feature #27009 [Cache] Add stampede protection via probabilistic early expiration (nicolas-grekas)

This PR was merged into the 4.2-dev branch.

Discussion
----------

[Cache] Add stampede protection via probabilistic early expiration

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        |

This PR implements [probabilistic early expiration](https://en.wikipedia.org/wiki/Cache_stampede#Probabilistic_early_expiration) on top of `$cache->get($key, $callback);`

It adds a 3rd arg to `CacheInterface::get`:
> float $beta A float that controls the likelyness of triggering early expiration. 0 disables it, INF forces immediate expiration. The default is implementation dependend but should typically be 1.0, which should provide optimal stampede protection.

Commits
-------

13523ad985 [Cache] Add stampede protection via probabilistic early expiration

UPGRADE-4.2.md

@@ -1,6 +1,11 @@
 UPGRADE FROM 4.1 to 4.2
 =======================
 
+Cache
+-----
+
+ * Deprecated `CacheItem::getPreviousTags()`, use `CacheItem::getMetadata()` instead.
+
 Security
 --------
 

UPGRADE-5.0.md

@@ -1,6 +1,11 @@
 UPGRADE FROM 4.x to 5.0
 =======================
 
+Cache
+-----
+
+ * Removed `CacheItem::getPreviousTags()`, use `CacheItem::getMetadata()` instead.
+
 Config
 ------
 

src/Symfony/Component/Cache/Adapter/AbstractAdapter.php

@@ -46,9 +46,18 @@ abstract class AbstractAdapter implements AdapterInterface, CacheInterface, Logg
             function ($key, $value, $isHit) use ($defaultLifetime) {
                 $item = new CacheItem();
                 $item->key = $key;
-                $item->value = $value;
+                $item->value = $v = $value;
                 $item->isHit = $isHit;
                 $item->defaultLifetime = $defaultLifetime;
+                // Detect wrapped values that encode for their expiry and creation duration
+                // For compactness, these values are packed in the key of an array using
+                // magic numbers in the form 9D-..-..-..-..-00-..-..-..-5F
+                if (\is_array($v) && 1 === \count($v) && 10 === \strlen($k = \key($v)) && "\x9D" === $k[0] && "\0" === $k[5] && "\x5F" === $k[9]) {
+                    $item->value = $v[$k];
+                    $v = \unpack('Ve/Nc', \substr($k, 1, -1));
+                    $item->metadata[CacheItem::METADATA_EXPIRY] = $v['e'] + CacheItem::METADATA_EXPIRY_OFFSET;
+                    $item->metadata[CacheItem::METADATA_CTIME] = $v['c'];
+                }
 
                 return $item;
             },
@@ -64,12 +73,18 @@ abstract class AbstractAdapter implements AdapterInterface, CacheInterface, Logg
 
                 foreach ($deferred as $key => $item) {
                     if (null === $item->expiry) {
-                        $byLifetime[0 < $item->defaultLifetime ? $item->defaultLifetime : 0][$getId($key)] = $item->value;
+                        $ttl = 0 < $item->defaultLifetime ? $item->defaultLifetime : 0;
                     } elseif ($item->expiry > $now) {
-                        $byLifetime[$item->expiry - $now][$getId($key)] = $item->value;
+                        $ttl = $item->expiry - $now;
                     } else {
                         $expiredIds[] = $getId($key);
+                        continue;
                     }
+                    if (isset(($metadata = $item->newMetadata)[CacheItem::METADATA_TAGS])) {
+                        unset($metadata[CacheItem::METADATA_TAGS]);
+                    }
+                    // For compactness, expiry and creation duration are packed in the key of a array, using magic numbers as separators
+                    $byLifetime[$ttl][$getId($key)] = $metadata ? array("\x9D".pack('VN', (int) $metadata[CacheItem::METADATA_EXPIRY] - CacheItem::METADATA_EXPIRY_OFFSET, $metadata[CacheItem::METADATA_CTIME])."\x5F" => $item->value) : $item->value;
                 }
 
                 return $byLifetime;

src/Symfony/Component/Cache/Adapter/ChainAdapter.php

@@ -64,8 +64,10 @@ class ChainAdapter implements AdapterInterface, CacheInterface, PruneableInterfa
                 $item->value = $sourceItem->value;
                 $item->expiry = $sourceItem->expiry;
                 $item->isHit = $sourceItem->isHit;
+                $item->metadata = $sourceItem->metadata;
 
