make the rules sample limiter and default sampling rate configurable

include/datadog/opentracing.h

@@ -66,21 +66,30 @@ struct TracerOptions {
   // The environment this trace belongs to. eg. "" (env:none), "staging", "prod". Can also be set
   // by the environment variable DD_ENV
   std::string environment = "";
-  // This option is deprecated and may be removed in future releases.
-  // It is equivalent to setting a sampling rule with only a "sample_rate".
-  // Values must be between 0.0 and 1.0 (inclusive)
+  // `sample_rate` is the default sampling rate for any trace unmatched by a
+  // sampling rule.  Setting `sample_rate` is equivalent to appending to
+  // `sampling_rules` a rule whose "sample_rate" is `sample_rate`.  If
+  // `sample_rate` is NaN, then no default rule is added, and traces not
+  // matching any sampling rule are subject to "priority sampling," where the
+  // sampling rate is determined by the Datadog trace agent.  This option is
+  // also configurable as the environment variable DD_TRACE_SAMPLE_RATE.
   double sample_rate = std::nan("");
   // This option is deprecated, and may be removed in future releases.
   bool priority_sampling = true;
-  // Rules sampling is applied when initiating traces to determine the sampling rate.
-  // Traces that do not match any rules fall back to using priority sampling, where the rate is
-  // determined by a combination of user-assigned priorities and configuration from the agent.
-  // Configuration is specified as a JSON array of objects. Each object must have a "sample_rate",
-  // and the "name" and "service" fields are optional. The "sample_rate" value must be between 0.0
-  // and 1.0 (inclusive). Rules are applied in configured order, so a specific match should be
-  // specified before a wider match. If any rules are invalid, they are ignored. Can also be set by
-  // the environment variable DD_TRACE_SAMPLING_RULES.
-  std::string sampling_rules = R"([{"sample_rate": 1.0}])";
+  // Rules sampling is applied when initiating traces to determine the sampling
+  // rate.  Configuration is specified as a JSON array of objects. Each object
+  // must have a "sample_rate", while the "name" and "service" fields are
+  // optional. The "sample_rate" value must be between 0.0 and 1.0 (inclusive).
+  // Rules are checked in order, so a more specific rule should be specified
+  // before a less specific rule.  Note that if the `sample_rate` field of this
+  // `TracerOptions` has a non-NaN value, then there is an implicit rule at the
+  // end of the list that matches any trace unmatched by other rules, and
+  // applies a sampling rate of `sample_rate`.  If no rule matches a trace,
+  // then "priority sampling" is applied instead, where the sample rate is
+  // determined by the Datadog trace agent.  If any rules are invalid, they are
+  // ignored. This option is also configurable as the environment variable
+  // DD_TRACE_SAMPLING_RULES.
+  std::string sampling_rules = "[]";
   // Max amount of time to wait between sending traces to agent, in ms. Agent discards traces older
   // than 10s, so that is the upper bound.
   int64_t write_period_ms = 1000;
@@ -134,6 +143,11 @@ struct TracerOptions {
     }
     std::cerr << level_str + ": " + message.data() + "\n";
   };
+  // `sampling_limit_per_second` is the limit on the number of rule-controlled
+  // traces that may be sampled per second.  This includes traces that match
+  // the implicit "catch-all" rule appended to `sampling_rules`.  This option
+  // is also configurable as the environment variable DD_TRACE_RATE_LIMIT.
+  double sampling_limit_per_second = 100;
 };
 
 // TraceEncoder exposes the data required to encode and submit traces to the

src/limiter.cpp

@@ -1,6 +1,7 @@
 #include "limiter.h"
 
 #include <algorithm>
+#include <cmath>
 #include <iostream>
 #include <numeric>
 
@@ -27,6 +28,9 @@ Limiter::Limiter(TimeProvider now_func, long max_tokens, double refresh_rate,
   previous_rates_sum_ = std::accumulate(previous_rates_.begin(), previous_rates_.end(), 0.0);
 }
 
+Limiter::Limiter(TimeProvider now_func, double allowed_per_second)
+    : Limiter(now_func, long(std::ceil(allowed_per_second)), allowed_per_second, 1) {}
+
 LimitResult Limiter::allow() { return allow(1); }
 
