[HttpClient] Add jitter to RetryBackoff

src/Symfony/Bundle/FrameworkBundle/DependencyInjection/Configuration.php

@@ -1641,8 +1641,8 @@ private function addHttpClientRetrySection()
                 ->addDefaultsIfNotSet()
                 ->beforeNormalization()
                     ->always(function ($v) {
-                        if (isset($v['backoff_service']) && (isset($v['delay']) || isset($v['multiplier']) || isset($v['max_delay']))) {
-                            throw new \InvalidArgumentException('The "backoff_service" option cannot be used along with the "delay", "multiplier" or "max_delay" options.');
+                        if (isset($v['backoff_service']) && (isset($v['delay']) || isset($v['multiplier']) || isset($v['max_delay']) || isset($v['jitter']))) {
+                            throw new \InvalidArgumentException('The "backoff_service" option cannot be used along with the "delay", "multiplier", "max_delay" or "jitter" options.');
                         }
                         if (isset($v['decider_service']) && (isset($v['http_codes']))) {
                             throw new \InvalidArgumentException('The "decider_service" option cannot be used along with the "http_codes" options.');
@@ -1670,6 +1670,7 @@ private function addHttpClientRetrySection()
                     ->integerNode('delay')->defaultValue(1000)->min(0)->info('Time in ms to delay (or the initial value when multiplier is used)')->end()
                     ->floatNode('multiplier')->defaultValue(2)->min(1)->info('If greater than 1, delay will grow exponentially for each retry: (delay * (multiple ^ retries))')->end()
                     ->integerNode('max_delay')->defaultValue(0)->min(0)->info('Max time in ms that a retry should ever be delayed (0 = infinite)')->end()
+                    ->floatNode('jitter')->defaultValue(0.1)->min(0)->max(1)->info('Randomness in percent (between 0 and 1)) to apply to the delay')->end()
                 ->end()
             ;
     }

src/Symfony/Bundle/FrameworkBundle/DependencyInjection/FrameworkExtension.php

@@ -2075,7 +2075,8 @@ private function registerHttpClientRetry(array $retryOptions, string $name, Cont
             $retryDefinition
                 ->replaceArgument(0, $retryOptions['delay'])
                 ->replaceArgument(1, $retryOptions['multiplier'])
-                ->replaceArgument(2, $retryOptions['max_delay']);
+                ->replaceArgument(2, $retryOptions['max_delay'])
+                ->replaceArgument(3, $retryOptions['jitter']);
             $container->setDefinition($retryServiceId, $retryDefinition);
 
             $backoffReference = new Reference($retryServiceId);

src/Symfony/Component/HttpClient/Retry/ExponentialBackOff.php

@@ -17,7 +17,7 @@
 /**
  * A retry backOff with a constant or exponential retry delay.
  *
- * For example, if $delayMilliseconds=10000 & $multiplier=1 (default),
+ * For example, if $delayMilliseconds=10000 & $multiplier=1,
  * each retry will wait exactly 10 seconds.
  *
  * But if $delayMilliseconds=10000 & $multiplier=2:
@@ -33,13 +33,15 @@ final class ExponentialBackOff implements RetryBackOffInterface
     private $delayMilliseconds;
     private $multiplier;
     private $maxDelayMilliseconds;
+    private $jitter;
 
     /**
      * @param int   $delayMilliseconds    Amount of time to delay (or the initial value when multiplier is used)
      * @param float $multiplier           Multiplier to apply to the delay each time a retry occurs
      * @param int   $maxDelayMilliseconds Maximum delay to allow (0 means no maximum)
+     * @param float $jitter               Probability of randomness int delay (0 = none, 1 = 100% random)
      */
-    public function __construct(int $delayMilliseconds = 1000, float $multiplier = 2.0, int $maxDelayMilliseconds = 0)
+    public function __construct(int $delayMilliseconds = 1000, float $multiplier = 2.0, int $maxDelayMilliseconds = 0, float $jitter = 0.1)
     {
         if ($delayMilliseconds < 0) {
             throw new InvalidArgumentException(sprintf('Delay must be greater than or equal to zero: "%s" given.', $delayMilliseconds));
@@ -55,11 +57,20 @@ public function __construct(int $delayMilliseconds = 1000, float $multiplier = 2
             throw new InvalidArgumentException(sprintf('Max delay must be greater than or equal to zero: "%s" given.', $maxDelayMilliseconds));
         }
         $this->maxDelayMilliseconds = $maxDelayMilliseconds;
+
+        if ($jitter < 0 || $jitter > 1) {
+            throw new InvalidArgumentException(sprintf('Jitter must be between 0 and 1: "%s" given.', $jitter));
+        }
+        $this->jitter = $jitter;
     }
 
     public function getDelay(int $retryCount, string $requestMethod, string $requestUrl, array $requestOptions, int $responseStatusCode, array $responseHeaders, ?string $responseContent, ?TransportExceptionInterface $exception): int
     {
         $delay = $this->delayMilliseconds * $this->multiplier ** $retryCount;
+        if ($this->jitter > 0) {
+            $randomness = $delay * $this->jitter;
+            $delay = $delay + random_int(-$randomness, +$randomness);
+        }
 
         if ($delay > $this->maxDelayMilliseconds && 0 !== $this->maxDelayMilliseconds) {
             return $this->maxDelayMilliseconds;

src/Symfony/Component/HttpClient/Tests/Retry/ExponentialBackOffTest.php

@@ -21,7 +21,7 @@ class ExponentialBackOffTest extends TestCase
      */
     public function testGetDelay(int $delay, int $multiplier, int $maxDelay, int $previousRetries, int $expectedDelay)
     {
-        $backOff = new ExponentialBackOff($delay, $multiplier, $maxDelay);
+        $backOff = new ExponentialBackOff($delay, $multiplier, $maxDelay, 0);
 
         self::assertSame($expectedDelay, $backOff->getDelay($previousRetries, 'GET', 'http://example.com/', [], 200, [], null, null));
     }
@@ -50,4 +50,22 @@ public function provideDelay(): iterable
         yield [0, 2, 10000, 0, 0];
         yield [0, 2, 10000, 1, 0];
     }
+
+    public function testJitter()
+    {
+        $backOff = new ExponentialBackOff(1000, 1, 0, 1);
+        $belowHalf = 0;
+        $aboveHalf = 0;
+        for ($i = 0; $i < 20; ++$i) {
+            $delay = $backOff->getDelay(0, 'GET', 'http://example.com/', [], 200, [], null, null);
+            if ($delay < 500) {
+                ++$belowHalf;
+            } elseif ($delay > 1500) {
+                ++$aboveHalf;
+            }
+        }
+
+        $this->assertGreaterThanOrEqual(1, $belowHalf);
+        $this->assertGreaterThanOrEqual(1, $aboveHalf);
+    }
 }