From 80262d53591e6b547d80fadc50f669e89792d653 Mon Sep 17 00:00:00 2001
From: Michael Hoffmann <michael.hoffmann@sentry.io>
Date: Fri, 22 Nov 2024 09:42:53 +0100
Subject: [PATCH] Propagated Sampling Rates

---
 src/State/Hub.php                      | 13 ++++++++++---
 src/Tracing/DynamicSamplingContext.php |  4 ++++
 src/Tracing/SamplingContext.php        | 11 +++++++++++
 src/Tracing/TransactionContext.php     |  8 ++++++++
 src/Tracing/TransactionMetadata.php    | 19 +++++++++++++++++++
 5 files changed, 52 insertions(+), 3 deletions(-)

diff --git a/src/State/Hub.php b/src/State/Hub.php
index 4fa9a4382..f34ff82f5 100644
--- a/src/State/Hub.php
+++ b/src/State/Hub.php
@@ -304,7 +304,14 @@ public function startTransaction(TransactionContext $context, array $customSampl
                 return $transaction;
             }
 
-            $transaction->setSampled($this->sample($sampleRate));
+            // The sample rand should only be used if the traces sampler is set.
+            // Otherwise, we use the configured traces sample rate.
+            $sampleRand = $context->getMetadata()->getSampleRand();
+            if ($tracesSampler !== null && $sampleRand !== null) {
+                $transaction->setSampled($sampleRand < $sampleRate);
+            } else {
+                $transaction->setSampled($this->sample($sampleRate));
+            }
         }
 
         if (!$transaction->getSampled()) {
@@ -376,11 +383,11 @@ 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;
diff --git a/src/Tracing/DynamicSamplingContext.php b/src/Tracing/DynamicSamplingContext.php
index cf159ed59..8146894cc 100644
--- a/src/Tracing/DynamicSamplingContext.php
+++ b/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;
diff --git a/src/Tracing/SamplingContext.php b/src/Tracing/SamplingContext.php
index bee540bfb..24a393cbc 100644
--- a/src/Tracing/SamplingContext.php
+++ b/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.
      */
diff --git a/src/Tracing/TransactionContext.php b/src/Tracing/TransactionContext.php
index e804a1bd9..04f42f0fb 100644
--- a/src/Tracing/TransactionContext.php
+++ b/src/Tracing/TransactionContext.php
@@ -196,6 +196,14 @@ private static function parseTraceAndBaggage(string $sentryTrace, string $baggag
             // The baggage header contains Dynamic Sampling Context data from an upstream SDK.
             // Propagate this Dynamic Sampling Context.
             $context->getMetadata()->setDynamicSamplingContext($samplingContext);
+
+            // Store the parent sample rate (sample_rand) if it exists,
+            // otherwise set it to 1.0 if the parent was sampled.
+            if ($samplingContext->has('sample_rand')) {
+                $context->getMetadata()->setSampleRand((float) $samplingContext->get('sample_rand'));
+            } elseif ($context->parentSampled === true) {
+                $context->getMetadata()->setSampleRand(1.0);
+            }
         }
 
         return $context;
diff --git a/src/Tracing/TransactionMetadata.php b/src/Tracing/TransactionMetadata.php
index 2a10923bc..670a1586c 100644
--- a/src/Tracing/TransactionMetadata.php
+++ b/src/Tracing/TransactionMetadata.php
@@ -21,6 +21,11 @@ final class TransactionMetadata
      */
     private $source;
 
+    /**
+     * @var float|null
+     */
+    private $sampleRand;
+
     /**
      * Constructor.
      *
@@ -36,6 +41,8 @@ public function __construct(
         $this->samplingRate = $samplingRate;
         $this->dynamicSamplingContext = $dynamicSamplingContext;
         $this->source = $source ?? TransactionSource::custom();
+
+        $this->sampleRand = round(mt_rand(0, mt_getrandmax() - 1) / mt_getrandmax(), 2);
     }
 
     /**
@@ -56,6 +63,18 @@ 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;