 LimitResult Limiter::allow(long tokens_requested) {

src/limiter.h

@@ -17,6 +17,7 @@ struct LimitResult {
 class Limiter {
  public:
   Limiter(TimeProvider clock, long max_tokens, double refresh_rate, long tokens_per_refresh);
+  Limiter(TimeProvider clock, double allowed_per_second);
 
   LimitResult allow();
   LimitResult allow(long tokens);

src/opentracing_agent.cpp

@@ -30,7 +30,7 @@ std::shared_ptr<ot::Tracer> makeTracer(const TracerOptions &options) {
   TracerOptions opts = maybe_options.value();
 
   auto logger = makeLogger(opts);
-  auto sampler = std::make_shared<RulesSampler>();
+  auto sampler = std::make_shared<RulesSampler>(opts.sampling_limit_per_second);
   auto writer = std::shared_ptr<Writer>{
       new AgentWriter(opts.agent_host, opts.agent_port, opts.agent_url,
                       std::chrono::milliseconds(llabs(opts.write_period_ms)), sampler, logger)};

src/opentracing_external.cpp

@@ -39,7 +39,7 @@ std::tuple<std::shared_ptr<ot::Tracer>, std::shared_ptr<TraceEncoder>> makeTrace
   }
   TracerOptions opts = maybe_options.value();
 
-  auto sampler = std::make_shared<RulesSampler>();
+  auto sampler = std::make_shared<RulesSampler>(opts.sampling_limit_per_second);
   auto writer = std::make_shared<ExternalWriter>(sampler, logger);
   auto encoder = writer->encoder();
   return std::tuple<std::shared_ptr<ot::Tracer>, std::shared_ptr<TraceEncoder>>{

src/sample.cpp

@@ -72,6 +72,9 @@ void PrioritySampler::configure(json config) {
 
 RulesSampler::RulesSampler() : sampling_limiter_(getRealTime, 100, 100.0, 1) {}
 
+RulesSampler::RulesSampler(double limit_per_second)
+    : sampling_limiter_(getRealTime, limit_per_second) {}
+
 RulesSampler::RulesSampler(TimeProvider clock, long max_tokens, double refresh_rate,
                            long tokens_per_refresh)
     : sampling_limiter_(clock, max_tokens, refresh_rate, tokens_per_refresh) {}

src/sample.h

@@ -69,6 +69,7 @@ using RuleFunc = std::function<RuleResult(const std::string&, const std::string&
 class RulesSampler {
  public:
   RulesSampler();
+  explicit RulesSampler(double limit_per_second);
   RulesSampler(TimeProvider clock, long max_tokens, double refresh_rate, long tokens_per_refresh);
   // Some of the member functions of this class are declared `virtual` so that
   // they can be overridden by `MockRulesSampler` for use in unit tests.

src/tracer.cpp

@@ -11,6 +11,7 @@
 #include <unistd.h>
 #endif
 
+#include <cmath>
 #include <fstream>
 #include <random>
 #include <sstream>
@@ -123,7 +124,11 @@ void startupLog(TracerOptions &options) {
   }
   j["analytics_enabled"] = options.analytics_enabled;
   j["analytics_sample_rate"] = options.analytics_rate;
+  if (!std::isnan(options.sample_rate)) {
+    j["sample_rate"] = options.sample_rate;
+  }
   j["sampling_rules"] = options.sampling_rules;
+  j["sampling_limit_per_second"] = options.sampling_limit_per_second;
   if (!options.tags.empty()) {
     j["tags"] = options.tags;
   }
@@ -167,71 +172,83 @@ uint64_t traceTagsPropagationMaxLength(const TracerOptions & /*options*/, const
 