                 $sourceItem->isTaggable = false;
+                unset($sourceItem->metadata[CacheItem::METADATA_TAGS]);
 
                 if (0 < $sourceItem->defaultLifetime && $sourceItem->defaultLifetime < $defaultLifetime) {
                     $defaultLifetime = $sourceItem->defaultLifetime;
@@ -84,19 +86,20 @@ class ChainAdapter implements AdapterInterface, CacheInterface, PruneableInterfa
     /**
      * {@inheritdoc}
      */
-    public function get(string $key, callable $callback)
+    public function get(string $key, callable $callback, float $beta = null)
     {
         $lastItem = null;
         $i = 0;
-        $wrap = function (CacheItem $item = null) use ($key, $callback, &$wrap, &$i, &$lastItem) {
+        $wrap = function (CacheItem $item = null) use ($key, $callback, $beta, &$wrap, &$i, &$lastItem) {
             $adapter = $this->adapters[$i];
             if (isset($this->adapters[++$i])) {
                 $callback = $wrap;
+                $beta = INF === $beta ? INF : 0;
             }
             if ($adapter instanceof CacheInterface) {
-                $value = $adapter->get($key, $callback);
+                $value = $adapter->get($key, $callback, $beta);
             } else {
-                $value = $this->doGet($adapter, $key, $callback);
+                $value = $this->doGet($adapter, $key, $callback, $beta ?? 1.0);
             }
             if (null !== $item) {
                 ($this->syncItem)($lastItem = $lastItem ?? $item, $item);

src/Symfony/Component/Cache/Adapter/PhpArrayAdapter.php

@@ -83,17 +83,17 @@ class PhpArrayAdapter implements AdapterInterface, CacheInterface, PruneableInte
     /**
      * {@inheritdoc}
      */
-    public function get(string $key, callable $callback)
+    public function get(string $key, callable $callback, float $beta = null)
     {
         if (null === $this->values) {
             $this->initialize();
         }
         if (null === $value = $this->values[$key] ?? null) {
             if ($this->pool instanceof CacheInterface) {
-                return $this->pool->get($key, $callback);
+                return $this->pool->get($key, $callback, $beta);
             }
 
-            return $this->doGet($this->pool, $key, $callback);
+            return $this->doGet($this->pool, $key, $callback, $beta ?? 1.0);
         }
         if ('N;' === $value) {
             return null;

src/Symfony/Component/Cache/Adapter/ProxyAdapter.php

@@ -31,6 +31,7 @@ class ProxyAdapter implements AdapterInterface, CacheInterface, PruneableInterfa
     private $namespace;
     private $namespaceLen;
     private $createCacheItem;
+    private $setInnerItem;
     private $poolHash;
 
     public function __construct(CacheItemPoolInterface $pool, string $namespace = '', int $defaultLifetime = 0)
@@ -43,11 +44,22 @@ class ProxyAdapter implements AdapterInterface, CacheInterface, PruneableInterfa
             function ($key, $innerItem) use ($defaultLifetime, $poolHash) {
                 $item = new CacheItem();
                 $item->key = $key;
-                $item->value = $innerItem->get();
+                $item->value = $v = $innerItem->get();
                 $item->isHit = $innerItem->isHit();
                 $item->defaultLifetime = $defaultLifetime;
                 $item->innerItem = $innerItem;
                 $item->poolHash = $poolHash;
+                // Detect wrapped values that encode for their expiry and creation duration
+                // For compactness, these values are packed in the key of an array using
+                // magic numbers in the form 9D-..-..-..-..-00-..-..-..-5F
+                if (\is_array($v) && 1 === \count($v) && 10 === \strlen($k = \key($v)) && "\x9D" === $k[0] && "\0" === $k[5] && "\x5F" === $k[9]) {
+                    $item->value = $v[$k];
+                    $v = \unpack('Ve/Nc', \substr($k, 1, -1));
+                    $item->metadata[CacheItem::METADATA_EXPIRY] = $v['e'] + CacheItem::METADATA_EXPIRY_OFFSET;
+                    $item->metadata[CacheItem::METADATA_CTIME] = $v['c'];
+                } elseif ($innerItem instanceof CacheItem) {
+                    $item->metadata = $innerItem->metadata;
+                }
                 $innerItem->set(null);
 
