Add ParentBasedAlwaysOnSampler as default sampler for OTLP trace export mode

dd-trace-core/src/main/java/datadog/trace/common/sampling/ParentBasedAlwaysOnSampler.java

@@ -0,0 +1,24 @@
+package datadog.trace.common.sampling;
+
+import datadog.trace.api.sampling.PrioritySampling;
+import datadog.trace.api.sampling.SamplingMechanism;
+import datadog.trace.core.CoreSpan;
+
+/**
+ * Implements the OpenTelemetry {@code parentbased_always_on} sampler.
+ *
+ * <p>Root spans are always sampled. Child spans inherit the sampling decision from their parent,
+ * which is handled by the context propagation layer.
+ */
+public class ParentBasedAlwaysOnSampler implements Sampler, PrioritySampler {
+
+  @Override
+  public <T extends CoreSpan<T>> boolean sample(final T span) {
+    return true;
+  }
+
+  @Override
+  public <T extends CoreSpan<T>> void setSamplingPriority(final T span) {
+    span.setSamplingPriority(PrioritySampling.SAMPLER_KEEP, SamplingMechanism.DEFAULT);
+  }
+}

dd-trace-core/src/main/java/datadog/trace/common/sampling/Sampler.java

@@ -80,6 +80,7 @@ public static Sampler forConfig(final Config config, final TraceConfig traceConf
             log.error("Invalid sampler configuration. Using AllSampler", e);
             sampler = new AllSampler();
           }
+          // TODO: if OTLP trace export enabled, select ParentBasedAlwaysOnSampler here
         } else if (config.isPrioritySamplingEnabled()) {
           if (KEEP.equalsIgnoreCase(config.getPrioritySamplingForce())) {
             log.debug("Force Sampling Priority to: SAMPLER_KEEP.");

dd-trace-core/src/test/java/datadog/trace/common/sampling/ParentBasedAlwaysOnSamplerTest.java

@@ -0,0 +1,124 @@
+package datadog.trace.common.sampling;
+
+import static datadog.trace.api.TracePropagationStyle.DATADOG;
+import static datadog.trace.api.sampling.PrioritySampling.SAMPLER_DROP;
+import static datadog.trace.api.sampling.PrioritySampling.SAMPLER_KEEP;
+import static datadog.trace.api.sampling.PrioritySampling.USER_DROP;
+import static datadog.trace.api.sampling.PrioritySampling.USER_KEEP;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import datadog.trace.api.DDTraceId;
+import datadog.trace.common.writer.ListWriter;
+import datadog.trace.common.writer.RemoteResponseListener;
+import datadog.trace.core.CoreTracer;
+import datadog.trace.core.DDSpan;
+import datadog.trace.core.propagation.ExtractedContext;
+import datadog.trace.core.propagation.PropagationTags;
+import java.util.stream.Stream;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+
+class ParentBasedAlwaysOnSamplerTest {
+
+  private final ListWriter writer = new ListWriter();
+  private CoreTracer tracer;
+
+  @AfterEach
+  void tearDown() {
+    if (tracer != null) {
+      tracer.close();
+    }
+  }
+
+  private CoreTracer buildTracer(ParentBasedAlwaysOnSampler sampler) {
+    tracer = CoreTracer.builder().writer(writer).sampler(sampler).build();
+    return tracer;
+  }
+
+  @Test
+  void alwaysSamplesSpans() {
+    ParentBasedAlwaysOnSampler sampler = new ParentBasedAlwaysOnSampler();
+    CoreTracer tracer = buildTracer(sampler);
+
+    DDSpan span = (DDSpan) tracer.buildSpan("test").start();
+    try {
+      assertTrue(sampler.sample(span));
+    } finally {
+      span.finish();
+    }
+  }
+
+  @Test
+  void setsSamplingPriorityToSamplerKeep() {
+    ParentBasedAlwaysOnSampler sampler = new ParentBasedAlwaysOnSampler();
+    CoreTracer tracer = buildTracer(sampler);
+
+    DDSpan span = (DDSpan) tracer.buildSpan("test").start();
+    try {
+      sampler.setSamplingPriority(span);
+      assertEquals(SAMPLER_KEEP, span.getSamplingPriority());
+    } finally {
+      span.finish();
+    }
+  }
+
+  @Test
+  void childSpanInheritsSamplingPriorityFromLocalParent() {
+    ParentBasedAlwaysOnSampler sampler = new ParentBasedAlwaysOnSampler();
+    CoreTracer tracer = buildTracer(sampler);
+
+    DDSpan rootSpan = (DDSpan) tracer.buildSpan("root").start();
+    sampler.setSamplingPriority(rootSpan);
+    DDSpan childSpan = (DDSpan) tracer.buildSpan("child").asChildOf(rootSpan.context()).start();
+    try {
+      assertEquals(SAMPLER_KEEP, rootSpan.getSamplingPriority());
+      assertEquals(SAMPLER_KEEP, childSpan.getSamplingPriority());
+    } finally {
+      childSpan.finish();
+      rootSpan.finish();
+    }
+  }
+
+  static Stream<Arguments> childSpanInheritsSamplingDecisionFromRemoteParentArguments() {
+    return Stream.of(
+        Arguments.arguments("sampler keep", SAMPLER_KEEP),
+        Arguments.arguments("sampler drop", SAMPLER_DROP),
+        Arguments.arguments("user keep", USER_KEEP),
+        Arguments.arguments("user drop", USER_DROP));
+  }
+
+  @ParameterizedTest(name = "child span inherits sampling decision from remote parent: {0}")
+  @MethodSource("childSpanInheritsSamplingDecisionFromRemoteParentArguments")
+  void childSpanInheritsSamplingDecisionFromRemoteParent(String scenario, int parentPriority) {
+    ParentBasedAlwaysOnSampler sampler = new ParentBasedAlwaysOnSampler();
+    CoreTracer tracer = buildTracer(sampler);
+
+    ExtractedContext extractedContext =
+        new ExtractedContext(
+            DDTraceId.ONE, 2, parentPriority, null, PropagationTags.factory().empty(), DATADOG);
+
+    DDSpan span = (DDSpan) tracer.buildSpan("child").asChildOf(extractedContext).start();
+    try {
+      assertEquals(parentPriority, span.getSamplingPriority());
+    } finally {
+      span.finish();
+    }
+  }
+
+  @Test
+  void isNotARemoteResponseListener() {
+    ParentBasedAlwaysOnSampler sampler = new ParentBasedAlwaysOnSampler();
+    assertFalse(sampler instanceof RemoteResponseListener);
+  }
+
+  @Test
+  void implementsPrioritySampler() {
+    ParentBasedAlwaysOnSampler sampler = new ParentBasedAlwaysOnSampler();
+    assertTrue(sampler instanceof PrioritySampler);
+  }
+}