 }  // namespace
 
-void Tracer::configureRulesSampler(std::shared_ptr<RulesSampler> sampler) noexcept try {
-  auto log_invalid_json = [&](const std::string &description, json &object) {
-    logger_->Log(LogLevel::info, description + ": " + object.get<std::string>());
-  };
-  json config = json::parse(opts_.sampling_rules);
-  for (auto &item : config.items()) {
-    auto rule = item.value();
-    if (!rule.is_object()) {
-      log_invalid_json("rules sampler: unexpected item in sampling rules", rule);
-      continue;
-    }
-    // "sample_rate" is mandatory
-    if (!rule.contains("sample_rate")) {
-      log_invalid_json("rules sampler: rule is missing 'sample_rate'", rule);
-      continue;
-    }
-    if (!rule.at("sample_rate").is_number()) {
-      log_invalid_json("rules sampler: invalid type for 'sample_rate' (expected number)", rule);
-      continue;
-    }
-    auto sample_rate = rule.at("sample_rate").get<json::number_float_t>();
-    if (!(sample_rate >= 0.0 && sample_rate <= 1.0)) {
-      log_invalid_json(
-          "rules sampler: invalid value for sample rate (expected value between 0.0 and 1.0)",
-          rule);
-    }
-    // "service" and "name" are optional
-    bool has_service = rule.contains("service") && rule.at("service").is_string();
-    bool has_name = rule.contains("name") && rule.at("name").is_string();
-    auto nan = std::nan("");
-    if (has_service && has_name) {
-      auto svc = rule.at("service").get<std::string>();
-      auto nm = rule.at("name").get<std::string>();
-      sampler->addRule([=](const std::string &service, const std::string &name) -> RuleResult {
-        if (service == svc && name == nm) {
-          return {true, sample_rate};
-        }
-        return {false, nan};
-      });
-    } else if (has_service) {
-      auto svc = rule.at("service").get<std::string>();
-      sampler->addRule([=](const std::string &service, const std::string &) -> RuleResult {
-        if (service == svc) {
-          return {true, sample_rate};
-        }
-        return {false, nan};
-      });
-    } else if (has_name) {
-      auto nm = rule.at("name").get<std::string>();
-      sampler->addRule([=](const std::string &, const std::string &name) -> RuleResult {
-        if (name == nm) {
+void Tracer::configureRulesSampler(std::shared_ptr<RulesSampler> sampler) noexcept {
+  try {
+    auto log_invalid_json = [&](const std::string &description, json &object) {
+      logger_->Log(LogLevel::info, description + ": " + object.get<std::string>());
+    };
+    json config = json::parse(opts_.sampling_rules);
+    for (auto &item : config.items()) {
+      auto rule = item.value();
+      if (!rule.is_object()) {
+        log_invalid_json("rules sampler: unexpected item in sampling rules", rule);
+        continue;
+      }
+      // "sample_rate" is mandatory
+      if (!rule.contains("sample_rate")) {
+        log_invalid_json("rules sampler: rule is missing 'sample_rate'", rule);
+        continue;
+      }
+      if (!rule.at("sample_rate").is_number()) {
+        log_invalid_json("rules sampler: invalid type for 'sample_rate' (expected number)", rule);
+        continue;
+      }
+      auto sample_rate = rule.at("sample_rate").get<json::number_float_t>();
+      if (!(sample_rate >= 0.0 && sample_rate <= 1.0)) {
+        log_invalid_json(
+            "rules sampler: invalid value for sample rate (expected value between 0.0 and 1.0)",
+            rule);
+      }
+      // "service" and "name" are optional
+      bool has_service = rule.contains("service") && rule.at("service").is_string();
+      bool has_name = rule.contains("name") && rule.at("name").is_string();
+      auto nan = std::nan("");
+      if (has_service && has_name) {
+        auto svc = rule.at("service").get<std::string>();
+        auto nm = rule.at("name").get<std::string>();
+        sampler->addRule([=](const std::string &service, const std::string &name) -> RuleResult {
+          if (service == svc && name == nm) {
+            return {true, sample_rate};
+          }
+          return {false, nan};
+        });
+      } else if (has_service) {
+        auto svc = rule.at("service").get<std::string>();
+        sampler->addRule([=](const std::string &service, const std::string &) -> RuleResult {
+          if (service == svc) {
+            return {true, sample_rate};
+          }
+          return {false, nan};
+        });
+      } else if (has_name) {
+        auto nm = rule.at("name").get<std::string>();
+        sampler->addRule([=](const std::string &, const std::string &name) -> RuleResult {
+          if (name == nm) {
+            return {true, sample_rate};
+          }
+          return {false, nan};
+        });
+      } else {
+        sampler->addRule([=](const std::string &, const std::string &) -> RuleResult {
           return {true, sample_rate};
-        }
-        return {false, nan};
-      });
-    } else {
-      sampler->addRule([=](const std::string &, const std::string &) -> RuleResult {
-        return {true, sample_rate};
-      });
+        });
+      }
     }
+  } catch (const json::parse_error &error) {
+    std::ostringstream message;
+    message << "rules sampler: unable to parse JSON config for rules sampler: " << error.what();
+    logger_->Log(LogLevel::error, message.str());
+  }
+
+  // If there is a configured overall sample rate, add an automatic "catch all"
+  // rule to the end that samples at that rate.  Otherwise, don't (unmatched
+  // traces will be subject to priority sampling).
+  const double sample_rate = opts_.sample_rate;
+  if (!std::isnan(sample_rate)) {
+    sampler->addRule([=](const std::string &, const std::string &) -> RuleResult {
+      return {true, sample_rate};
+    });
   }
-} catch (const json::parse_error &error) {
-  std::ostringstream message;
-  message << "rules sampler: unable to parse JSON config for rules sampler: " << error.what();
-  logger_->Log(LogLevel::error, message.str());
 }
 