                 return $item;
@@ -55,20 +67,43 @@ class ProxyAdapter implements AdapterInterface, CacheInterface, PruneableInterfa
             null,
             CacheItem::class
         );
+        $this->setInnerItem = \Closure::bind(
+            /**
+             * @param array $item A CacheItem cast to (array); accessing protected properties requires adding the \0*\0" PHP prefix
+             */
+            function (CacheItemInterface $innerItem, array $item) {
+                // Tags are stored separately, no need to account for them when considering this item's newly set metadata
+                if (isset(($metadata = $item["\0*\0newMetadata"])[CacheItem::METADATA_TAGS])) {
+                    unset($metadata[CacheItem::METADATA_TAGS]);
+                }
+                if ($metadata) {
+                    // For compactness, expiry and creation duration are packed in the key of a array, using magic numbers as separators
+                    $item["\0*\0value"] = array("\x9D".pack('VN', (int) $metadata[CacheItem::METADATA_EXPIRY] - CacheItem::METADATA_EXPIRY_OFFSET, $metadata[CacheItem::METADATA_CTIME])."\x5F" => $item["\0*\0value"]);
+                }
+                $innerItem->set($item["\0*\0value"]);
+                $innerItem->expiresAt(null !== $item["\0*\0expiry"] ? \DateTime::createFromFormat('U', $item["\0*\0expiry"]) : null);
+            },
+            null,
+            CacheItem::class
+        );
     }
 
     /**
      * {@inheritdoc}
      */
-    public function get(string $key, callable $callback)
+    public function get(string $key, callable $callback, float $beta = null)
     {
         if (!$this->pool instanceof CacheInterface) {
-            return $this->doGet($this->pool, $key, $callback);
+            return $this->doGet($this, $key, $callback, $beta ?? 1.0);
         }
 
         return $this->pool->get($this->getId($key), function ($innerItem) use ($key, $callback) {
-            return $callback(($this->createCacheItem)($key, $innerItem));
+            $item = ($this->createCacheItem)($key, $innerItem);
-        });
+            $item->set($value = $callback($item));
+            ($this->setInnerItem)($innerItem, (array) $item);
+
+            return $value;
+        }, $beta);
     }
 
     /**
@@ -164,13 +199,11 @@ class ProxyAdapter implements AdapterInterface, CacheInterface, PruneableInterfa
             return false;
         }
         $item = (array) $item;
-        $expiry = $item["\0*\0expiry"];
+        if (null === $item["\0*\0expiry"] && 0 < $item["\0*\0defaultLifetime"]) {
-        if (null === $expiry && 0 < $item["\0*\0defaultLifetime"]) {
+            $item["\0*\0expiry"] = time() + $item["\0*\0defaultLifetime"];
-            $expiry = time() + $item["\0*\0defaultLifetime"];
         }
         $innerItem = $item["\0*\0poolHash"] === $this->poolHash ? $item["\0*\0innerItem"] : $this->pool->getItem($this->namespace.$item["\0*\0key"]);
-        $innerItem->set($item["\0*\0value"]);
+        ($this->setInnerItem)($innerItem, $item);
-        $innerItem->expiresAt(null !== $expiry ? \DateTime::createFromFormat('U', $expiry) : null);
 
         return $this->pool->$method($innerItem);
     }

src/Symfony/Component/Cache/Adapter/TagAwareAdapter.php

@@ -67,7 +67,7 @@ class TagAwareAdapter implements CacheInterface, TagAwareAdapterInterface, Prune
                 }
                 if (isset($itemTags[$key])) {
                     foreach ($itemTags[$key] as $tag => $version) {
-                        $item->prevTags[$tag] = $tag;
+                        $item->metadata[CacheItem::METADATA_TAGS][$tag] = $tag;
                     }
                     unset($itemTags[$key]);
                 } else {
@@ -84,7 +84,7 @@ class TagAwareAdapter implements CacheInterface, TagAwareAdapterInterface, Prune
             function ($deferred) {
                 $tagsByKey = array();
                 foreach ($deferred as $key => $item) {
-                    $tagsByKey[$key] = $item->tags;
+                    $tagsByKey[$key] = $item->newMetadata[CacheItem::METADATA_TAGS] ?? array();
                 }
 
