseCacheTagsChecksum.php
index 784294e..4375b45 100644
--- a/core/lib/Drupal/Core/Cache/DatabaseCacheTagsChecksum.php
+++ b/core/lib/Drupal/Core/Cache/DatabaseCacheTagsChecksum.php
@@ -82,7 +82,9 @@ public function invalidateTags(array $tags) {
    */
   public function getCurrentChecksum(array $tags) {
     // Remove tags that were already invalidated during this request from the
-    // static caches so that another deletion or invalidation can occur.
+    // static caches so that another invalidation can occur later in the same
+    // request. Without that, written cache items would not be invalidated
+    // correctly.
     foreach ($tags as $tag) {
       unset($this->invalidatedTags[$tag]);
     }
diff --git a/core/lib/Drupal/Core/Cache/PhpBackend.php b/core/lib/Drupal/Core/Cache/PhpBackend.php
index df6ffa7..b665e86 100644
--- a/core/lib/Drupal/Core/Cache/PhpBackend.php
+++ b/core/lib/Drupal/Core/Cache/PhpBackend.php
@@ -148,16 +148,13 @@ protected function prepareItem($cache, $allow_invalid) {
    * {@inheritdoc}
    */
   public function set($cid, $data, $expire = Cache::PERMANENT, array $tags = array()) {
-    if ($tags) {
-      $tags = array_unique($tags);
-      Cache::validateTags($tags);
-    }
+    Cache::validateTags($tags);
     $item = (object) array(
       'cid' => $cid,
       'data' => $data,
       'created' => round(microtime(TRUE), 3),
       'expire' => $expire,
-      'tags' => $tags,
+      'tags' => array_unique($tags),
       'checksum' => $this->checksumProvider->getCurrentChecksum($tags),
     );
     $this->writeItem($this->normalizeCid($cid), $item);
@@ -218,18 +215,6 @@ public function invalidateMultiple(array $cids) {
   /**
    * {@inheritdoc}
    */
-  public function invalidateTags(array $tags) {
-    foreach ($this->storage()->listAll() as $cidhash) {
-      $item = $this->getByHash($cidhash);
-      if ($item && array_intersect($tags, $item->tags)) {
-        $this->invalidate($item->cid);
-      }
-    }
-  }
-
-  /**
-   * {@inheritdoc}
-   */
   public function invalidateAll() {
     foreach($this->storage()->listAll() as $cidhash) {
       $this->invalidatebyHash($cidhash);
diff --git a/core/lib/Drupal/Core/Field/FieldStorageDefinitionInterface.php b/core/lib/Drupal/Core/Field/FieldStorageDefinitionInterface.php
index d18a8c7..9b0eb65 100644
--- a/core/lib/Drupal/Core/Field/FieldStorageDefinitionInterface.php
+++ b/core/lib/Drupal/Core/Field/FieldStorageDefinitionInterface.php
@@ -8,7 +8,6 @@
 namespace Drupal\Core\Field;
 
 use Drupal\Core\Entity\FieldableEntityInterface;
-use Drupal\Core\TypedData\DataDefinitionInterface;
 
 /**
  * Defines an interface for entity field storage definitions.
diff --git a/core/lib/Drupal/Core/Installer/InstallerServiceProvider.php b/core/lib/Drupal/Core/Installer/InstallerServiceProvider.php
index 15a7e79..9862552 100644
--- a/core/lib/Drupal/Core/Installer/InstallerServiceProvider.php
+++ b/core/lib/Drupal/Core/Installer/InstallerServiceProvider.php
@@ -52,7 +52,7 @@ public function register(ContainerBuilder $container) {
     // Remove the cache tags invalidator tag from the cache tags storage, so
     // that we don't call it when cache tags are invalidated very early in the
     // installer.
-    $container->getDefinition('cache_tags.checksum')
+    $container->getDefinition('cache_tags.invalidator.checksum')
       ->clearTag('cache_tags.invalidator');
 
     // Replace the route builder with an empty implementation.
diff --git a/core/modules/config/src/Tests/Storage/CachedStorageTest.php b/core/modules/config/src/Tests/Storage/CachedStorageTest.php
index f7dd32e..66f6225 100644
--- a/core/modules/config/src/Tests/Storage/CachedStorageTest.php
+++ b/core/modules/config/src/Tests/Storage/CachedStorageTest.php
@@ -91,7 +91,7 @@ public function containerBuild(ContainerBuilder $container) {
     // Use the regular database cache backend to aid testing.
     $container->register('cache_factory', 'Drupal\Core\Cache\DatabaseBackendFactory')
       ->addArgument(new Reference('database'))
-      ->addArgument(new Reference('cache_tags.checksum'));
+      ->addArgument(new Reference('cache_tags.invalidator.checksum'));
   }
 
 }
diff --git a/core/modules/locale/src/Tests/LocaleTranslationUiTest.php b/core/modules/locale/src/Tests/LocaleTranslationUiTest.php
index f918e1e..55d48e7 100644
--- a/core/modules/locale/src/Tests/LocaleTranslationUiTest.php
+++ b/core/modules/locale/src/Tests/LocaleTranslationUiTest.php
@@ -137,7 +137,7 @@ public function testStringTranslation() {
 
     // Reset the tag cache on the tester side in order to pick up the call to
     // Cache::invalidateTags() on the tested side.
-    \Drupal::service('cache_tags.checksum')->reset();
+    \Drupal::service('cache_tags.invalidator.checksum')->reset();
 
