include parentSampled as separate entry in transaction context

Author: lobsterkatie

packages/tracing/src/hubextensions.ts

@@ -29,15 +29,15 @@ function traceHeaders(this: Hub): { [key: string]: string } {
 }
 
 /**
- * Uses existing sampling decision if available; falls back to user-provided static rate.
+ * Implements sampling inheritance and falls back to user-provided static rate if no parent decision is available.
  *
- * @param existingDecision: The transaction's existing sampling decision, if any (likely inherited).
+ * @param parentSampled: The parent transaction's sampling decision, if any.
  * @param givenRate: The rate to use if no parental decision is available.
  *
- * @returns The existing sampling decision (if one exists), or the provided static rate
+ * @returns The parent's sampling decision (if one exists), or the provided static rate
  */
-function _useExistingDecisionOrGivenRate(existingDecision: boolean | undefined, givenRate: unknown): boolean | unknown {
-  return existingDecision !== undefined ? existingDecision : givenRate;
+function _inheritOrUseGivenRate(parentSampled: boolean | undefined, givenRate: unknown): boolean | unknown {
+  return parentSampled !== undefined ? parentSampled : givenRate;
 }
 
 /**
@@ -67,7 +67,7 @@ function sample<T extends Transaction>(hub: Hub, transaction: T, samplingContext
   const sampleRate =
     typeof options.tracesSampler === 'function'
       ? options.tracesSampler(samplingContext)
-      : _useExistingDecisionOrGivenRate(samplingContext.transactionContext.sampled, options.tracesSampleRate);
+      : _inheritOrUseGivenRate(samplingContext.parentSampled, options.tracesSampleRate);
 
   // Since this is coming from the user (or from a function provided by the user), who knows what we might get. (The
   // only valid values are booleans or numbers between 0 and 1.)
@@ -117,7 +117,9 @@ function sample<T extends Transaction>(hub: Hub, transaction: T, samplingContext
  * @returns The default sample context
  */
 function getDefaultSamplingContext(transactionContext: TransactionContext): SamplingContext {
-  const defaultSamplingContext: SamplingContext = { transactionContext };
+  // promote parent sampling decision (if any) for easy access
+  const { parentSampled } = transactionContext;
+  const defaultSamplingContext: SamplingContext = { transactionContext, parentSampled };
 
   if (isNodeEnv()) {
     const domain = getActiveDomain();

packages/tracing/test/hub.test.ts

@@ -149,13 +149,29 @@ describe('Hub', () => {
         const transactionContext = {
           name: 'dogpark',
           parentSpanId: '12312012',
-          sampled: true,
+          parentSampled: true,
         };
 
         hub.startTransaction(transactionContext);
 
         expect(tracesSampler).toHaveBeenLastCalledWith(expect.objectContaining({ transactionContext }));
       });
+
+      it("should add parent's sampling decision to default sample context", () => {
+        const tracesSampler = jest.fn();
+        const hub = new Hub(new BrowserClient({ tracesSampler }));
+        const parentSamplingDecsion = false;
+
+        hub.startTransaction({
+          name: 'dogpark',
+          parentSpanId: '12312012',
+          parentSampled: parentSamplingDecsion,
+        });
+
+        expect(tracesSampler).toHaveBeenLastCalledWith(
+          expect.objectContaining({ parentSampled: parentSamplingDecsion }),
+        );
+      });
     });
 
     describe('sample()', () => {
@@ -289,7 +305,7 @@ describe('Hub', () => {
         const transaction = hub.startTransaction({
           name: 'dogpark',
           parentSpanId: '12312012',
-          sampled: parentSamplingDecsion,
+          parentSampled: parentSamplingDecsion,
         });
 
         expect(transaction.sampled).toBe(parentSamplingDecsion);
@@ -306,7 +322,7 @@ describe('Hub', () => {
         const transaction = hub.startTransaction({
           name: 'dogpark',
           parentSpanId: '12312012',
-          sampled: parentSamplingDecsion,
+          parentSampled: parentSamplingDecsion,
         });
 
         expect(transaction.sampled).not.toBe(parentSamplingDecsion);

packages/types/src/transaction.ts

@@ -13,6 +13,11 @@ export interface TransactionContext extends SpanContext {
    * transaction after a given "idle time" and we don't want this "idle time" to be part of the transaction.
    */
   trimEnd?: boolean;
+
+  /**
+   * If this transaction has a parent, the parent's sampling decision
+   */
+  parentSampled?: boolean;
 }
 
 /**
@@ -68,6 +73,11 @@ export interface SamplingContext extends CustomSamplingContext {
    */
   transactionContext: TransactionContext;
 
+  /**
+   * Sampling decision from the parent transaction, if any.
+   */
+  parentSampled?: boolean;
+
   /**
    * Object representing the URL of the current page or worker script. Passed by default in a browser or service worker
    * context.