Propagated Sampling Rates

src/State/Hub.php

@@ -269,6 +269,7 @@
         $samplingContext->setAdditionalContext($customSamplingContext);
 
         $sampleSource = 'context';
+        $sampleRand = $context->getMetadata()->getSampleRand();
 
         if ($transaction->getSampled() === null) {
             $tracesSampler = $options->getTracesSampler();
@@ -278,12 +279,17 @@
 
                 $sampleSource = 'config:traces_sampler';
             } else {
-                $sampleRate = $this->getSampleRate(
-                    $samplingContext->getParentSampled(),
-                    $options->getTracesSampleRate() ?? 0
-                );
-
-                $sampleSource = $samplingContext->getParentSampled() ? 'parent' : 'config:traces_sample_rate';
+                $parentSampleRate = (float) $context->getMetadata()->getDynamicSamplingContext()->get('sample_rate');
+                if ($parentSampleRate !== null) {
+                    $sampleRate = $parentSampleRate;
+                    $sampleSource = 'parent';
+                } else {
+                    $sampleRate = $this->getSampleRate(
+                        $samplingContext->getParentSampled(),
+                        $options->getTracesSampleRate() ?? 0
+                    );
+                    $sampleSource = $samplingContext->getParentSampled() ? 'parent' : 'config:traces_sample_rate';
+                }
             }
 
             if (!$this->isValidSampleRate($sampleRate)) {
@@ -294,6 +300,7 @@
                 return $transaction;
             }
 
+            // TODO Why are we doing this?
             $transaction->getMetadata()->setSamplingRate($sampleRate);
 
             if ($sampleRate === 0.0) {
@@ -304,7 +311,8 @@
                 return $transaction;
             }
 
-            $transaction->setSampled($this->sample($sampleRate));
+            $sampleRand = $context->getMetadata()->getSampleRand();
+            $transaction->setSampled($sampleRand < $sampleRate);
         }
 
         if (!$transaction->getSampled()) {
@@ -376,11 +384,11 @@
     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 @@
             $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 @@
     {
         $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

@@ -33,6 +33,11 @@
      */
     private $dynamicSamplingContext;
 
+    /**
+     * @var float|null
+     */
+    private $sampleRand;
+
     private function __construct()
     {
     }
@@ -46,6 +51,9 @@
         $context->parentSpanId = null;
         $context->dynamicSamplingContext = null;
 
+        // TODO check if this is precise enough
+        $context->sampleRand = round(mt_rand(0, mt_getrandmax() - 1) / mt_getrandmax(), 6);
+
         return $context;
     }
 
@@ -159,6 +167,7 @@
         return $this;
     }
 
+    // TODO add same logic as in TransactionContext
     private static function parseTraceparentAndBaggage(string $traceparent, string $baggage): self
     {
         $context = self::fromDefaults();

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 +198,24 @@
             $context->getMetadata()->setDynamicSamplingContext($samplingContext);
         }
 
+        if ($samplingContext->has('sample_rand')) {
+            // TODO check for 1e13 etc.
+            $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() * $samplingContext->get('sample-rate'), 6));
+                } else {
+                    // [rate, 1]
+                    $context->getMetadata()->setSampleRand(round(mt_rand(0, mt_getrandmax() - 1) / mt_getrandmax() * (1 - $samplingContext->get('sample-rate')) + $samplingContext->get('sample-rate'), 6));
+                }
+            } elseif ($context->parentSampled !== null) {
+                // [0, 1)
+                $context->getMetadata()->setSampleRand(round(mt_rand(0, mt_getrandmax() - 1) / mt_getrandmax(), 6));
+            }
+        }
+
         return $context;
     }
 }

src/Tracing/TransactionMetadata.php

@@ -21,6 +21,11 @@ final class TransactionMetadata
      */
     private $source;
 
+    /**
+     * @var float|null
+     */
+    private $sampleRand;
+
     /**
      * Constructor.
      *
@@ -36,6 +41,9 @@ public function __construct(
         $this->samplingRate = $samplingRate;
         $this->dynamicSamplingContext = $dynamicSamplingContext;
         $this->source = $source ?? TransactionSource::custom();
+
+        // TODO check if this is precise enough
+        $this->sampleRand = round(mt_rand(0, mt_getrandmax() - 1) / mt_getrandmax(), 6);
     }
 
     /**
@@ -56,6 +64,18 @@ public function setSamplingRate($samplingRate): self
         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;