feat(tracing): add span link support by MilanGarnier · Pull Request #330 · DataDog/dd-trace-cpp

MilanGarnier · 2026-06-30T12:28:31Z

Description

Adds OpenTelemetry-style span links to dd-trace-cpp.

ExtractedContext struct (include/datadog/extracted_context.h)
SpanLink struct (include/datadog/span_link.h): 128-bit trace_id, span_id, optional tracestate, string attributes, optional flags.
msgpack serialization (src/datadog/span_link.cpp): per-link map with the exact field names and omission rules used by dd-trace-go / dd-trace-py / dd-trace-rs (trace_id, trace_id_high only when non-zero, span_id, attributes only when non-empty, tracestate only when non-empty, flags with high bit set when present).
SpanData::span_links (src/datadog/span_data.h/.cpp): std::vector<SpanLink> field; the span encoder emits a span_links array only when non-empty (matches omitempty behaviour of other tracers).
Span::add_link(const SpanLink&) (include/datadog/span.h, src/datadog/span.cpp): public API to attach a link to a live span.
Parametric endpoint POST /trace/span/add_link (test/system-tests/): wires up the cross-language system-test client contract.

Motivation

This feature is supported by all other tracers and is a dependency for future implementations, such as DD_TRACE_PROPAGATION_BEHAVIOR_EXTRACT config (planned).

Additional Notes

Unit tests in test/test_span_link.cpp and test/test_span.cpp exercise serialization and the public API end-to-end.
System tests : parametric tests enabled in Enable span links parametric tests for cpp [cpp@milan.garnier/span-links-support] system-tests#7238
Jira ticket: APMAPI-1941

pr-commenter · 2026-06-30T12:36:31Z

Benchmarks

Benchmark execution time: 2026-07-06 11:07:17

Comparing candidate commit 5311345 in PR branch milan.garnier/span-links-support with baseline commit d7ea3a8 in branch main.

Found 2 performance improvements and 3 performance regressions! Performance is the same for 3 metrics, 0 unstable metrics.

Explanation

This is an A/B test comparing a candidate commit's performance against that of a baseline commit. Performance changes are noted in the tables below as:

🟩 = significantly better candidate vs. baseline
🟥 = significantly worse candidate vs. baseline

We compute a confidence interval (CI) over the relative difference of means between metrics from the candidate and baseline commits, considering the baseline as the reference.

If the CI is entirely outside the configured SIGNIFICANT_IMPACT_THRESHOLD (or the deprecated UNCONFIDENCE_THRESHOLD), the change is considered significant.

Feel free to reach out to #apm-benchmarking-platform on Slack if you have any questions.

More details about the CI and significant changes

You can imagine this CI as a range of values that is likely to contain the true difference of means between the candidate and baseline commits.

CIs of the difference of means are often centered around 0%, because often changes are not that big:

---------------------------------(------|---^--------)-------------------------------->
                              -0.6%    0%  0.3%     +1.2%
                                 |          |        |
         lower bound of the CI --'          |        |
sample mean (center of the CI) -------------'        |
         upper bound of the CI ----------------------'

As described above, a change is considered significant if the CI is entirely outside the configured SIGNIFICANT_IMPACT_THRESHOLD (or the deprecated UNCONFIDENCE_THRESHOLD).

For instance, for an execution time metric, this confidence interval indicates a significantly worse performance:

----------------------------------------|---------|---(---------^---------)---------->
                                       0%        1%  1.3%      2.2%      3.1%
                                                  |   |         |         |
       significant impact threshold --------------'   |         |         |
                      lower bound of CI --------------'         |         |
       sample mean (center of the CI) --------------------------'         |
                      upper bound of CI ----------------------------------'

scenario:BM_HexPadded_uint64/NoPadding

🟥 execution_time [+2.433ns; +2.497ns] or [+4.529%; +4.649%]

scenario:BM_HexPadded_uint64/WorstCasePadding

🟥 execution_time [+1.656ns; +1.758ns] or [+3.368%; +3.576%]

scenario:BM_Hex_uint64

🟩 execution_time [-0.991ns; -0.966ns] or [-4.186%; -4.080%]

scenario:BM_TraceID_ParseHex/128bit

🟩 execution_time [-3.988ns; -3.969ns] or [-3.171%; -3.156%]

scenario:BM_TraceTinyCCSource

🟥 execution_time [+2.426ms; +2.656ms] or [+3.260%; +3.569%]

chatgpt-codex-connector

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 1a36ade09f

ℹ️ About Codex in GitHub

Codex has been enabled to automatically review pull requests in this repo. Reviews are triggered when you

