Introduce a configurable sample rate

lib/spandex.ex

@@ -503,12 +503,14 @@ defmodule Spandex do
   defp do_start_trace(name, opts) do
     strategy = opts[:strategy]
     adapter = opts[:adapter]
+    sample_rate = Keyword.get(opts, :sample_rate, 1.0)
+    priority = calculate_priority(sample_rate)
     trace_id = adapter.trace_id()
     span_context = %SpanContext{trace_id: trace_id}
 
     with {:ok, span} <- span(name, opts, span_context, adapter) do
       Logger.metadata(trace_id: to_string(trace_id), span_id: to_string(span.id))
-      trace = %Trace{spans: [], stack: [span], id: trace_id}
+      trace = %Trace{spans: [], stack: [span], id: trace_id, priority: priority}
       strategy.put_trace(opts[:trace_key], trace)
     end
   end
@@ -548,6 +550,7 @@ defmodule Spandex do
     |> Keyword.put(:parent_id, span_context.parent_id)
     |> Keyword.put(:start, adapter.now())
     |> Keyword.put(:id, adapter.span_id())
+    |> Keyword.put(:priority, span_context.priority)
     |> Span.new()
   end
 
@@ -557,4 +560,13 @@ defmodule Spandex do
       {:ok, span} -> span
     end
   end
+
+  defp calculate_priority(sample_rate) do
+    cond do
+      sample_rate == 0 -> 0
+      sample_rate == 1 -> 1
+      :rand.uniform() <= sample_rate -> 1
+      true -> 0
+    end
+  end
 end

lib/tracer.ex

@@ -51,6 +51,7 @@ defmodule Spandex.Tracer do
                    service: :atom,
                    disabled?: :boolean,
                    env: :string,
+                   sample_rate: :float,
                    service_version: :string,
                    services: {:keyword, :atom},
                    strategy: :atom,
@@ -61,7 +62,8 @@ defmodule Spandex.Tracer do
                  defaults: [
                    disabled?: false,
                    services: [],
-                   strategy: Spandex.Strategy.Pdict
+                   strategy: Spandex.Strategy.Pdict,
+                   sample_rate: 1.0
                  ],
                  describe: [
                    adapter: "The third party adapter to use",
@@ -73,7 +75,8 @@ defmodule Spandex.Tracer do
                    disabled?: "Allows for wholesale disabling a tracer",
                    env: "A name used to identify the environment name, e.g `prod` or `development`",
                    services: "A mapping of service name to the default span types.",
-                   strategy: "The storage and tracing strategy. Currently only supports local process dictionary."
+                   strategy: "The storage and tracing strategy. Currently only supports local process dictionary.",
+                   sample_rate: "The rate at which to sample traces. 1.0 means 100% of traces are sampled."
                  ]
                )
 

test/spandex_test.exs

@@ -60,6 +60,33 @@ defmodule Spandex.Test.SpandexTest do
       assert {:service, "is required"} in validation_errors
     end
 
+    test "sets priority to 1 if no priority is set" do
+      assert {:ok, %Trace{priority: 1}} =
+               Spandex.start_trace("root_span", @base_opts ++ @span_opts ++ [sample_rate: nil])
+    end
+
+    test "sets priority to 1 if priority is set to 1" do
+      assert {:ok, %Trace{priority: 1}} = Spandex.start_trace("root_span", @base_opts ++ @span_opts ++ [sample_rate: 1])
+    end
+
+    test "sets priority to 0 if no priority is set to 0" do
+      assert {:ok, %Trace{priority: 0}} = Spandex.start_trace("root_span", @base_opts ++ @span_opts ++ [sample_rate: 0])
+    end
+
+    test "sets priority based on sample rate value" do
+      sample_rate = 0.10
+
+      total_priority =
+        Enum.reduce(1..5000, 0, fn _, acc ->
+          {:ok, trace} = Spandex.start_trace("root_span", @base_opts ++ @span_opts ++ [sample_rate: sample_rate])
+          Spandex.finish_trace(@base_opts)
+          acc + trace.priority
+        end)
+
+      diff = abs(total_priority - 500)
+      assert diff <= 50
+    end
+
     test "adds span_id, trace_id to log metadata" do
       opts = @base_opts ++ @span_opts