fix: `_dd.p.ksr` formatting to use 6 significant digits without trailing zeros by bm1549 · Pull Request #288 · DataDog/dd-trace-cpp

bm1549 · 2026-03-11T02:07:05Z

Description

Fix _dd.p.ksr formatting to use 6 significant digits without trailing zeros, using std::to_chars with std::chars_format::general. Also skip setting _dd.p.ksr when the sampling mechanism is DEFAULT (no agent configuration received yet).

Changes:

Replace snprintf with std::to_chars for _dd.p.ksr formatting
Use std::chars_format::general with 6 significant digits (matches Go tracer behavior)
Skip setting _dd.p.ksr when SamplingMechanism::DEFAULT is active
Add error handling for std::to_chars conversion failures
Update tests to expect the new formatting

Motivation

The previous snprintf approach with %g format produced inconsistent output compared to other tracers. std::to_chars with std::chars_format::general gives us precise control over significant digits while automatically removing trailing zeros.

Additionally, setting _dd.p.ksr with the DEFAULT mechanism is meaningless since no sampling configuration has been received from the agent yet.

Additional Notes

Jira ticket: [PROJ-IDENT]

🤖 Generated with Claude Code

… zeros Replace std::to_string() (which uses sprintf "%f" producing 6 trailing decimal places) with snprintf "%.6g" which produces up to 6 significant digits with no trailing zeros. This matches the behavior of Python's f"{rate:.6g}" and Go's strconv.FormatFloat(rate, 'g', 6, 64). Examples: 1.0 -> "1", 0.5 -> "0.5", 0.0 -> "0" Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pr-commenter · 2026-03-11T02:08:12Z

Benchmarks

Benchmark execution time: 2026-03-12 15:33:47

Comparing candidate commit d9275b8 in PR branch brian.marks/fix-ksr-formatting with baseline commit f8c3913 in branch main.

Found 1 performance improvements and 0 performance regressions! Performance is the same for 0 metrics, 0 unstable metrics.

scenario:BM_TraceTinyCCSource

🟩 execution_time [-2.357ms; -2.111ms] or [-3.017%; -2.701%]

Reformat the emplace_back call to match clang-format's expected style (arguments on one line with alignment) to fix the verify CI job. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

datadog-datadog-prod-us1 · 2026-03-11T02:20:17Z

🎯 Code Coverage (details)
• Patch Coverage: 58.33%
• Overall Coverage: 87.70% (-0.07%)

_{This comment will be updated automatically if new data arrives.

🔗 Commit SHA: d9275b8 | Docs | Datadog PR Page | Was this helpful? React with 👍/👎 or give us feedback!}

codecov-commenter · 2026-03-11T02:24:19Z

Codecov Report

❌ Patch coverage is 58.33333% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.70%. Comparing base (f8c3913) to head (d9275b8).

Files with missing lines	Patch %	Lines
src/datadog/trace_segment.cpp	58.33%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #288      +/-   ##
==========================================
- Coverage   87.76%   87.70%   -0.07%     
==========================================
  Files          84       84              
  Lines        5658     5668      +10     
==========================================
+ Hits         4966     4971       +5     
- Misses        692      697       +5

☔ View full report in Codecov by Sentry.
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The ksr trace tag should only be set when the sampling decision comes from an explicit source (agent rate, rule, or remote rule). When the DEFAULT mechanism is used — meaning no agent configuration has been received yet — the rate is a hardcoded 100% and ksr is meaningless. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/datadog/trace_segment.cpp

dmehala

Well done.

EDIT: I'm trying to enforce semantic commits, please prefix your commit message with fix: during the merge. Thanks.

Co-authored-by: Damien Mehala <damien.mehala@datadoghq.com>

src/datadog/trace_segment.cpp

Fix two issues introduced by GitHub suggestion commits: 1. Correct the error condition: `ec == std::errc()` means success, not failure — changed to `ec != std::errc()` to properly detect errors. 2. Fix indentation from 4-space to 2-space to match project style and pass clang-format checks. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

# What does this PR do? Adds `_dd.p.ksr` (Knuth Sampling Rate) as a propagated tag set when agent-based or rule-based sampling decisions are made. The tag is stored in span `meta` (string type) with up to 6 significant digits and no trailing zeros. `format_sampling_rate` now returns `Option<String>` and guards against invalid inputs (negative, >1.0, NaN, infinity), returning `None` instead of producing garbage output. # Motivation To enable consistent sampling across tracers and backend retention filters, the backend needs to know the sampling rate applied by the tracer. Without transmitting the tracer's rate via `_dd.p.ksr`, backend resampling cannot correctly compute effective rates in multi-stage sampling scenarios. See RFC: "Transmit Knuth sampling rate to backend" # Additional Notes Key files changed: - `datadog-opentelemetry/src/core/constants.rs` — Added `SAMPLING_KNUTH_RATE_TAG_KEY` constant - `datadog-opentelemetry/src/sampling/datadog_sampler.rs` — Added `format_sampling_rate()` helper (returns `Option<String>`, defensive against invalid rates) and set ksr in agent/rule sampling paths - Updated 2 snapshot JSON files Related PRs across tracers: - Java: DataDog/dd-trace-java#10802 - .NET: DataDog/dd-trace-dotnet#8287 - Ruby: DataDog/dd-trace-rb#5436 - Node.js: DataDog/dd-trace-js#7741 - PHP: DataDog/dd-trace-php#3701 - C++: DataDog/dd-trace-cpp#288 - System tests: DataDog/system-tests#6466 🤖 Generated with [Claude Code](https://claude.com/claude-code) --------- Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

