Write and use sample rand on propagation context

Author: Luca Forstner

docs/migration/v8-to-v9.md

@@ -186,6 +186,7 @@ Sentry.init({
 - The `addRequestDataToEvent` method has been removed. Use `httpRequestToRequestData` instead and put the resulting object directly on `event.request`.
 - The `extractPathForTransaction` method has been removed. There is no replacement.
 - The `addNormalizedRequestDataToEvent` method has been removed. Use `httpRequestToRequestData` instead and put the resulting object directly on `event.request`.
+- A `sampleRand` field on `PropagationContext` is now required. This is relevant if you used `scope.setPropagationContext(...)`
 
 #### Other/Internal Changes
 

packages/browser/src/tracing/browserTracingIntegration.ts

@@ -452,8 +452,8 @@ export function startBrowserTracingPageLoadSpan(
  * This will only do something if a browser tracing integration has been setup.
  */
 export function startBrowserTracingNavigationSpan(client: Client, spanOptions: StartSpanOptions): Span | undefined {
-  getIsolationScope().setPropagationContext({ traceId: generateTraceId() });
-  getCurrentScope().setPropagationContext({ traceId: generateTraceId() });
+  getIsolationScope().setPropagationContext({ traceId: generateTraceId(), sampleRand: Math.random() });
+  getCurrentScope().setPropagationContext({ traceId: generateTraceId(), sampleRand: Math.random() });
 
   client.emit('startNavigationSpan', spanOptions);
 

packages/core/src/scope.ts

@@ -164,6 +164,7 @@ export class Scope {
     this._sdkProcessingMetadata = {};
     this._propagationContext = {
       traceId: generateTraceId(),
+      sampleRand: Math.random(),
     };
   }
 
@@ -456,7 +457,7 @@ export class Scope {
     this._session = undefined;
     _setSpanForScope(this, undefined);
     this._attachments = [];
-    this.setPropagationContext({ traceId: generateTraceId() });
+    this.setPropagationContext({ traceId: generateTraceId(), sampleRand: Math.random() });
 
     this._notifyScopeListeners();
     return this;

packages/core/src/tracing/dynamicSamplingContext.ts

@@ -116,6 +116,7 @@ export function getDynamicSamplingContextFromSpan(span: Span): Readonly<Partial<
   // So we end up with an active span that is not sampled (neither positively nor negatively)
   if (hasTracingEnabled()) {
     dsc.sampled = String(spanIsSampled(rootSpan));
+    // TODO(lforst): Grab scope off of root span and put sample_rand on dsc
   }
 
   client.emit('createDsc', dsc, rootSpan);

packages/core/src/tracing/sampling.ts

@@ -1,6 +1,5 @@
 import type { Options, SamplingContext } from '../types-hoist';
 
-import { getIsolationScope } from '../currentScopes';
 import { DEBUG_BUILD } from '../debug-build';
 import { logger } from '../utils-hoist/logger';
 import { hasTracingEnabled } from '../utils/hasTracingEnabled';
@@ -15,26 +14,20 @@ import { parseSampleRate } from '../utils/parseSampleRate';
 export function sampleSpan(
   options: Pick<Options, 'tracesSampleRate' | 'tracesSampler' | 'enableTracing'>,
   samplingContext: SamplingContext,
+  sampleRand: number,
 ): [sampled: boolean, sampleRate?: number] {
   // nothing to do if tracing is not enabled
   if (!hasTracingEnabled(options)) {
     return [false];
   }
 
