Propagated Sampling Rates

src/State/Hub.php

@@ -269,6 +269,7 @@ public function startTransaction(TransactionContext $context, array $customSampl
         $samplingContext->setAdditionalContext($customSamplingContext);
 
         $sampleSource = 'context';
+        $sampleRand = $context->getMetadata()->getSampleRand();
 
         if ($transaction->getSampled() === null) {
             $tracesSampler = $options->getTracesSampler();
@@ -278,12 +279,17 @@ public function startTransaction(TransactionContext $context, array $customSampl
 
                 $sampleSource = 'config:traces_sampler';
             } else {
-                $sampleRate = $this->getSampleRate(
-                    $samplingContext->getParentSampled(),
-                    $options->getTracesSampleRate() ?? 0
-                );
-
-                $sampleSource = $samplingContext->getParentSampled() ? 'parent' : 'config:traces_sample_rate';
+                $parentSampleRate = $context->getMetadata()->getParentSamplingRate();
+                if ($parentSampleRate !== null) {
+                    $sampleRate = $parentSampleRate;
+                    $sampleSource = 'parent:sample_rate';
+                } else {
+                    $sampleRate = $this->getSampleRate(
+                        $samplingContext->getParentSampled(),
+                        $options->getTracesSampleRate() ?? 0
+                    );
+                    $sampleSource = $samplingContext->getParentSampled() ? 'parent:sampling_decision' : 'config:traces_sample_rate';
+                }
             }
 
             if (!$this->isValidSampleRate($sampleRate)) {
@@ -304,7 +310,7 @@ public function startTransaction(TransactionContext $context, array $customSampl
                 return $transaction;
             }
 
-            $transaction->setSampled($this->sample($sampleRate));
+            $transaction->setSampled($sampleRand < $sampleRate);
         }
 
         if (!$transaction->getSampled()) {
@@ -376,11 +382,11 @@ private function getStackTop(): Layer
     private function getSampleRate(?bool $hasParentBeenSampled, float $fallbackSampleRate): float
     {
         if ($hasParentBeenSampled === true) {
-            return 1;
+            return 1.0;
         }
 
         if ($hasParentBeenSampled === false) {
-            return 0;
+            return 0.0;
         }
 
         return $fallbackSampleRate;

src/Tracing/DynamicSamplingContext.php

@@ -186,6 +186,10 @@ public static function fromTransaction(Transaction $transaction, HubInterface $h
             $samplingContext->set('sampled', $transaction->getSampled() ? 'true' : 'false');
         }
 
+        if ($transaction->getMetadata()->getSampleRand() !== null) {
+            $samplingContext->set('sample_rand', (string) $transaction->getMetadata()->getSampleRand());
+        }
+
         $samplingContext->freeze();
 
         return $samplingContext;
@@ -195,6 +199,7 @@ public static function fromOptions(Options $options, Scope $scope): self
     {
         $samplingContext = new self();
         $samplingContext->set('trace_id', (string) $scope->getPropagationContext()->getTraceId());
+        $samplingContext->set('sample_rand', (string) $scope->getPropagationContext()->getSampleRand());
 
         if ($options->getTracesSampleRate() !== null) {
             $samplingContext->set('sample_rate', (string) $options->getTracesSampleRate());

src/Tracing/PropagationContext.php

@@ -28,6 +28,16 @@ final class PropagationContext
      */
     private $parentSpanId;
 
+    /**
+     * @var bool|null The parent's sampling decision
+     */
+    private $parentSampled;
+
+    /**
+     * @var float|null
+     */
+    private $sampleRand;
+
     /**
      * @var DynamicSamplingContext|null The dynamic sampling context
      */
@@ -44,6 +54,8 @@ public static function fromDefaults(): self
         $context->traceId = TraceId::generate();
         $context->spanId = SpanId::generate();
         $context->parentSpanId = null;
+        $context->parentSampled = null;
+        $context->sampleRand = round(mt_rand(0, mt_getrandmax() - 1) / mt_getrandmax(), 6);
         $context->dynamicSamplingContext = null;
 
         return $context;
@@ -159,6 +171,19 @@ public function setDynamicSamplingContext(DynamicSamplingContext $dynamicSamplin
         return $this;
     }
 