     $this->assertTrue($name != $translation && t($name, array(), array('langcode' => $langcode)) == $translation, 't() works for non-English.');
     // Refresh the locale() cache to get fresh data from t() below. We are in
diff --git a/core/modules/simpletest/src/WebTestBase.php b/core/modules/simpletest/src/WebTestBase.php
index 51e5940..4c993b2 100644
--- a/core/modules/simpletest/src/WebTestBase.php
+++ b/core/modules/simpletest/src/WebTestBase.php
@@ -1151,7 +1151,7 @@ protected function resetAll() {
    */
   protected function refreshVariables() {
     // Clear the tag cache.
-    \Drupal::service('cache_tags.checksum')->reset();
+    \Drupal::service('cache_tags.invalidator.checksum')->reset();
     foreach (Cache::getBins() as $backend) {
       if (is_callable(array($backend, 'reset'))) {
         $backend->reset();
diff --git a/core/modules/system/src/Tests/Cache/ApcuBackendUnitTest.php b/core/modules/system/src/Tests/Cache/ApcuBackendUnitTest.php
index 14bb16c..20fb3a4 100644
--- a/core/modules/system/src/Tests/Cache/ApcuBackendUnitTest.php
+++ b/core/modules/system/src/Tests/Cache/ApcuBackendUnitTest.php
@@ -34,8 +34,7 @@ protected function checkRequirements() {
   }
 
   protected function createCacheBackend($bin) {
-    $backend = new ApcuBackend($bin, $this->databasePrefix, \Drupal::service('cache_tags.checksum'));
-    return $backend;
+    return new ApcuBackend($bin, $this->databasePrefix, \Drupal::service('cache_tags.invalidator.checksum'));
   }
 
   protected function tearDown() {
diff --git a/core/modules/system/src/Tests/Cache/ChainedFastBackendUnitTest.php b/core/modules/system/src/Tests/Cache/ChainedFastBackendUnitTest.php
index 961c10a..c34563c 100644
--- a/core/modules/system/src/Tests/Cache/ChainedFastBackendUnitTest.php
+++ b/core/modules/system/src/Tests/Cache/ChainedFastBackendUnitTest.php
@@ -25,8 +25,8 @@ class ChainedFastBackendUnitTest extends GenericCacheBackendUnitTestBase {
    *   A new ChainedFastBackend object.
    */
   protected function createCacheBackend($bin) {
-    $consistent_backend = new DatabaseBackend(\Drupal::service('database'), \Drupal::service('cache_tags.checksum'), $bin);
-    $fast_backend = new PhpBackend($bin, \Drupal::service('cache_tags.checksum'));
+    $consistent_backend = new DatabaseBackend(\Drupal::service('database'), \Drupal::service('cache_tags.invalidator.checksum'), $bin);
+    $fast_backend = new PhpBackend($bin, \Drupal::service('cache_tags.invalidator.checksum'));
     $backend = new ChainedFastBackend($consistent_backend, $fast_backend, $bin);
     // Explicitly register the cache bin as it can not work through the
     // cache bin list in the container.
diff --git a/core/modules/system/src/Tests/Cache/DatabaseBackendUnitTest.php b/core/modules/system/src/Tests/Cache/DatabaseBackendUnitTest.php
index 91009f8..e54198b 100644
--- a/core/modules/system/src/Tests/Cache/DatabaseBackendUnitTest.php
+++ b/core/modules/system/src/Tests/Cache/DatabaseBackendUnitTest.php
@@ -30,7 +30,7 @@ class DatabaseBackendUnitTest extends GenericCacheBackendUnitTestBase {
    *   A new DatabaseBackend object.
    */
   protected function createCacheBackend($bin) {
-    return new DatabaseBackend($this->container->get('database'), $this->container->get('cache_tags.checksum'), $bin);
+    return new DatabaseBackend($this->container->get('database'), $this->container->get('cache_tags.invalidator.checksum'), $bin);
   }
 
 }
diff --git a/core/modules/system/src/Tests/Cache/PhpBackendUnitTest.php b/core/modules/system/src/Tests/Cache/PhpBackendUnitTest.php
index 507c16f..1751883 100644
--- a/core/modules/system/src/Tests/Cache/PhpBackendUnitTest.php
+++ b/core/modules/system/src/Tests/Cache/PhpBackendUnitTest.php
@@ -23,7 +23,7 @@ class PhpBackendUnitTest extends GenericCacheBackendUnitTestBase {
    *   A new MemoryBackend object.
    */
   protected function createCacheBackend($bin) {
-    $backend = new PhpBackend($bin, \Drupal::service('cache_tags.checksum'));
+    $backend = new PhpBackend($bin, \Drupal::service('cache_tags.invalidator.checksum'));
     return $backend;
   }
 
diff --git a/core/tests/Drupal/Tests/Core/Cache/CacheTagsInvalidatorTest.php b/core/tests/Drupal/Tests/Core/Cache/CacheTagsInvalidatorTest.php
index adfb7a3..1424b1e 100644
--- a/core/tests/Drupal/Tests/Core/Cache/CacheTagsInvalidatorTest.php
+++ b/core/tests/Drupal/Tests/Core/Cache/CacheTagsInvalidatorTest.php
@@ -17,7 +17,6 @@
  */
 class CacheTagsInvalidatorTest extends UnitTestCase {
 
-
   /**
    * @covers ::invalidateTags
    *
@@ -48,7 +47,7 @@ public function testInvalidateTags() {
     // We do not have to define that invalidateTags() is never called as the
     // interface does not define that method, trying to call it would result in
     // a fatal error.
-    $non_invalidator_cache_bin = $this->getMock('\Drupal\Core\Cache\CacheBackendI');
+    $non_invalidator_cache_bin = $this->getMock('\Drupal\Core\Cache\CacheBackendInterface');
 
     $container = new Container();
     $container->set('cache.invalidator_cache_bin', $invalidator_cache_bin);