-  const normalizedRequest = getIsolationScope().getScopeData().sdkProcessingMetadata.normalizedRequest;
-
-  const enhancedSamplingContext = {
-    ...samplingContext,
-    normalizedRequest: samplingContext.normalizedRequest || normalizedRequest,
-  };
-
   // we would have bailed already if neither `tracesSampler` nor `tracesSampleRate` nor `enableTracing` were defined, so one of these should
   // work; prefer the hook if so
   let sampleRate;
   if (typeof options.tracesSampler === 'function') {
-    sampleRate = options.tracesSampler(enhancedSamplingContext);
-  } else if (enhancedSamplingContext.parentSampled !== undefined) {
-    sampleRate = enhancedSamplingContext.parentSampled;
+    sampleRate = options.tracesSampler(samplingContext);
+  } else if (samplingContext.parentSampled !== undefined) {
+    sampleRate = samplingContext.parentSampled;
   } else if (typeof options.tracesSampleRate !== 'undefined') {
     sampleRate = options.tracesSampleRate;
   } else {
@@ -66,7 +59,7 @@ export function sampleSpan(
 
   // Now we roll the dice. Math.random is inclusive of 0, but not of 1, so strict < is safe here. In case sampleRate is
   // a boolean, the < comparison will cause it to be automatically cast to 1 if it's true and 0 if it's false.
-  const shouldSample = Math.random() < parsedSampleRate;
+  const shouldSample = sampleRand < parsedSampleRate;
 
   // if we're not going to keep it, we're done
   if (!shouldSample) {

packages/core/src/tracing/trace.ts

@@ -279,7 +279,10 @@ export function suppressTracing<T>(callback: () => T): T {
  */
 export function startNewTrace<T>(callback: () => T): T {
   return withScope(scope => {
-    scope.setPropagationContext({ traceId: generateTraceId() });
+    scope.setPropagationContext({
+      traceId: generateTraceId(),
+      sampleRand: Math.random(),
+    });
     DEBUG_BUILD && logger.info(`Starting a new trace with id ${scope.getPropagationContext().traceId}`);
     return withActiveSpan(null, callback);
   });
@@ -402,17 +405,23 @@ function _startRootSpan(spanArguments: SentrySpanArguments, scope: Scope, parent
   const options: Partial<ClientOptions> = client?.getOptions() || {};
 
   const { name = '', attributes } = spanArguments;
+  const sampleRand = scope.getPropagationContext().sampleRand;
   const [sampled, sampleRate] = scope.getScopeData().sdkProcessingMetadata[SUPPRESS_TRACING_KEY]
     ? [false]
-    : sampleSpan(options, {
-        name,
-        parentSampled,
-        attributes,
-        transactionContext: {
+    : sampleSpan(
+        options,
+        {
           name,
           parentSampled,
+          attributes,
+          transactionContext: {
+            name,
+            parentSampled,
+          },
+          // parentSampleRate: 'TODO',
         },
-      });
+        sampleRand,
+      );
 
   const rootSpan = new SentrySpan({
     ...spanArguments,

packages/core/src/types-hoist/envelope.ts

@@ -23,6 +23,7 @@ export type DynamicSamplingContext = {
   transaction?: string;
   replay_id?: string;
   sampled?: string;
+  sample_rand?: string;
 };
 
 // https://github.com/getsentry/relay/blob/311b237cd4471042352fa45e7a0824b8995f216f/relay-server/src/envelope.rs#L154

packages/core/src/types-hoist/samplingcontext.ts

@@ -29,6 +29,11 @@ export interface SamplingContext extends CustomSamplingContext {
    */
   parentSampled?: boolean;
 
+  /**
+   * Sample rate that is coming from an incoming trace (if there is one).
+   */
+  parentSampleRate?: number;
+
   /**
    * Object representing the URL of the current page or worker script. Passed by default when using the `BrowserTracing`
    * integration.

packages/core/src/types-hoist/tracing.ts

@@ -16,6 +16,11 @@ export interface PropagationContext {
    */
   traceId: string;
 
+  /**
+   * TODO
+   */
+  sampleRand: number;
+
   /**
    * Represents the sampling decision of the incoming trace.
    *

packages/core/src/utils-hoist/propagationContext.ts

@@ -9,6 +9,7 @@ import { uuid4 } from './misc';
 export function generatePropagationContext(): PropagationContext {
   return {
     traceId: generateTraceId(),
+    sampleRand: Math.random(),
   };
 }