Store both the raw and scaled probability on spans

Author: imjoehaines

packages/core/lib/batch-processor.ts

@@ -132,7 +132,7 @@ export class BatchProcessor<C extends Configuration> implements Processor {
     const probability = this.sampler.spanProbability
 
     for (const span of this.batch) {
-      if (span.samplingProbability < probability) {
+      if (span.samplingProbability.raw < probability.raw) {
         span.samplingProbability = probability
       }
 

packages/core/lib/sampler.ts

@@ -1,10 +1,10 @@
-import { type SpanEnded, type SpanProbability } from './span'
+import { type SpanEnded, type ScaledProbability, type SpanProbability } from './span'
 
 // sampling rates are stored as a number between 0 and 2^32 - 1 (i.e. they are
 // u32s) so we need to scale the probability value to match this range as they
 // are stored as values between 0 and 1
-function scaleProbabilityToMatchSamplingRate (probability: number): SpanProbability {
-  return Math.floor(probability * 0xffffffff) as SpanProbability
+function scaleProbabilityToMatchSamplingRate (probability: number): ScaledProbability {
+  return Math.floor(probability * 0xffffffff) as ScaledProbability
 }
 
 interface ReadonlySampler {
@@ -25,7 +25,7 @@ class Sampler {
    *
    * @see scaleProbabilityToMatchSamplingRate
    */
-  private scaledProbability: SpanProbability
+  private scaledProbability: ScaledProbability
 
   constructor (initialProbability: number) {
     // we could just do 'this.probability = initialProbability' but TypeScript
@@ -57,11 +57,14 @@ class Sampler {
    * @see scaleProbabilityToMatchSamplingRate
    */
   get spanProbability (): SpanProbability {
-    return this.scaledProbability
+    return {
+      raw: this._probability,
+      scaled: this.scaledProbability
+    }
   }
 
   sample (span: SpanEnded): boolean {
-    return span.samplingRate <= span.samplingProbability
+    return span.samplingRate <= span.samplingProbability.scaled
   }
 }
 

packages/core/lib/span.ts

@@ -26,8 +26,13 @@ export const enum Kind {
 // this prevents the wrong kind of number being assigned to the span's
 // samplingProbability
 // this exists only in the type system; at runtime it's a regular number
-declare const validSpanProbability: unique symbol
-export type SpanProbability = number & { [validSpanProbability]: true }
+declare const validScaledProbability: unique symbol
+export type ScaledProbability = number & { [validScaledProbability]: true }
+
+export interface SpanProbability {
+  readonly scaled: ScaledProbability
+  readonly raw: number
+}
 
 export interface SpanEnded {
   readonly id: string // 64 bit random string

packages/test-utilities/lib/create-span.ts

@@ -1,6 +1,6 @@
 import {
   type SpanEnded,
-  type SpanProbability,
+  type ScaledProbability,
   SpanAttributes,
   traceIdToSamplingRate,
   SpanEvents
@@ -20,7 +20,10 @@ export function createEndedSpan (overrides: Partial<SpanEnded> = {}): SpanEnded
     traceId,
     samplingRate: traceIdToSamplingRate(traceId),
     endTime: 23456,
-    samplingProbability: Math.floor(0.5 * 0xffffffff) as SpanProbability,
+    samplingProbability: {
+      raw: 0.5,
+      scaled: Math.floor(0.5 * 0xffffffff) as ScaledProbability
+    },
     ...overrides
   }
 }