Proposal: create metrics from traces and agent based sampling

opentelemetry-datadog/CHANGELOG.md

@@ -12,6 +12,12 @@
 
 - Bump opentelemetry version to 0.22, opentelemetry_sdk version to 0.22
 
+### Changed
+
+- allow send all traces to `datadog-agent` with `agent-sampling` feature.
+- allow `datadog-agent` generate metrics from spans for [APM](https://docs.datadoghq.com/tracing/metrics/).
+
+
 ## v0.9.0
 
 ### Changed

opentelemetry-datadog/Cargo.toml

@@ -19,6 +19,7 @@ all-features = true
 rustdoc-args = ["--cfg", "docsrs"]
 
 [features]
+agent-sampling = []
 reqwest-blocking-client = ["reqwest/blocking", "opentelemetry-http/reqwest"]
 reqwest-client = ["reqwest", "opentelemetry-http/reqwest"]
 
@@ -49,3 +50,8 @@ opentelemetry_sdk = { workspace = true, features = ["trace", "testing"] }
 [[example]]
 name = "datadog"
 path = "examples/datadog.rs"
+
+[[example]]
+name = "agent_sampling"
+path = "examples/agent_sampling.rs"
+required-features = ["agent-sampling"]

opentelemetry-datadog/README.md

@@ -24,6 +24,7 @@ to [`Datadog`].
 
 `opentelemetry-datadog` supports following features:
 
+- `agent-sampling`: move decision making about sampling to `datadog-agent` (see `agent_sampling.rs` example).
 - `reqwest-blocking-client`: use `reqwest` blocking http client to send spans.
 - `reqwest-client`: use `reqwest` http client to send spans.
 - `surf-client`: use `surf` http client to send spans.

opentelemetry-datadog/examples/agent_sampling.rs

@@ -0,0 +1,78 @@
+use opentelemetry::{
+    global::{self, shutdown_tracer_provider},
+    trace::{SamplingResult, Span, TraceContextExt, Tracer},
+    Key,
+};
+use opentelemetry_datadog::{new_pipeline, ApiVersion, DatadogTraceStateBuilder};
+use opentelemetry_sdk::trace::{self, RandomIdGenerator, ShouldSample};
+use std::thread;
+use std::time::Duration;
+
+fn bar() {
+    let tracer = global::tracer("component-bar");
+    let mut span = tracer.start("bar");
+    span.set_attribute(Key::new("span.type").string("sql"));
+    span.set_attribute(Key::new("sql.query").string("SELECT * FROM table"));
+    thread::sleep(Duration::from_millis(6));
+    span.end()
+}
+
+#[derive(Debug, Clone)]
+struct AgentBasedSampler;
+
+impl ShouldSample for AgentBasedSampler {
+    fn should_sample(
+        &self,
+        parent_context: Option<&opentelemetry::Context>,
+        _trace_id: opentelemetry::trace::TraceId,
+        _name: &str,
+        _span_kind: &opentelemetry::trace::SpanKind,
+        _attributes: &[opentelemetry::KeyValue],
+        _links: &[opentelemetry::trace::Link],
+    ) -> opentelemetry::trace::SamplingResult {
+        let trace_state = parent_context
+            .map(
+                |parent_context| parent_context.span().span_context().trace_state().clone(), // inherit sample decision from parent span
+            )
+            .unwrap_or_else(|| {
+                DatadogTraceStateBuilder::default()
+                    .with_priority_sampling(true) // always sample root span(span without remote or local parent)
+                    .with_measuring(true) // datadog-agent will create metric for this span for APM
+                    .build()
+            });
+
+        SamplingResult {
+            decision: opentelemetry::trace::SamplingDecision::RecordAndSample, // send all spans to datadog-agent
+            attributes: vec![],
+            trace_state,
+        }
+    }
+}
+
+fn main() -> Result<(), Box<dyn std::error::Error + Send + Sync + 'static>> {
+    let tracer = new_pipeline()
+        .with_service_name("agent-sampling-demo")
+        .with_api_version(ApiVersion::Version05)
+        .with_trace_config(
+            trace::config()
+                .with_sampler(AgentBasedSampler)
+                .with_id_generator(RandomIdGenerator::default()),
+        )
+        .install_simple()?;
+
+    tracer.in_span("foo", |cx| {
+        let span = cx.span();
+        span.set_attribute(Key::new("span.type").string("web"));
+        span.set_attribute(Key::new("http.url").string("http://localhost:8080/foo"));
+        span.set_attribute(Key::new("http.method").string("GET"));
+        span.set_attribute(Key::new("http.status_code").i64(200));
+
+        thread::sleep(Duration::from_millis(6));
+        bar();
+        thread::sleep(Duration::from_millis(6));
+    });
+
+    shutdown_tracer_provider();
+
+    Ok(())
+}

