Probability Sampler (gabime#136)

sdk/include/opentelemetry/sdk/trace/samplers/probability.h

@@ -0,0 +1,50 @@
+#pragma once
+
+#include "opentelemetry/sdk/trace/sampler.h"
+
+OPENTELEMETRY_BEGIN_NAMESPACE
+namespace sdk
+{
+namespace trace
+{
+namespace trace_api = opentelemetry::trace;
+/**
+ * The probability sampler, based on it's configuration, should either defer the
+ * decision to sample to it's parent, or compute and return a decision based on
+ * the provided trace_id and probability.
+ */
+class ProbabilitySampler : public Sampler
+{
+public:
+  /**
+   * @param probability a required value, 1.0 >= probability >= 0.0, that given any
+   * random trace_id, ShouldSample will return RECORD_AND_SAMPLE
+   * @throws invalid_argument if probability is out of bounds [0.0, 1.0]
+   */
+  explicit ProbabilitySampler(double probability);
+
+  /**
+   * @return Returns either RECORD_AND_SAMPLE or NOT_RECORD based on current
+   * sampler configuration and provided parent_context / tracer_id. tracer_id
+   * is used as a pseudorandom value in conjunction with the predefined
+   * threshold to determine whether this trace should be sampled
+   */
+  SamplingResult ShouldSample(
+    const trace_api::SpanContext *parent_context,
+    trace_api::TraceId trace_id,
+    nostd::string_view /*name*/,
+    trace_api::SpanKind /*span_kind*/,
+    const trace_api::KeyValueIterable & /*attributes*/) noexcept override;
+
+  /**
+   * @return Description MUST be ProbabilitySampler{0.000100}
+   */
+  std::string GetDescription() const noexcept override;
+
+private:
+  std::string sampler_description_;
+  const uint64_t threshold_;
+};
+}  // namespace trace
+}  // namespace sdk
+OPENTELEMETRY_END_NAMESPACE

sdk/src/trace/CMakeLists.txt

@@ -1 +1,2 @@
-add_library(opentelemetry_trace tracer_provider.cc tracer.cc span.cc samplers/parent_or_else.cc)
+add_library(opentelemetry_trace tracer_provider.cc tracer.cc span.cc
+	        samplers/parent_or_else.cc samplers/probability.cc)

sdk/src/trace/samplers/probability.cc

@@ -0,0 +1,114 @@
+// Copyright 2020, Open Telemetry Authors
+// Copyright 2017, OpenCensus Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "opentelemetry/sdk/trace/samplers/probability.h"
+
+#include <cmath>
+#include <cstdint>
+#include <stdexcept>
+
+namespace trace_api = opentelemetry::trace;
+
+namespace
+{
+  /**
+   * Converts a probability in [0, 1] to a threshold in [0, UINT64_MAX]
+   *
+   * @param probability a required value top be converted to uint64_t. is
+   * bounded by 1 >= probability >= 0.
+   * @return Returns threshold value computed after converting probability to
+   * uint64_t datatype
+   */
+  uint64_t CalculateThreshold(double probability) noexcept
+  {
+    if (probability <= 0.0) return 0; 
+    if (probability >= 1.0) return UINT64_MAX;
+
+    // We can't directly return probability * UINT64_MAX.
+    //
+    // UINT64_MAX is (2^64)-1, but as a double rounds up to 2^64.
+    // For probabilities >= 1-(2^-54), the product wraps to zero!
+    // Instead, calculate the high and low 32 bits separately.
+    const double product = UINT32_MAX * probability;
+    double hi_bits, lo_bits = ldexp(modf(product, &hi_bits), 32) + product;
+    return (static_cast<uint64_t>(hi_bits) << 32) +
+           static_cast<uint64_t>(lo_bits);
+  }
+
+  /**
+   * @param trace_id a required value to be converted to uint64_t. trace_id must
+   * at least 8 bytes long
+   * @return Returns threshold value computed after converting trace_id to
+   * uint64_t datatype
+   */
+  uint64_t CalculateThresholdFromBuffer(const trace_api::TraceId& trace_id) noexcept
+  {
+    // We only use the first 8 bytes of TraceId.
+    static_assert(trace_api::TraceId::kSize >= 8, "TraceID must be at least 8 bytes long.");
+
+    uint64_t res = 0;
+    std::memcpy(&res, &trace_id, 8);
+
+    double probability = (double) res / UINT64_MAX;
+
+    return CalculateThreshold(probability);
+  }
+} // namespace
+
+OPENTELEMETRY_BEGIN_NAMESPACE
+namespace sdk
+{
+namespace trace
+{
+ProbabilitySampler::ProbabilitySampler(double probability)
+: threshold_(CalculateThreshold(probability))
+{
+    if (probability > 1.0) probability = 1.0;
+    if (probability < 0.0) probability = 0.0;
+    sampler_description_ = "ProbabilitySampler{" + std::to_string(probability) + "}";
+  }
+
+SamplingResult ProbabilitySampler::ShouldSample(
+  const trace_api::SpanContext *parent_context,
+  trace_api::TraceId trace_id,
+  nostd::string_view /*name*/,
+  trace_api::SpanKind /*span_kind*/,
+  const trace_api::KeyValueIterable & /*attributes*/) noexcept
+{
+  if (parent_context && !parent_context->HasRemoteParent()) {
+    if (parent_context->IsSampled()) {
+      return { Decision::RECORD_AND_SAMPLE, nullptr };
+    } else {
+      return { Decision::NOT_RECORD, nullptr };
+    }
+  }
+
+  if (threshold_ == 0) return { Decision::NOT_RECORD, nullptr };
+
+  if (CalculateThresholdFromBuffer(trace_id) <= threshold_)
+  {
+    return { Decision::RECORD_AND_SAMPLE, nullptr };
+  }
+
+  return { Decision::NOT_RECORD, nullptr };
+}
+
+std::string ProbabilitySampler::GetDescription() const noexcept
+{
+  return sampler_description_;
+}
+}  // namespace trace
+}  // namespace sdk
+OPENTELEMETRY_END_NAMESPACE

sdk/test/trace/BUILD

@@ -74,3 +74,15 @@ cc_test(
         "@com_google_googletest//:gtest_main",
     ],
 )
+
+cc_test(
+    name = "probability_sampler_test",
+    srcs = [
+        "probability_sampler_test.cc",
+    ],
+    deps = [
+        "//sdk/src/trace",
+        "//sdk/src/common:random",
+        "@com_google_googletest//:gtest_main",
+    ],
+)

sdk/test/trace/CMakeLists.txt

@@ -1,8 +1,8 @@
 foreach(testname tracer_provider_test span_data_test simple_processor_test
                  tracer_test always_off_sampler_test always_on_sampler_test 
-                 parent_or_else_sampler_test)
+                 parent_or_else_sampler_test probability_sampler_test)
   add_executable(${testname} "${testname}.cc")
   target_link_libraries(${testname} ${GTEST_BOTH_LIBRARIES}
-                        ${CMAKE_THREAD_LIBS_INIT} opentelemetry_trace)
+                        ${CMAKE_THREAD_LIBS_INIT} opentelemetry_common opentelemetry_trace)
   gtest_add_tests(TARGET ${testname} TEST_PREFIX trace. TEST_LIST ${testname})
 endforeach()