+    public function getSampleRand(): ?float
+    {
+        return $this->sampleRand;
+    }
+
+    public function setSampleRand(?float $sampleRand): self
+    {
+        $this->sampleRand = $sampleRand;
+
+        return $this;
+    }
+
+    // TODO add same logic as in TransactionContext
     private static function parseTraceparentAndBaggage(string $traceparent, string $baggage): self
     {
         $context = self::fromDefaults();
@@ -174,6 +199,11 @@ private static function parseTraceparentAndBaggage(string $traceparent, string $
                 $context->parentSpanId = new SpanId($matches['span_id']);
                 $hasSentryTrace = true;
             }
+
+            if (isset($matches['sampled'])) {
+                $context->parentSampled = $matches['sampled'] === '1';
+                $hasSentryTrace = true;
+            }
         } elseif (preg_match(self::W3C_TRACEPARENT_HEADER_REGEX, $traceparent, $matches)) {
             if (!empty($matches['trace_id'])) {
                 $context->traceId = new TraceId($matches['trace_id']);
@@ -184,6 +214,11 @@ private static function parseTraceparentAndBaggage(string $traceparent, string $
                 $context->parentSpanId = new SpanId($matches['span_id']);
                 $hasSentryTrace = true;
             }
+
+            if (isset($matches['sampled'])) {
+                $context->parentSampled = $matches['sampled'] === '01';
+                $hasSentryTrace = true;
+            }
         }
 
         $samplingContext = DynamicSamplingContext::fromHeader($baggage);
@@ -201,6 +236,24 @@ private static function parseTraceparentAndBaggage(string $traceparent, string $
             $context->dynamicSamplingContext = $samplingContext;
         }
 
+        // Store the propagated trace sample rand or generate a new one
+        if ($samplingContext->has('sample_rand')) {
+            $context->sampleRand = (float) $samplingContext->get('sample_rand');
+        } else {
+            if ($samplingContext->has('sample_rate') && $context->parentSampled !== null) {
+                if ($context->parentSampled === true) {
+                    // [0, rate)
+                    $context->sampleRand = round(mt_rand(0, mt_getrandmax() - 1) / mt_getrandmax() * (float) $samplingContext->get('sample_rate'), 6);
+                } else {
+                    // [rate, 1)
+                    $context->sampleRand = round(mt_rand(0, mt_getrandmax() - 1) / mt_getrandmax() * (1 - (float) $samplingContext->get('sample_rate')) + (float) $samplingContext->get('sample-rate'), 6);
+                }
+            } elseif ($context->parentSampled !== null) {
+                // [0, 1)
+                $context->sampleRand = round(mt_rand(0, mt_getrandmax() - 1) / mt_getrandmax(), 6);
+            }
+        }
+
         return $context;
     }
 }

src/Tracing/SamplingContext.php

@@ -16,6 +16,11 @@ final class SamplingContext
      */
     private $parentSampled;
 
+    /**
+     * @var float|null The parent sample rate
+     */
+    private $sampleRand;
+
     /**
      * @var array<string, mixed>|null Additional context, depending on where the SDK runs
      */
@@ -29,6 +34,7 @@ public static function getDefault(TransactionContext $transactionContext): self
         $context = new self();
         $context->transactionContext = $transactionContext;
         $context->parentSampled = $transactionContext->getParentSampled();
+        $context->sampleRand = $transactionContext->getMetadata()->getSampleRand();
 
         return $context;
     }
@@ -46,6 +52,11 @@ public function getParentSampled(): ?bool
         return $this->parentSampled;
     }
 
+    public function getSampleRand(): ?float
+    {
+        return $this->sampleRand;
+    }
+
     /**
      * Sets the sampling decision from the parent transaction, if any.
      */

src/Tracing/TransactionContext.php

@@ -198,6 +198,29 @@ private static function parseTraceAndBaggage(string $sentryTrace, string $baggag
             $context->getMetadata()->setDynamicSamplingContext($samplingContext);
         }
 
+        // Store the propagated traces sample rate
+        if ($samplingContext->has('sample_rate')) {
+            $context->getMetadata()->setParentSamplingRate((float) $samplingContext->get('sample_rate'));
+        }
+
+        // Store the propagated trace sample rand or generate a new one
+        if ($samplingContext->has('sample_rand')) {
+            $context->getMetadata()->setSampleRand((float) $samplingContext->get('sample_rand'));
+        } else {
+            if ($samplingContext->has('sample_rate') && $context->parentSampled !== null) {
+                if ($context->parentSampled === true) {
+                    // [0, rate)
+                    $context->getMetadata()->setSampleRand(round(mt_rand(0, mt_getrandmax() - 1) / mt_getrandmax() * (float) $samplingContext->get('sample_rate'), 6));
+                } else {
+                    // [rate, 1)
+                    $context->getMetadata()->setSampleRand(round(mt_rand(0, mt_getrandmax() - 1) / mt_getrandmax() * (1 - (float) $samplingContext->get('sample_rate')) + (float) $samplingContext->get('sample-rate'), 6));
+                }
+            } elseif ($context->parentSampled !== null) {
+                // [0, 1)
+                $context->getMetadata()->setSampleRand(round(mt_rand(0, mt_getrandmax() - 1) / mt_getrandmax(), 6));
+            }
+        }
+
         return $context;
     }
 }

src/Tracing/TransactionMetadata.php

@@ -21,21 +21,37 @@ final class TransactionMetadata
      */
     private $source;
 