opentelemetry-datadog/src/exporter/model/mod.rs

@@ -20,6 +20,9 @@ mod v05;
 // https://github.com/DataDog/dd-trace-js/blob/c89a35f7d27beb4a60165409376e170eacb194c5/packages/dd-trace/src/constants.js#L4
 static SAMPLING_PRIORITY_KEY: &str = "_sampling_priority_v1";
 
+// https://github.com/DataDog/datadog-agent/blob/ec96f3c24173ec66ba235bda7710504400d9a000/pkg/trace/traceutil/span.go#L20
+static DD_MEASURED_KEY: &str = "_dd.measured";
+
 /// Custom mapping between opentelemetry spans and datadog spans.
 ///
 /// User can provide custom function to change the mapping. It currently supports customizing the following

opentelemetry-datadog/src/exporter/model/v05.rs

@@ -1,13 +1,15 @@
 use crate::exporter::intern::StringInterner;
-use crate::exporter::model::SAMPLING_PRIORITY_KEY;
+use crate::exporter::model::{DD_MEASURED_KEY, SAMPLING_PRIORITY_KEY};
 use crate::exporter::{Error, ModelConfig};
+use crate::propagator::DatadogTraceState;
 use opentelemetry::trace::Status;
 use opentelemetry_sdk::export::trace::SpanData;
 use std::time::SystemTime;
 
 use super::unified_tags::{UnifiedTagField, UnifiedTags};
 
 const SPAN_NUM_ELEMENTS: u32 = 12;
+const METRICS_LEN: u32 = 2;
 const GIT_META_TAGS_COUNT: u32 = if matches!(
     (
         option_env!("DD_GIT_REPOSITORY_URL"),
@@ -125,6 +127,28 @@
     Ok(())
 }
 
+#[cfg(not(feature = "agent-sampling"))]
+fn get_sampling_priority(_span: &SpanData) -> f64 {
+    1.0
+}
+
+#[cfg(feature = "agent-sampling")]
+fn get_sampling_priority(span: &SpanData) -> f64 {
+    if span.span_context.trace_state().priority_sampling_enabled() {
+        1.0
+    } else {
+        0.0
+    }
+}
+
+fn get_measuring(span: &SpanData) -> f64 {
+    if span.span_context.trace_state().measuring_enabled() {
+        1.0
+    } else {
+        0.0
+    }
+}
+
 fn encode_traces<S, N, R>(
     interner: &mut StringInterner,
     model_config: &ModelConfig,
@@ -228,16 +252,14 @@
                 rmp::encode::write_u32(&mut encoded, interner.intern(commit_sha))?;
             }
 
-            rmp::encode::write_map_len(&mut encoded, 1)?;
+            rmp::encode::write_map_len(&mut encoded, METRICS_LEN)?;
             rmp::encode::write_u32(&mut encoded, interner.intern(SAMPLING_PRIORITY_KEY))?;
-            rmp::encode::write_f64(
-                &mut encoded,
-                if span.span_context.is_sampled() {
-                    1.0
-                } else {
-                    0.0
-                },
-            )?;
+            let sampling_priority = get_sampling_priority(&span);
+            rmp::encode::write_f64(&mut encoded, sampling_priority)?;
+
+            rmp::encode::write_u32(&mut encoded, interner.intern(DD_MEASURED_KEY))?;
+            let measuring = get_measuring(&span);
+            rmp::encode::write_f64(&mut encoded, measuring)?;
             rmp::encode::write_u32(&mut encoded, span_type)?;
         }
     }