 Tracer::Tracer(TracerOptions options, std::shared_ptr<SpanBuffer> buffer, TimeProvider get_time,

src/tracer_factory.cpp

@@ -10,7 +10,7 @@ namespace opentracing {
 
 ot::expected<TracerOptions> optionsFromConfig(const char *configuration,
                                               std::string &error_message) {
-  TracerOptions options{"localhost", 8126, "", "web", "", 1.0};
+  TracerOptions options;
   json config;
   try {
     config = json::parse(configuration);
@@ -85,6 +85,9 @@ ot::expected<TracerOptions> optionsFromConfig(const char *configuration,
     if (config.find("dd.trace.analytics-sample-rate") != config.end()) {
       config.at("dd.trace.analytics-sample-rate").get_to(options.analytics_rate);
     }
+    if (config.find("sampling_limit_per_second") != config.end()) {
+      config.at("sampling_limit_per_second").get_to(options.sampling_limit_per_second);
+    }
   } catch (const nlohmann::detail::type_error &) {
     error_message = "configuration has an argument with an incorrect type";
     return ot::make_unexpected(std::make_error_code(std::errc::invalid_argument));

src/tracer_factory.h

@@ -19,28 +19,9 @@ ot::expected<TracerOptions> optionsFromConfig(const char *configuration,
 template <class TracerImpl>
 class TracerFactory : public ot::TracerFactory {
  public:
-  // Accepts configuration in JSON format, with the following keys:
-  // "service": Required. A string, the name of the service.
-  // "agent_host": A string, defaults to localhost. Can also be set by the environment variable
-  //     DD_AGENT_HOST
-  // "agent_port": A number, defaults to 8126. "type": A string, defaults to web. Can also be set
-  //     by the environment variable DD_TRACE_AGENT_PORT
-  // "type": A string, defaults to web.
-  // "environment": A string, defaults to "". The environment this trace belongs to.
-  //     eg. "" (env:none), "staging", "prod". Can also be set by the environment variable
-  //     DD_ENV
-  // "sample_rate": A double, defaults to 1.0.
-  // "operation_name_override": A string, if not empty it overrides the operation name (and the
-  //     overridden operation name is recorded in the tag "operation").
-  // "propagation_style_extract": A list of strings, each string is one of "Datadog", "B3".
-  //     Defaults to ["Datadog"]. The type of headers to use to propagate
-  //     distributed traces. Can also be set by the environment variable
-  //     DD_PROPAGATION_STYLE_EXTRACT.
-  // "propagation_style_inject": A list of strings, each string is one of "Datadog", "B3". Defaults
-  //     to ["Datadog"]. The type of headers to use to receive distributed traces. Can also be set
-  //     by the environment variable DD_PROPAGATION_STYLE_INJECT.
-  //
-  // Extra keys will be ignored.
+  // Accepts configuration as a JSON object.  See `optionsFromConfig` in
+  // tracer_factory.cpp for a list of supported attributes.  Unsupported
+  // attributes are ignored.
   ot::expected<std::shared_ptr<ot::Tracer>> MakeTracer(const char *configuration,
                                                        std::string &error_message) const
       noexcept override;