                 return $tagsByKey;

src/Symfony/Component/Cache/Adapter/TraceableAdapter.php

@@ -37,7 +37,7 @@ class TraceableAdapter implements AdapterInterface, CacheInterface, PruneableInt
     /**
      * {@inheritdoc}
      */
-    public function get(string $key, callable $callback)
+    public function get(string $key, callable $callback, float $beta = null)
     {
         if (!$this->pool instanceof CacheInterface) {
             throw new \BadMethodCallException(sprintf('Cannot call "%s::get()": this class doesn\'t implement "%s".', get_class($this->pool), CacheInterface::class));
@@ -52,7 +52,7 @@ class TraceableAdapter implements AdapterInterface, CacheInterface, PruneableInt
 
         $event = $this->start(__FUNCTION__);
         try {
-            $value = $this->pool->get($key, $callback);
+            $value = $this->pool->get($key, $callback, $beta);
             $event->result[$key] = \is_object($value) ? \get_class($value) : gettype($value);
         } finally {
             $event->end = microtime(true);

src/Symfony/Component/Cache/CHANGELOG.md

@@ -4,8 +4,9 @@ CHANGELOG
 4.2.0
 -----
 
- * added `CacheInterface`, which should become the preferred way to use a cache
+ * added `CacheInterface`, which provides stampede protection via probabilistic early expiration and should become the preferred way to use a cache
  * throw `LogicException` when `CacheItem::tag()` is called on an item coming from a non tag-aware pool
+ * deprecated `CacheItem::getPreviousTags()`, use `CacheItem::getMetadata()` instead
 
 3.4.0
 -----
@@ -19,7 +20,7 @@ CHANGELOG
 3.3.0
 -----
 
- * [EXPERIMENTAL] added CacheItem::getPreviousTags() to get bound tags coming from the pool storage if any
+ * added CacheItem::getPreviousTags() to get bound tags coming from the pool storage if any
  * added PSR-16 "Simple Cache" implementations for all existing PSR-6 adapters
  * added Psr6Cache and SimpleCacheAdapter for bidirectional interoperability between PSR-6 and PSR-16
  * added MemcachedAdapter (PSR-6) and MemcachedCache (PSR-16)

src/Symfony/Component/Cache/CacheInterface.php

@@ -26,8 +26,12 @@ interface CacheInterface
 {
     /**
      * @param callable(CacheItem):mixed $callback Should return the computed value for the given key/item
+     * @param float|null                $beta     A float that controls the likeliness of triggering early expiration.
+     *                                            0 disables it, INF forces immediate expiration.
+     *                                            The default (or providing null) is implementation dependent but should
+     *                                            typically be 1.0, which should provide optimal stampede protection.
      *
      * @return mixed The value corresponding to the provided key
      */
-    public function get(string $key, callable $callback);
+    public function get(string $key, callable $callback, float $beta = null);
 }

src/Symfony/Component/Cache/CacheItem.php

@@ -21,13 +21,30 @@ use Symfony\Component\Cache\Exception\LogicException;
  */
 final class CacheItem implements CacheItemInterface
 {
+    /**
+     * References the Unix timestamp stating when the item will expire.
+     */
+    const METADATA_EXPIRY = 'expiry';
+
+    /**
+     * References the time the item took to be created, in milliseconds.
+     */
+    const METADATA_CTIME = 'ctime';
+
+    /**
+     * References the list of tags that were assigned to the item, as string[].
+     */
+    const METADATA_TAGS = 'tags';
+
+    private const METADATA_EXPIRY_OFFSET = 1527506807;
+
     protected $key;
     protected $value;
     protected $isHit = false;
     protected $expiry;
     protected $defaultLifetime;
-    protected $tags = array();
+    protected $metadata = array();
-    protected $prevTags = array();
+    protected $newMetadata = array();
     protected $innerItem;
     protected $poolHash;
     protected $isTaggable = false;
@@ -121,7 +138,7 @@ final class CacheItem implements CacheItemInterface
             if (!\is_string($tag)) {
                 throw new InvalidArgumentException(sprintf('Cache tag must be string, "%s" given', is_object($tag) ? get_class($tag) : gettype($tag)));
             }
-            if (isset($this->tags[$tag])) {
+            if (isset($this->newMetadata[self::METADATA_TAGS][$tag])) {
                 continue;
             }
             if ('' === $tag) {
@@ -130,7 +147,7 @@ final class CacheItem implements CacheItemInterface
             if (false !== strpbrk($tag, '{}()/\@:')) {
                 throw new InvalidArgumentException(sprintf('Cache tag "%s" contains reserved characters {}()/\@:', $tag));
             }
-            $this->tags[$tag] = $tag;
+            $this->newMetadata[self::METADATA_TAGS][$tag] = $tag;
         }
 