Open a pull request for review
Mark a draft as ready
Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

When you sign up for Codex through ChatGPT, Codex can also answer questions or update the PR, like "@codex address that feedback".

chatgpt-codex-connector

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 6c2f121762

ℹ️ About Codex in GitHub

Codex has been enabled to automatically review pull requests in this repo. Reviews are triggered when you

Open a pull request for review
Mark a draft as ready
Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

When you sign up for Codex through ChatGPT, Codex can also answer questions or update the PR, like "@codex address that feedback".

xlamorlette-datadog

This is only a first very light and superficial review, I should review in detail in a few days.

xlamorlette-datadog · 2026-07-06T09:22:39Z

+#include "optional.h"
+#include "trace_id.h"
+
+namespace datadog {


nit: please rather use nested namespace syntax

xlamorlette-datadog · 2026-07-06T09:23:52Z

+  // Capture injected headers (tracestate, traceparent flags) into a map.
+  struct : DictWriter {
+    std::unordered_map<std::string, std::string> map;
+    void set(StringView k, StringView v) override {


nit, clean code: please use meaningful variable names (this is not Go ;-) )

xlamorlette-datadog · 2026-07-06T09:25:51Z

+  const bool has_links = !span.span_links.empty();
+
+  // 12 always-present fields, plus span_links when there are any.
+  auto result = msgpack::pack_map(destination, has_links ? 13u : 12u);


nit: could we name these magic numbers?

xlamorlette-datadog · 2026-07-06T09:27:54Z

+        });
+    if (!result) return result;
+  }
  // clang-format on


opt: to be checked (I could do it): why do we need this clang-format exclusion?

this was already present before and the reason is that msgpack::pack_map_suffix(...) was not properly formatted. I can totally remove the comments and stage the formatted version instead

xlamorlette-datadog · 2026-07-06T09:28:33Z

+namespace datadog {
+namespace tracing {
+
+Expected<void> msgpack_encode(std::string& destination, const SpanLink& link) {


nit, clean code: please try to split this big function

xlamorlette-datadog · 2026-07-06T09:29:02Z

  res.status = 200;
 }

+void RequestHandler::on_add_link(const httplib::Request& req,


nit, clean code: please try to split this big function

xlamorlette-datadog · 2026-07-06T09:30:08Z

-  };
+  const auto upstream_id = span->parent_id().value_or(0);
+
+  StoredLinkContext stored;


nit, clean code: please try to extract this additional part to avoid make this function, which is already too big, grow

MilanGarnier added 8 commits June 30, 2026 10:39

add span link struct and test

fad515b

enable span link tests

0796973

implement Span::add_link

0fecaca

add span link to msg pack encoding

f254562

add spn link test for the Span class

58cf95d

implenent add_link endpoint for parametric tests

84a7ef4

flatten array

65382c4

cleaner spanlink construction

ecfb7cc

This comment has been minimized.

