Feature: configurable span tracking

config/config.exs

@@ -8,6 +8,11 @@ if Mix.env() == :test do
     report_dir: "reports/exunit"
 
   config :opencensus,
+    process_contexts: [
+      Opencensus.Unstable.ProcessContext.SeqTrace,
+      Opencensus.Unstable.ProcessContext.ProcessDictionary,
+      Opencensus.Unstable.ProcessContext.ProcessDictionaryWithRecovery
+    ],
     reporters: [{Opencensus.TestSupport.SpanCaptureReporter, []}],
     send_interval_ms: 100
 end

lib/opencensus/trace.ex

@@ -45,6 +45,7 @@ defmodule Opencensus.Trace do
   end
   ```
   """
+
   defmacro with_child_span(label, attributes \\ quote(do: %{}), do: block) do
     line = __CALLER__.line
     module = __CALLER__.module
@@ -60,21 +61,23 @@ defmodule Opencensus.Trace do
       })
 
     quote do
-      parent_span_ctx = :ocp.current_span_ctx()
+      previous_span_ctx = Opencensus.Unstable.current_span_ctx()
+      parent_span_ctx = Opencensus.Unstable.recover_span_ctx()
 
       new_span_ctx =
         :oc_trace.start_span(unquote(label), parent_span_ctx, %{
           :attributes => unquote(computed_attributes)
         })
 
-      _ = :ocp.with_span_ctx(new_span_ctx)
+      _ = Opencensus.Unstable.with_span_ctx(new_span_ctx)
+      ^new_span_ctx = Opencensus.Unstable.current_span_ctx()
       Opencensus.Logger.set_logger_metadata()
 
       try do
         unquote(block)
       after
         _ = :oc_trace.finish_span(new_span_ctx)
-        _ = :ocp.with_span_ctx(parent_span_ctx)
+        _ = Opencensus.Unstable.with_span_ctx(previous_span_ctx)
         Opencensus.Logger.set_logger_metadata()
       end
     end

lib/opencensus/unstable.ex

@@ -0,0 +1,158 @@
+defmodule Opencensus.Unstable do
+  @moduledoc """
+  Experimental higher-level API built on proposed `ot_ctx` behaviour.
+  """
+
+  @doc """
+  Get the current span context.
+
+  Uses the first configured `process_context` only to ensure the value is safe to pass to
+  `with_span_ctx/1` and `with_span_ctx/2` after you've finished your work.
+  """
+  @spec current_span_ctx() :: :opencensus.span_ctx() | :undefined
+  def current_span_ctx do
+    process_contexts()
+    |> hd
+    |> get_span_ctx_via()
+  end
+
+  @doc """
+  Recovers the span context.
+
+  Uses all configured `process_context`.
+  Results MAY be used as the parent of a new span.
+  Results MUST NOT be passed to `with_span_ctx/1` or `with_span_ctx/2`.
+  """
+  @spec recover_span_ctx() :: :opencensus.span_ctx() | :undefined
+  def recover_span_ctx do
+    process_contexts()
+    |> Enum.find_value(:undefined, &get_span_ctx_via/1)
+  end
+
+  @doc """
+  Sets the span context. Replaces `:ocp.with_span_ctx/1`.
+
+  Uses all configured `process_context`.
+  Returns the previous value of `current_span_ctx/0`.
+  """
+  @spec with_span_ctx(span_ctx :: :opencensus.span_ctx() | :undefined) ::
+          :opencensus.span_ctx() | :undefined
+  def with_span_ctx(span_ctx) do
+    return_span_ctx = current_span_ctx()
+    process_contexts() |> Enum.each(&put_span_ctx_via(&1, span_ctx))
+    return_span_ctx
+  end
+
+  defp get_span_ctx_via(module) do
+    apply(module, :get, [span_ctx_key()])
+    |> case do
+      nil -> :undefined
+      span_ctx -> span_ctx
+    end
+  end
+
+  defp put_span_ctx_via(module, value) do
+    apply(module, :with_value, [span_ctx_key(), value])
+  end
+
+  @spec span_ctx_key() :: atom()
+  defp span_ctx_key do
+    Application.get_env(:opencensus, :span_ctx_key, :oc_span_ctx_key)
+  end
+
+  @spec process_contexts() :: list(module())
+  defp process_contexts do
+    Application.get_env(:opencensus, :process_contexts, [
+      Opencensus.Unstable.ProcessContext.SeqTrace,
+      Opencensus.Unstable.ProcessContext.ProcessDictionary,
+      Opencensus.Unstable.ProcessContext.ProcessDictionaryWithRecovery
+    ])
+  end
+end
+
+defmodule Opencensus.Unstable.ProcessContext do
+  @moduledoc "Abstraction over process-local storage."
+
+  @doc "Get a value."
+  @callback get(key :: atom()) :: any() | nil
+
+  @doc "Put a value."
+  @callback with_value(key :: atom, value :: any()) :: :ok
+end
+
+defmodule Opencensus.Unstable.ProcessContext.SeqTrace do
+  @moduledoc """
+  Process-local storage using `seq_trace`.
