diff --git a/src/Symfony/Component/Cache/Adapter/AbstractAdapter.php b/src/Symfony/Component/Cache/Adapter/AbstractAdapter.php
index 9bc05fd2b3..6b38991b77 100644
--- a/src/Symfony/Component/Cache/Adapter/AbstractAdapter.php
+++ b/src/Symfony/Component/Cache/Adapter/AbstractAdapter.php
@@ -38,7 +38,7 @@ abstract class AbstractAdapter implements AdapterInterface, LoggerAwareInterface
      */
     protected function __construct($namespace = '', $defaultLifetime = 0)
     {
-        $this->namespace = '' === $namespace ? '' : $this->getId($namespace).':';
+        $this->namespace = '' === $namespace ? '' : CacheItem::validateKey($namespace).':';
         if (null !== $this->maxIdLength && strlen($namespace) > $this->maxIdLength - 24) {
             throw new InvalidArgumentException(sprintf('Namespace must be %d chars max, %d given ("%s")', $this->maxIdLength - 24, strlen($namespace), $namespace));
         }
diff --git a/src/Symfony/Component/Cache/Adapter/ProxyAdapter.php b/src/Symfony/Component/Cache/Adapter/ProxyAdapter.php
index 4f37ffd731..cd310be062 100644
--- a/src/Symfony/Component/Cache/Adapter/ProxyAdapter.php
+++ b/src/Symfony/Component/Cache/Adapter/ProxyAdapter.php
@@ -35,7 +35,7 @@ class ProxyAdapter implements AdapterInterface
     {
         $this->pool = $pool;
         $this->poolHash = $poolHash = spl_object_hash($pool);
-        $this->namespace = '' === $namespace ? '' : $this->getId($namespace);
+        $this->namespace = '' === $namespace ? '' : CacheItem::validateKey($namespace);
         $this->namespaceLen = strlen($namespace);
         $this->createCacheItem = \Closure::bind(
             function ($key, $innerItem) use ($defaultLifetime, $poolHash) {
diff --git a/src/Symfony/Component/Cache/CacheItem.php b/src/Symfony/Component/Cache/CacheItem.php
index 55e25de9a9..d6f629b2ab 100644
--- a/src/Symfony/Component/Cache/CacheItem.php
+++ b/src/Symfony/Component/Cache/CacheItem.php
@@ -148,6 +148,8 @@ final class CacheItem implements CacheItemInterface
      *
      * @param string $key The key to validate
      *
+     * @return string
+     *
      * @throws InvalidArgumentException When $key is not valid
      */
     public static function validateKey($key)
@@ -161,6 +163,8 @@ final class CacheItem implements CacheItemInterface
         if (false !== strpbrk($key, '{}()/\@:')) {
             throw new InvalidArgumentException(sprintf('Cache key "%s" contains reserved characters {}()/\@:', $key));
         }
+
+        return $key;
     }
 
     /**
diff --git a/src/Symfony/Component/Cache/Simple/AbstractCache.php b/src/Symfony/Component/Cache/Simple/AbstractCache.php
index e4046463f1..264eb60653 100644
--- a/src/Symfony/Component/Cache/Simple/AbstractCache.php
+++ b/src/Symfony/Component/Cache/Simple/AbstractCache.php
@@ -37,7 +37,7 @@ abstract class AbstractCache implements CacheInterface, LoggerAwareInterface
     protected function __construct($namespace = '', $defaultLifetime = 0)
     {
         $this->defaultLifetime = max(0, (int) $defaultLifetime);
-        $this->namespace = '' === $namespace ? '' : $this->getId($namespace).':';
+        $this->namespace = '' === $namespace ? '' : CacheItem::validateKey($namespace).':';
         if (null !== $this->maxIdLength && strlen($namespace) > $this->maxIdLength - 24) {
             throw new InvalidArgumentException(sprintf('Namespace must be %d chars max, %d given ("%s")', $this->maxIdLength - 24, strlen($namespace), $namespace));
         }
diff --git a/src/Symfony/Component/Cache/Tests/CacheItemTest.php b/src/Symfony/Component/Cache/Tests/CacheItemTest.php
index 4e455c64a4..daca925fd5 100644
--- a/src/Symfony/Component/Cache/Tests/CacheItemTest.php
+++ b/src/Symfony/Component/Cache/Tests/CacheItemTest.php
@@ -18,7 +18,7 @@ class CacheItemTest extends TestCase
 {
     public function testValidKey()
     {
-        $this->assertNull(CacheItem::validateKey('foo'));
+        $this->assertSame('foo', CacheItem::validateKey('foo'));
     }
 
     /**
diff --git a/src/Symfony/Component/Cache/Traits/AbstractTrait.php b/src/Symfony/Component/Cache/Traits/AbstractTrait.php
index 375ccf7620..108ac67c8e 100644
--- a/src/Symfony/Component/Cache/Traits/AbstractTrait.php
+++ b/src/Symfony/Component/Cache/Traits/AbstractTrait.php
@@ -24,6 +24,8 @@ trait AbstractTrait
     use LoggerAwareTrait;
 
     private $namespace;
+    private $namespaceVersion = '';
+    private $versioningIsEnabled = false;
     private $deferred = array();
 