Sign in to view

add extracted context struct

eecfe93

cleaning up (move logic to span.cpp)

c4ae457

MilanGarnier changed the title ~~feat(tracing): add support for span links~~ feat(tracing): add span link support Jun 30, 2026

format

86d189e

MilanGarnier force-pushed the milan.garnier/span-links-support branch from cbb3ed8 to 86d189e Compare June 30, 2026 13:23

MilanGarnier temporarily deployed to dev June 30, 2026 13:25 — with GitHub Actions Inactive

MilanGarnier temporarily deployed to dev June 30, 2026 13:28 — with GitHub Actions Inactive

MilanGarnier temporarily deployed to dev June 30, 2026 13:35 — with GitHub Actions Inactive

MilanGarnier force-pushed the milan.garnier/span-links-support branch 2 times, most recently from 003399b to 28432fe Compare June 30, 2026 13:38

MilanGarnier marked this pull request as draft June 30, 2026 15:25

chatgpt-codex-connector Bot reviewed Jun 30, 2026

View reviewed changes

Comment thread include/datadog/span.h

Comment thread src/datadog/span_data.cpp

resolve #330 (comment)

6c2f121

MilanGarnier temporarily deployed to dev June 30, 2026 15:34 — with GitHub Actions Inactive

MilanGarnier temporarily deployed to dev June 30, 2026 15:37 — with GitHub Actions Inactive

MilanGarnier marked this pull request as ready for review July 1, 2026 10:58

chatgpt-codex-connector Bot reviewed Jul 1, 2026

View reviewed changes

Comment thread include/datadog/span.h Outdated

Comment thread test/system-tests/request_handler.cpp Outdated

change scope of ExtractedHeaders struct (restrict to parametric server)

5c93eca

MilanGarnier force-pushed the milan.garnier/span-links-support branch from 8492993 to 5c93eca Compare July 2, 2026 08:55

MilanGarnier temporarily deployed to dev July 2, 2026 08:56 — with GitHub Actions Inactive

MilanGarnier had a problem deploying to dev July 2, 2026 08:56 — with GitHub Actions Failure

MilanGarnier temporarily deployed to dev July 2, 2026 08:56 — with GitHub Actions Inactive

MilanGarnier temporarily deployed to dev July 2, 2026 09:38 — with GitHub Actions Inactive

MilanGarnier temporarily deployed to dev July 2, 2026 09:41 — with GitHub Actions Inactive

case insensitive lookup for headers

5311345

MilanGarnier temporarily deployed to dev July 6, 2026 09:14 — with GitHub Actions Inactive

xlamorlette-datadog reviewed Jul 6, 2026

View reviewed changes

Uh oh!

Conversation

MilanGarnier commented Jun 30, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Description

Motivation

Additional Notes

Uh oh!

This comment has been minimized.

pr-commenter Bot commented Jun 30, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Benchmarks

Explanation

More details about the CI and significant changes

scenario:BM_HexPadded_uint64/NoPadding

scenario:BM_HexPadded_uint64/WorstCasePadding

scenario:BM_Hex_uint64

scenario:BM_TraceID_ParseHex/128bit

scenario:BM_TraceTinyCCSource

Uh oh!

chatgpt-codex-connector Bot left a comment

Choose a reason for hiding this comment

💡 Codex Review

Uh oh!

Uh oh!

Uh oh!

chatgpt-codex-connector Bot left a comment

Choose a reason for hiding this comment

💡 Codex Review

Uh oh!

Uh oh!

Uh oh!

xlamorlette-datadog left a comment

Choose a reason for hiding this comment

Uh oh!

xlamorlette-datadog Jul 6, 2026

Choose a reason for hiding this comment

Uh oh!

xlamorlette-datadog Jul 6, 2026

Choose a reason for hiding this comment

Uh oh!

xlamorlette-datadog Jul 6, 2026

Choose a reason for hiding this comment

Uh oh!

xlamorlette-datadog Jul 6, 2026

Choose a reason for hiding this comment

Uh oh!

MilanGarnier Jul 6, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

xlamorlette-datadog Jul 6, 2026

Choose a reason for hiding this comment

Uh oh!

xlamorlette-datadog Jul 6, 2026

Choose a reason for hiding this comment

Uh oh!

xlamorlette-datadog Jul 6, 2026

Choose a reason for hiding this comment

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

2 participants

MilanGarnier commented Jun 30, 2026 •

edited

Loading

pr-commenter Bot commented Jun 30, 2026 •

edited

Loading

MilanGarnier Jul 6, 2026 •

edited

Loading