fix(tracing): format _dd.p.ksr with decimal notation instead of scientific notation (#7846)

bm1549 · claude · BridgeAR · web-flow · commit f657b07188b3 · 2026-03-25T18:49:06.000+01:00
* fix(tracing): format _dd.p.ksr with decimal notation instead of scientific notation Very small sampling rates (e.g. 0.0000001) were formatted using toPrecision(6) + toString() which outputs scientific notation like "1e-7". This changes to explicit rounding at the integer level and toFixed(6) formatting to always produce decimal notation with up to 6 decimal digits, trailing zeros stripped (e.g. "0.000001"). Fixes APMAPI-1869 Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com> * fix(tracing): replace regex with loop in formatKnuthRate for hot-path perf Addresses PR review feedback: replaces the regex-based trailing-zero strip with a manual loop to avoid regex overhead on the hot path. Also adds Number() coercion as defensive measure per reviewer suggestion. Math.round pre-rounding is intentionally kept — toFixed(6) has imprecise rounding for sub-precision values in V8 (e.g. 0.0000005.toFixed(6) returns '0.000000' in Node 23). JSDoc updated to explain this. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com> * fix(tracing): simplify formatKnuthRate loop per review feedback Drop Math.round pre-rounding — the edge case (0.0000005 rounding down in V8) is acceptable imprecision. Test updated to use 0.00000051 which rounds up unambiguously without Math.round. Drop indexOf('.') — check for '.' directly in the loop instead, which handles integer rates (0, 1) without the overhead of a separate scan. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com> * Update packages/dd-trace/src/priority_sampler.js Co-authored-by: Ruben Bridgewater <ruben@bridgewater.de> * fix(tracing): resolve lint errors in formatKnuthRate - Flip negated conditions to positive checks (unicorn/no-negated-condition) - Remove @returns tag since linter can't statically verify loop always returns (jsdoc/require-returns-check) - Preserve BridgeAR's single-check-per-iteration optimization Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com> --------- Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com> Co-authored-by: Ruben Bridgewater <ruben@bridgewater.de>
diff --git a/packages/dd-trace/src/priority_sampler.js b/packages/dd-trace/src/priority_sampler.js
@@ -37,13 +37,16 @@ const {
 const DEFAULT_KEY = 'service:,env:'
 
 /**
- * Formats a sampling rate as a string with up to 6 significant digits and no trailing zeros.
+ * Formats a sampling rate as a string with up to 6 decimal digits and no trailing zeros.
  *
  * @param {number} rate
- * @returns {string}
  */
 function formatKnuthRate (rate) {
-  return Number(rate.toPrecision(6)).toString()
+  const string = Number(rate).toFixed(6)
+  for (let i = string.length - 1; i > 0; i--) {
+    if (string[i] === '0') continue
+    return string.slice(0, i + (string[i] === '.' ? 0 : 1))
+  }
 }
 
 const defaultSampler = new Sampler(AUTO_KEEP)
diff --git a/packages/dd-trace/test/priority_sampler.spec.js b/packages/dd-trace/test/priority_sampler.spec.js
@@ -421,6 +421,45 @@ describe('PrioritySampler', () => {
       assert.strictEqual(typeof context._trace.tags[SAMPLING_KNUTH_RATE], 'string')
     })
 
+    it('should format _dd.p.ksr with decimal notation for very small rates', () => {
+      Sampler.withArgs(0.000001).returns({
+        isSampled: sinon.stub().returns(true),
+        rate: sinon.stub().returns(0.000001),
+      })
+      prioritySampler = new PrioritySampler('test', {
+        sampleRate: 0.000001,
+      })
+      prioritySampler.sample(span)
+
+      assert.strictEqual(context._trace.tags[SAMPLING_KNUTH_RATE], '0.000001')
+    })
+
+    it('should round _dd.p.ksr to zero when rate is below 6 decimal precision', () => {
+      Sampler.withArgs(0.0000001).returns({
+        isSampled: sinon.stub().returns(true),
+        rate: sinon.stub().returns(0.0000001),
+      })
+      prioritySampler = new PrioritySampler('test', {
+        sampleRate: 0.0000001,
+      })
+      prioritySampler.sample(span)
+
+      assert.strictEqual(context._trace.tags[SAMPLING_KNUTH_RATE], '0')
+    })
+
+    it('should round _dd.p.ksr up to 0.000001 when rate rounds up', () => {
+      Sampler.withArgs(0.00000051).returns({
+        isSampled: sinon.stub().returns(true),
+        rate: sinon.stub().returns(0.00000051),
+      })
+      prioritySampler = new PrioritySampler('test', {
+        sampleRate: 0.00000051,
+      })
+      prioritySampler.sample(span)
+
+      assert.strictEqual(context._trace.tags[SAMPLING_KNUTH_RATE], '0.000001')
+    })
+
     it('should not set _dd.p.ksr tag for manual sampling', () => {
       context._tags[MANUAL_KEEP] = undefined
 