         return $this;
@@ -140,10 +157,24 @@ final class CacheItem implements CacheItemInterface
      * Returns the list of tags bound to the value coming from the pool storage if any.
      *
      * @return array
+     *
+     * @deprecated since Symfony 4.2, use the "getMetadata()" method instead.
      */
     public function getPreviousTags()
     {
-        return $this->prevTags;
+        @trigger_error(sprintf('The "%s" method is deprecated since Symfony 4.2, use the "getMetadata()" method instead.', __METHOD__), E_USER_DEPRECATED);
+
+        return $this->metadata[self::METADATA_TAGS] ?? array();
+    }
+
+    /**
+     * Returns a list of metadata info that were saved alongside with the cached value.
+     *
+     * See public CacheItem::METADATA_* consts for keys potentially found in the returned array.
+     */
+    public function getMetadata(): array
+    {
+        return $this->metadata;
     }
 
     /**

src/Symfony/Component/Cache/Tests/Adapter/AdapterTestCase.php

@@ -45,6 +45,42 @@ abstract class AdapterTestCase extends CachePoolTest
 
         $item = $cache->getItem('foo');
         $this->assertSame($value, $item->get());
+
+        $isHit = true;
+        $this->assertSame($value, $cache->get('foo', function (CacheItem $item) use (&$isHit) { $isHit = false; }, 0));
+        $this->assertTrue($isHit);
+
+        $this->assertNull($cache->get('foo', function (CacheItem $item) use (&$isHit, $value) {
+            $isHit = false;
+            $this->assertTrue($item->isHit());
+            $this->assertSame($value, $item->get());
+        }, INF));
+        $this->assertFalse($isHit);
+    }
+
+    public function testGetMetadata()
+    {
+        if (isset($this->skippedTests[__FUNCTION__])) {
+            $this->markTestSkipped($this->skippedTests[__FUNCTION__]);
+        }
+
+        $cache = $this->createCachePool(0, __FUNCTION__);
+
+        $cache->deleteItem('foo');
+        $cache->get('foo', function ($item) {
+            $item->expiresAfter(10);
+            sleep(1);
+
+            return 'bar';
+        });
+
+        $item = $cache->getItem('foo');
+
+        $expected = array(
+            CacheItem::METADATA_EXPIRY => 9 + time(),
+            CacheItem::METADATA_CTIME => 1000,
+        );
+        $this->assertSame($expected, $item->getMetadata());
     }
 
     public function testDefaultLifeTime()

src/Symfony/Component/Cache/Tests/Adapter/ArrayAdapterTest.php

@@ -19,6 +19,7 @@ use Symfony\Component\Cache\Adapter\ArrayAdapter;
 class ArrayAdapterTest extends AdapterTestCase
 {
     protected $skippedTests = array(
+        'testGetMetadata' => 'ArrayAdapter does not keep metadata.',
         'testDeferredSaveWithoutCommit' => 'Assumes a shared cache which ArrayAdapter is not.',
         'testSaveWithoutExpire' => 'Assumes a shared cache which ArrayAdapter is not.',
     );

src/Symfony/Component/Cache/Tests/Adapter/ChainAdapterTest.php

@@ -24,8 +24,12 @@ use Symfony\Component\Cache\Tests\Fixtures\ExternalAdapter;
  */
 class ChainAdapterTest extends AdapterTestCase
 {
-    public function createCachePool($defaultLifetime = 0)
+    public function createCachePool($defaultLifetime = 0, $testMethod = null)
     {
+        if ('testGetMetadata' === $testMethod) {
+            return new ChainAdapter(array(new FilesystemAdapter('', $defaultLifetime)), $defaultLifetime);
+        }
+
         return new ChainAdapter(array(new ArrayAdapter($defaultLifetime), new ExternalAdapter(), new FilesystemAdapter('', $defaultLifetime)), $defaultLifetime);
     }
 