     /**
@@ -102,10 +104,18 @@ trait AbstractTrait
      */
     public function clear()
     {
+        if ($cleared = $this->versioningIsEnabled) {
+            $this->namespaceVersion = 2;
+            foreach ($this->doFetch(array('@'.$this->namespace)) as $v) {
+                $this->namespaceVersion = 1 + (int) $v;
+            }
+            $this->namespaceVersion .= ':';
+            $cleared = $this->doSave(array('@'.$this->namespace => $this->namespaceVersion), 0);
+        }
         $this->deferred = array();
 
         try {
-            return $this->doClear($this->namespace);
+            return $this->doClear($this->namespace) || $cleared;
         } catch (\Exception $e) {
             CacheItem::log($this->logger, 'Failed to clear the cache', array('exception' => $e));
 
@@ -158,6 +168,27 @@ trait AbstractTrait
         return $ok;
     }
 
+    /**
+     * Enables/disables versioning of items.
+     *
+     * When versioning is enabled, clearing the cache is atomic and doesn't require listing existing keys to proceed,
+     * but old keys may need garbage collection and extra round-trips to the back-end are required.
+     *
+     * Calling this method also clears the memoized namespace version and thus forces a resynchonization of it.
+     *
+     * @param bool $enable
+     *
+     * @return bool the previous state of versioning
+     */
+    public function enableVersioning($enable = true)
+    {
+        $wasEnabled = $this->versioningIsEnabled;
+        $this->versioningIsEnabled = (bool) $enable;
+        $this->namespaceVersion = '';
+
+        return $wasEnabled;
+    }
+
     /**
      * Like the native unserialize() function but throws an exception if anything goes wrong.
      *
@@ -189,11 +220,18 @@ trait AbstractTrait
     {
         CacheItem::validateKey($key);
 
-        if (null === $this->maxIdLength) {
-            return $this->namespace.$key;
+        if ($this->versioningIsEnabled && '' === $this->namespaceVersion) {
+            $this->namespaceVersion = '1:';
+            foreach ($this->doFetch(array('@'.$this->namespace)) as $v) {
+                $this->namespaceVersion = $v;
+            }
         }
-        if (strlen($id = $this->namespace.$key) > $this->maxIdLength) {
-            $id = $this->namespace.substr_replace(base64_encode(hash('sha256', $key, true)), ':', -22);
+
+        if (null === $this->maxIdLength) {
+            return $this->namespace.$this->namespaceVersion.$key;
+        }
+        if (strlen($id = $this->namespace.$this->namespaceVersion.$key) > $this->maxIdLength) {
+            $id = $this->namespace.$this->namespaceVersion.substr_replace(base64_encode(hash('sha256', $key, true)), ':', -22);
         }
 
         return $id;
diff --git a/src/Symfony/Component/Cache/Traits/MemcachedTrait.php b/src/Symfony/Component/Cache/Traits/MemcachedTrait.php
index 8a836bac18..6dc46e9dd8 100644
--- a/src/Symfony/Component/Cache/Traits/MemcachedTrait.php
+++ b/src/Symfony/Component/Cache/Traits/MemcachedTrait.php
@@ -54,6 +54,7 @@ trait MemcachedTrait
         }
 
         parent::__construct($namespace, $defaultLifetime);
+        $this->enableVersioning();
     }
 
     /**
@@ -242,7 +243,7 @@ trait MemcachedTrait
      */
     protected function doClear($namespace)
     {
-        return $this->checkResultCode($this->getClient()->flush());
+        return false;
     }
 
     private function checkResultCode($result)
diff --git a/src/Symfony/Component/Cache/Traits/RedisTrait.php b/src/Symfony/Component/Cache/Traits/RedisTrait.php
index b45d65adc8..6202051a8d 100644
--- a/src/Symfony/Component/Cache/Traits/RedisTrait.php
+++ b/src/Symfony/Component/Cache/Traits/RedisTrait.php
@@ -46,7 +46,9 @@ trait RedisTrait
         if (preg_match('#[^-+_.A-Za-z0-9]#', $namespace, $match)) {
             throw new InvalidArgumentException(sprintf('RedisAdapter namespace contains "%s" but only characters in [-+_.A-Za-z0-9] are allowed.', $match[0]));
         }
-        if (!$redisClient instanceof \Redis && !$redisClient instanceof \RedisArray && !$redisClient instanceof \RedisCluster && !$redisClient instanceof \Predis\Client) {
+        if ($redisClient instanceof \RedisCluster) {
+            $this->enableversioning();
+        } elseif (!$redisClient instanceof \Redis && !$redisClient instanceof \RedisArray && !$redisClient instanceof \Predis\Client) {
             throw new InvalidArgumentException(sprintf('%s() expects parameter 1 to be Redis, RedisArray, RedisCluster or Predis\Client, %s given', __METHOD__, is_object($redisClient) ? get_class($redisClient) : gettype($redisClient)));
         }
         $this->redis = $redisClient;
@@ -171,8 +173,8 @@ trait RedisTrait
      */
     protected function doClear($namespace)
     {
-        // When using a native Redis cluster, clearing the cache cannot work and always returns false.
-        // Clearing the cache should then be done by any other means (e.g. by restarting the cluster).
+        // When using a native Redis cluster, clearing the cache is done by versioning in AbstractTrait::clear().
+        // This means old keys are not really removed until they expire and may need gargage collection.
 
         $cleared = true;
         $hosts = array($this->redis);