### What does this PR do? Fixes `_dd.p.ksr` (Knuth Sampling Rate) to only be set on spans when the agent has provided sampling rates via `readRatesJSON()`. Previously, ksr was unconditionally set in `prioritySampler.apply()`, including when the rate was the initial client-side default (1.0) before any agent response arrived. Also refactors `prioritySampler` to consolidate lock acquisitions: extracts `getRateLocked()` so `apply()` acquires `ps.mu.RLock` only once to read both the rate and `agentRatesLoaded`. ### Motivation Cross-language consistency: Python, Java, PHP, and other tracers only set ksr when actual agent rates or sampling rules are applied, not for the default fallback. This aligns Go with that behavior. See RFC: "Transmit Knuth sampling rate to backend" ### Additional Notes - Added `agentRatesLoaded` bool field to `prioritySampler`, set to `true` in `readRatesJSON()` - `apply()` now gates ksr behind `agentRatesLoaded` check - Extracted `getRateLocked()` to avoid double lock acquisition in `apply()` - Rule-based sampling path (`applyTraceRuleSampling` in span.go) unchanged — correctly always sets ksr - Tests added: `ksr-not-set-without-agent-rates` and `ksr-set-after-agent-rates-received` Related PRs across tracers: - Java: DataDog/dd-trace-java#10802 - .NET: DataDog/dd-trace-dotnet#8287 - Ruby: DataDog/dd-trace-rb#5436 - Node.js: DataDog/dd-trace-js#7741 - PHP: DataDog/dd-trace-php#3701 - Rust: DataDog/dd-trace-rs#180 - C++: DataDog/dd-trace-cpp#288 - System tests: DataDog/system-tests#6466 ### Reviewer's Checklist - [x] Changed code has unit tests for its functionality at or near 100% coverage. - [x] [System-Tests](https://github.com/DataDog/system-tests/) covering this feature have been added and enabled with the va.b.c-dev version tag. - [ ] There is a benchmark for any new code, or changes to existing code. - [x] If this interacts with the agent in a new way, a system test has been added. - [x] New code is free of linting errors. You can check this by running `make lint` locally. - [x] New code doesn't break existing tests. You can check this by running `make test` locally. - [ ] Add an appropriate team label so this PR gets put in the right place for the release notes. - [ ] All generated files are up to date. You can check this by running `make generate` locally. - [ ] Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild. Make sure all nested modules are up to date by running `make fix-modules` locally. Unsure? Have a question? Request a review! 🤖 Generated with [Claude Code](https://claude.com/claude-code) --------- Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com> Co-authored-by: Dario Castañé <dario.castane@datadoghq.com> Co-authored-by: Mikayla Toffler <46911781+mtoffl01@users.noreply.github.com>

Fix clang-format violation in trace_segment.cpp

28fee0b

Reformat the emplace_back call to match clang-format's expected style (arguments on one line with alignment) to fix the verify CI job. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

bm1549 temporarily deployed to dev March 11, 2026 02:14 — with GitHub Actions Inactive

bm1549 added the AI Generated Largely based on code generated by an AI or LLM. This label is the same across all dd-trace-* repos label Mar 11, 2026

bm1549 temporarily deployed to dev March 11, 2026 02:17 — with GitHub Actions Inactive

bm1549 mentioned this pull request Mar 11, 2026

fix(tracer): only set _dd.p.ksr after agent rates are received DataDog/dd-trace-go#4523

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bm1549 marked this pull request as ready for review March 11, 2026 19:51

bm1549 requested a review from a team as a code owner March 11, 2026 19:51

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dmehala reviewed Mar 12, 2026

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dmehala changed the title ~~Fix _dd.p.ksr formatting to use 6 significant digits without trailing zeros~~ fix: _dd.p.ksr formatting to use 6 significant digits without trailing zeros Mar 12, 2026

Update src/datadog/trace_segment.cpp

230dcef

Co-authored-by: Damien Mehala <damien.mehala@datadoghq.com>

bm1549 commented Mar 12, 2026

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bm1549 and others added 2 commits March 12, 2026 11:27

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bm1549 merged commit 1572290 into main Mar 12, 2026
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bm1549 deleted the brian.marks/fix-ksr-formatting branch March 12, 2026 15:50

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fix: `_dd.p.ksr` formatting to use 6 significant digits without trailing zeros#288

fix: `_dd.p.ksr` formatting to use 6 significant digits without trailing zeros#288
bm1549 merged 7 commits intomainfrom
brian.marks/fix-ksr-formatting

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