src/Symfony/Component/Cache/Tests/Adapter/NamespacedProxyAdapterTest.php

@@ -12,6 +12,7 @@
 namespace Symfony\Component\Cache\Tests\Adapter;
 
 use Symfony\Component\Cache\Adapter\ArrayAdapter;
+use Symfony\Component\Cache\Adapter\FilesystemAdapter;
 use Symfony\Component\Cache\Adapter\ProxyAdapter;
 
 /**
@@ -19,8 +20,12 @@ use Symfony\Component\Cache\Adapter\ProxyAdapter;
  */
 class NamespacedProxyAdapterTest extends ProxyAdapterTest
 {
-    public function createCachePool($defaultLifetime = 0)
+    public function createCachePool($defaultLifetime = 0, $testMethod = null)
     {
+        if ('testGetMetadata' === $testMethod) {
+            return new ProxyAdapter(new FilesystemAdapter(), 'foo', $defaultLifetime);
+        }
+
         return new ProxyAdapter(new ArrayAdapter($defaultLifetime), 'foo', $defaultLifetime);
     }
 }

src/Symfony/Component/Cache/Tests/Adapter/PhpArrayAdapterTest.php

@@ -12,6 +12,7 @@
 namespace Symfony\Component\Cache\Tests\Adapter;
 
 use Psr\Cache\CacheItemInterface;
+use Symfony\Component\Cache\Adapter\FilesystemAdapter;
 use Symfony\Component\Cache\Adapter\NullAdapter;
 use Symfony\Component\Cache\Adapter\PhpArrayAdapter;
 
@@ -68,8 +69,12 @@ class PhpArrayAdapterTest extends AdapterTestCase
         }
     }
 
-    public function createCachePool()
+    public function createCachePool($defaultLifetime = 0, $testMethod = null)
     {
+        if ('testGetMetadata' === $testMethod) {
+            return new PhpArrayAdapter(self::$file, new FilesystemAdapter());
+        }
+
         return new PhpArrayAdapterWrapper(self::$file, new NullAdapter());
     }
 

src/Symfony/Component/Cache/Tests/Adapter/ProxyAdapterTest.php

@@ -13,6 +13,7 @@ namespace Symfony\Component\Cache\Tests\Adapter;
 
 use Psr\Cache\CacheItemInterface;
 use Symfony\Component\Cache\Adapter\ArrayAdapter;
+use Symfony\Component\Cache\Adapter\FilesystemAdapter;
 use Symfony\Component\Cache\Adapter\ProxyAdapter;
 use Symfony\Component\Cache\CacheItem;
 
@@ -27,8 +28,12 @@ class ProxyAdapterTest extends AdapterTestCase
         'testPrune' => 'ProxyAdapter just proxies',
     );
 
-    public function createCachePool($defaultLifetime = 0)
+    public function createCachePool($defaultLifetime = 0, $testMethod = null)
     {
+        if ('testGetMetadata' === $testMethod) {
+            return new ProxyAdapter(new FilesystemAdapter(), '', $defaultLifetime);
+        }
+
         return new ProxyAdapter(new ArrayAdapter(), '', $defaultLifetime);
     }
 

src/Symfony/Component/Cache/Tests/Adapter/TagAwareAdapterTest.php

@@ -14,6 +14,7 @@ namespace Symfony\Component\Cache\Tests\Adapter;
 use Symfony\Component\Cache\Adapter\AdapterInterface;
 use Symfony\Component\Cache\Adapter\FilesystemAdapter;
 use Symfony\Component\Cache\Adapter\TagAwareAdapter;
+use Symfony\Component\Cache\CacheItem;
 
 /**
  * @group time-sensitive
@@ -138,6 +139,9 @@ class TagAwareAdapterTest extends AdapterTestCase
         $this->assertFalse($pool->getItem('foo')->isHit());
     }
 