+    /**
+     * @var float|int|null
+     */
+    private $parentSamplingRate;
+
+    /**
+     * @var float|int|null
+     */
+    private $sampleRand;
+
     /**
      * Constructor.
      *
      * @param float|int|null              $samplingRate           The sampling rate
      * @param DynamicSamplingContext|null $dynamicSamplingContext The Dynamic Sampling Context
      * @param TransactionSource|null      $source                 The transaction source
+     * @param float|null                  $parentSamplingRate     The parent sampling rate
+     * @param float|null                  $sampleRand             The trace sample rand
      */
     public function __construct(
         $samplingRate = null,
         ?DynamicSamplingContext $dynamicSamplingContext = null,
-        ?TransactionSource $source = null
+        ?TransactionSource $source = null,
+        ?float $parentSamplingRate = null,
+        ?float $sampleRand = null
     ) {
         $this->samplingRate = $samplingRate;
         $this->dynamicSamplingContext = $dynamicSamplingContext;
         $this->source = $source ?? TransactionSource::custom();
+        $this->parentSamplingRate = $parentSamplingRate;
+        $this->sampleRand = $sampleRand ?? round(mt_rand(0, mt_getrandmax() - 1) / mt_getrandmax(), 6);
     }
 
     /**
@@ -56,6 +72,30 @@ public function setSamplingRate($samplingRate): self
         return $this;
     }
 
+    public function getParentSamplingRate(): ?float
+    {
+        return $this->parentSamplingRate;
+    }
+
+    public function setParentSamplingRate(?float $parentSamplingRate): self
+    {
+        $this->parentSamplingRate = $parentSamplingRate;
+
+        return $this;
+    }
+
+    public function getSampleRand(): ?float
+    {
+        return $this->sampleRand;
+    }
+
+    public function setSampleRand(?float $sampleRand): self
+    {
+        $this->sampleRand = $sampleRand;
+
+        return $this;
+    }
+
     public function getDynamicSamplingContext(): ?DynamicSamplingContext
     {
         return $this->dynamicSamplingContext;

tests/FunctionsTest.php

@@ -503,6 +503,7 @@ public function testBaggageWithTracingDisabled(): void
     {
         $propagationContext = PropagationContext::fromDefaults();
         $propagationContext->setTraceId(new TraceId('566e3688a61d4bc888951642d6f14a19'));
+        $propagationContext->setSampleRand(0.25);
 
         $scope = new Scope($propagationContext);
 
@@ -520,7 +521,7 @@ public function testBaggageWithTracingDisabled(): void
 
         $baggage = getBaggage();
 
-        $this->assertSame('sentry-trace_id=566e3688a61d4bc888951642d6f14a19,sentry-release=1.0.0,sentry-environment=development', $baggage);
+        $this->assertSame('sentry-trace_id=566e3688a61d4bc888951642d6f14a19,sentry-sample_rand=0.25,sentry-release=1.0.0,sentry-environment=development', $baggage);
     }
 
     public function testBaggageWithTracingEnabled(): void
@@ -541,6 +542,7 @@ public function testBaggageWithTracingEnabled(): void
         $transactionContext = new TransactionContext();
         $transactionContext->setName('Test');
         $transactionContext->setTraceId(new TraceId('566e3688a61d4bc888951642d6f14a19'));
+        $transactionContext->getMetadata()->setSampleRand(0.25);
 
         $transaction = startTransaction($transactionContext);
 
@@ -552,7 +554,7 @@ public function testBaggageWithTracingEnabled(): void
 
         $baggage = getBaggage();
 
-        $this->assertSame('sentry-trace_id=566e3688a61d4bc888951642d6f14a19,sentry-sample_rate=1,sentry-transaction=Test,sentry-release=1.0.0,sentry-environment=development,sentry-sampled=true', $baggage);
+        $this->assertSame('sentry-trace_id=566e3688a61d4bc888951642d6f14a19,sentry-sample_rate=1,sentry-transaction=Test,sentry-release=1.0.0,sentry-environment=development,sentry-sampled=true,sentry-sample_rand=0.25', $baggage);
     }
 
     public function testContinueTrace(): void