+
+  Shares well with any other use that maintains a namespace in the second element of a 2-tuple
+  `{:shared_label, _map}`. Otherwise leaves the trace label alone to avoid disrupting the other
+  usage.
+  """
+
+  @behaviour Opencensus.Unstable.ProcessContext
+
+  @doc "Get a value from the shared `seq_trace` label."
+  @impl Opencensus.Unstable.ProcessContext
+  def get(key) do
+    case :seq_trace.get_token(:label) do
+      {:label, {:shared_label, %{^key => value}}} ->
+        value
+
+      _ ->
+        nil
+    end
+  end
+
+  @doc "Put a value to the shared `seq_trace` label if safe."
+  @impl Opencensus.Unstable.ProcessContext
+  def with_value(key, value) do
+    case :seq_trace.get_token(:label) do
+      [] ->
+        :seq_trace.set_token(:label, {:shared_label, %{key => value}})
+
+      {:label, {:shared_label, map}} when is_map(map) ->
+        :seq_trace.set_token(:label, {:shared_label, Map.put(map, key, value)})
+
+      _ ->
+        nil
+    end
+
+    :ok
+  end
+end
+
+defmodule Opencensus.Unstable.ProcessContext.ProcessDictionary do
+  @moduledoc """
+  Process-local storage using the process dictionary.
+  """
+
+  @behaviour Opencensus.Unstable.ProcessContext
+
+  @doc "Get a value from the process dictionary."
+  @impl Opencensus.Unstable.ProcessContext
+  def get(key), do: Process.get(key)
+
+  @impl Opencensus.Unstable.ProcessContext
+  def with_value(key, value) do
+    Process.put(key, value)
+    :ok
+  end
+end
+
+defmodule Opencensus.Unstable.ProcessContext.ProcessDictionaryWithRecovery do
+  @moduledoc """
+  Process-local storage using the process dictionary.
+  """
+
+  @behaviour Opencensus.Unstable.ProcessContext
+
+  @doc "Get a value from the process dictionary."
+  @impl Opencensus.Unstable.ProcessContext
+  def get(key) do
+    [self() | Process.get(:"$callers", [])]
+    |> Enum.find_value(fn pid -> pid |> Process.info() |> get_in([:dictionary, key]) end)
+  end
+
+  @impl Opencensus.Unstable.ProcessContext
+  defdelegate with_value(key, value), to: Opencensus.Unstable.ProcessContext.ProcessDictionary
+end

test/opencensus_test.exs

@@ -11,12 +11,12 @@ defmodule OpencensusTest do
     on_exit(make_ref(), &detach/0)
 
     assert Logger.metadata() == []
-    assert :ocp.current_span_ctx() == :undefined
+    assert Opencensus.Unstable.current_span_ctx() == :undefined
 
     with_child_span "child_span" do
       :do_something
 
-      assert :ocp.current_span_ctx() != :undefined
+      assert Opencensus.Unstable.current_span_ctx() != :undefined
 
       assert Logger.metadata() |> Keyword.keys() |> Enum.sort() == [
                :span_id,

test/opencensus_trace_async_test.exs

@@ -7,15 +7,15 @@ defmodule Opencensus.AsyncTest do
   alias Opencensus.Trace
 
   test "Trace.async/1" do
-    assert :ocp.current_span_ctx() == :undefined
+    assert Opencensus.Unstable.current_span_ctx() == :undefined
 
     {inner, outer} =
       Trace.with_child_span "outside" do
-        outer = :ocp.current_span_ctx() |> Span.load()
+        outer = Opencensus.Unstable.current_span_ctx() |> Span.load()
 
         Trace.async(fn ->
           Trace.with_child_span "inside" do
-            inner = :ocp.current_span_ctx() |> Span.load()
+            inner = Opencensus.Unstable.current_span_ctx() |> Span.load()
             {inner, outer}
           end
         end)
@@ -30,19 +30,19 @@ defmodule Opencensus.AsyncTest do
   defmodule M do
     def f(outer) do
       Trace.with_child_span "inside" do
-        inner = :ocp.current_span_ctx() |> Span.load()
+        inner = Opencensus.Unstable.current_span_ctx() |> Span.load()
         {inner, outer}
       end
     end
   end
 
   test "Trace.async/3" do
-    assert :ocp.current_span_ctx() == :undefined
+    assert Opencensus.Unstable.current_span_ctx() == :undefined
 
     {inner, outer} =
       Trace.with_child_span "outside" do
-        outer = :ocp.current_span_ctx() |> Span.load()
-        Trace.async(M, :f, [outer]) |> Trace.await(10)
+        outer = Opencensus.Unstable.current_span_ctx() |> Span.load()
+        M |> Trace.async(:f, [outer]) |> Trace.await(10)
       end
 
     assert inner.trace_id == outer.trace_id