+    /**
+     * @group legacy
+     */
     public function testGetPreviousTags()
     {
         $pool = $this->createCachePool();
@@ -149,6 +153,17 @@ class TagAwareAdapterTest extends AdapterTestCase
         $this->assertSame(array('foo' => 'foo'), $i->getPreviousTags());
     }
 
+    public function testGetMetadata()
+    {
+        $pool = $this->createCachePool();
+
+        $i = $pool->getItem('k');
+        $pool->save($i->tag('foo'));
+
+        $i = $pool->getItem('k');
+        $this->assertSame(array('foo' => 'foo'), $i->getMetadata()[CacheItem::METADATA_TAGS]);
+    }
+
     public function testPrune()
     {
         $cache = new TagAwareAdapter($this->getPruneableMock());

src/Symfony/Component/Cache/Tests/CacheItemTest.php

@@ -63,7 +63,7 @@ class CacheItemTest extends TestCase
         $this->assertSame($item, $item->tag(array('bar', 'baz')));
 
         call_user_func(\Closure::bind(function () use ($item) {
-            $this->assertSame(array('foo' => 'foo', 'bar' => 'bar', 'baz' => 'baz'), $item->tags);
+            $this->assertSame(array('foo' => 'foo', 'bar' => 'bar', 'baz' => 'baz'), $item->newMetadata[CacheItem::METADATA_TAGS]);
         }, $this, CacheItem::class));
     }
 

src/Symfony/Component/Cache/Traits/GetTrait.php

@@ -11,9 +11,15 @@
 
 namespace Symfony\Component\Cache\Traits;
 
+use Psr\Cache\CacheItemInterface;
 use Psr\Cache\CacheItemPoolInterface;
+use Symfony\Component\Cache\CacheItem;
 
 /**
+ * An implementation for CacheInterface that provides stampede protection via probabilistic early expiration.
+ *
+ * @see https://en.wikipedia.org/wiki/Cache_stampede
+ *
  * @author Nicolas Grekas <p@tchwork.com>
  *
  * @internal
@@ -23,21 +29,58 @@ trait GetTrait
     /**
      * {@inheritdoc}
      */
-    public function get(string $key, callable $callback)
+    public function get(string $key, callable $callback, float $beta = null)
     {
-        return $this->doGet($this, $key, $callback);
+        return $this->doGet($this, $key, $callback, $beta ?? 1.0);
     }
 
-    private function doGet(CacheItemPoolInterface $pool, string $key, callable $callback)
+    private function doGet(CacheItemPoolInterface $pool, string $key, callable $callback, float $beta)
     {
+        $t = 0;
         $item = $pool->getItem($key);
+        $recompute = !$item->isHit() || INF === $beta;
 
-        if ($item->isHit()) {
+        if ($item instanceof CacheItem && 0 < $beta) {
+            if ($recompute) {
+                $t = microtime(true);
+            } else {
+                $metadata = $item->getMetadata();
+                $expiry = $metadata[CacheItem::METADATA_EXPIRY] ?? false;
+                $ctime = $metadata[CacheItem::METADATA_CTIME] ?? false;
+
+                if ($ctime && $expiry) {
+                    $t = microtime(true);
+                    $recompute = $expiry <= $t - $ctime / 1000 * $beta * log(random_int(1, PHP_INT_MAX) / PHP_INT_MAX);
+                }
+            }
+            if ($recompute) {
+                // force applying defaultLifetime to expiry
+                $item->expiresAt(null);
+            }
+        }
+
+        if (!$recompute) {
             return $item->get();
         }
 
-        $pool->save($item->set($value = $callback($item)));
+        static $save = null;
 
-        return $value;
+        if (null === $save) {
+            $save = \Closure::bind(
+                function (CacheItemPoolInterface $pool, CacheItemInterface $item, $value, float $startTime) {
+                    if ($item instanceof CacheItem && $startTime && $item->expiry > $endTime = microtime(true)) {
+                        $item->newMetadata[CacheItem::METADATA_EXPIRY] = $item->expiry;
+                        $item->newMetadata[CacheItem::METADATA_CTIME] = 1000 * (int) ($endTime - $startTime);
+                    }
+                    $pool->save($item->set($value));
+
+                    return $value;
+                },
+                null,
+                CacheItem::class
+            );
+        }
+
+        return $save($pool, $item, $callback($item), $t);
     }
 }