Add option to inherit service name from the parent span

Signed-off-by: Marco Costa <marco.costa@datadoghq.com>

docs/GettingStarted.md

@@ -1935,6 +1935,7 @@ For example, if `tracing.sampling.default_rate` is configured by [Remote Configu
 | `tracing.propagation_style`                            | `DD_TRACE_PROPAGATION_STYLE`                            | `Array`                               | `nil`                        | Distributed tracing propagation formats to extract and inject. See [Distributed Tracing](#distributed-tracing) for more details.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
 | `tracing.enabled`                                      | `DD_TRACE_ENABLED`                                      | `Bool`                                | `true`                       | Enables or disables tracing. If set to `false` instrumentation will still run, but no traces are sent to the trace agent.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
 | `tracing.header_tags`                                  | `DD_TRACE_HEADER_TAGS`                                  | `Array`                               | `nil`                        | Record HTTP headers as span tags. See [Applying header tags to root spans][header tags] for more information.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
+| `tracing.inherit_parent_service`                       |                                                         | `Bool`                                | `false`                      | Inherit the parent span's service name when creating a new child span. |
 | `tracing.instrument(<integration-name>, <options...>)` |                                                         |                                       |                              | Activates instrumentation for a specific library. See [Integration instrumentation](#integration-instrumentation) for more details.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
 | `tracing.log_injection`                                | `DD_LOGS_INJECTION`                                     | `Bool`                                | `true`                       | Injects [Trace Correlation](#trace-correlation) information into Rails logs if present. Supports the default logger (`ActiveSupport::TaggedLogging`), `lograge`, and `semantic_logger`.                                                                                                                                                                                                                                                                                                                                                                                                                                                |
 | `tracing.partial_flush.enabled`                        |                                                         | `Bool`                                | `false`                      | Enables or disables partial flushing. Partial flushing submits completed portions of a trace to the agent. Used when tracing instruments long running tasks (e.g. jobs) with many spans.                                                                                                                                                                                                                                                                                                                                                                                                                                               |

lib/datadog/tracing/component.rb

@@ -54,6 +54,7 @@ def build_tracer(settings, agent_settings, logger:)
           span_sampler: build_span_sampler(settings),
           writer: writer,
           tags: build_tracer_tags(settings),
+          inherit_parent_service: settings.tracing.inherit_parent_service
         )
       end
 

lib/datadog/tracing/configuration/settings.rb

@@ -144,6 +144,21 @@ def self.extended(base)
                 o.setter { |header_tags, _| Configuration::HTTP::HeaderTags.new(header_tags) }
               end
 
+              # Whether spans inherit their parent's service name, when span `service` is not explicitly set.
+              # If `true`, the spans will use the service name from its parent span.
+              # If `false`, the spans will use the global service name (`c.service`).
+              #
+              # DEV3.0: All other tracer languages inherit the parent's service by default.
+              # DEV3.0: This option should be set to `true` by default in the next major version,
+              # DEV3.0: with a removal deprecation warning, then removed in 4.0.
+              #
+              # @default `false`
+              # @return [Boolean]
+              option :inherit_parent_service do |o|
+                o.default false
+                o.type :bool
+              end
+
               # Enable 128 bit trace id generation.
               #
               # @default `DD_TRACE_128_BIT_TRACEID_GENERATION_ENABLED` environment variable, otherwise `true`

lib/datadog/tracing/span_operation.rb

@@ -48,7 +48,8 @@ def initialize(
         tags: nil,
         trace_id: nil,
         type: nil,
-        links: nil
+        links: nil,
+        inherit_parent_service: false
       )
         # Ensure dynamically created strings are UTF-8 encoded.
         #
@@ -90,6 +91,8 @@ def initialize(
         # Subscribe :on_error event
         @events.on_error.wrap_default(&on_error) if on_error.is_a?(Proc)
 
+        @inherit_parent_service = inherit_parent_service
+
         # Start the span with start time, if given.
         start(start_time) if start_time
       end
@@ -428,7 +431,8 @@ def message
       # it has been finished.
       attr_reader \
         :events,
-        :span
+        :span,
+        :inherit_parent_service
 
       # Stored only for `service_entry` calculation.
       # Use `parent_id` for the effective parent span id.
@@ -439,6 +443,11 @@ def message
       # mutation by reference; when this span is returned,
       # we don't want this SpanOperation to modify it further.
       def build_span
+        # Use parent service if no service is set and @inherit_parent_service is set
+        service = self.service || (@parent&.service if @inherit_parent_service)
+
+        service_entry = @parent.nil? || (service && @parent.service != service)
+
         Span.new(
           @name,
           duration: duration,
@@ -448,13 +457,13 @@ def build_span
           metrics: Core::Utils::SafeDup.frozen_or_dup(metrics),
           parent_id: @parent_id,
           resource: @resource,
-          service: @service,
+          service: service,
           start_time: @start_time,
           status: @status,
           type: @type,
           trace_id: @trace_id,
           links: @links,
-          service_entry: parent.nil? || (service && parent.service != service)
+          service_entry: service_entry
         )
       end
 

lib/datadog/tracing/trace_operation.rb

@@ -70,19 +70,22 @@ def initialize(
         sampled: nil,
         sampling_priority: nil,
         service: nil,
+        default_service: nil,
         profiling_enabled: nil,
         tags: nil,
         metrics: nil,
         trace_state: nil,
         trace_state_unknown_fields: nil,
-        remote_parent: false
+        remote_parent: false,
+        inherit_parent_service: false
       )
         # Attributes
         @id = id || Tracing::Utils::TraceId.next_id
         @max_length = max_length || DEFAULT_MAX_LENGTH
         @parent_span_id = parent_span_id
         @sampled = sampled.nil? ? true : sampled
         @remote_parent = remote_parent
+        @inherit_parent_service = inherit_parent_service
 
         # Tags
         @agent_sample_rate = agent_sample_rate
@@ -95,6 +98,7 @@ def initialize(
         @sample_rate = sample_rate
         @sampling_priority = sampling_priority
         @service = service
+        @default_service = default_service
         @profiling_enabled = profiling_enabled
         @trace_state = trace_state
         @trace_state_unknown_fields = trace_state_unknown_fields
@@ -169,7 +173,7 @@ def resource_override?
       end
 
       def service
-        @service || (root_span && root_span.service)
+        @service || (root_span && root_span.service) || @default_service
       end
 
       def measure(
@@ -249,7 +253,8 @@ def build_span(
             start_time: start_time,
             tags: tags,
             trace_id: trace_id,
-            type: type
+            type: type,
+            inherit_parent_service: @inherit_parent_service
           )
         rescue StandardError => e
           Datadog.logger.debug { "Failed to build new span: #{e}" }

lib/datadog/tracing/tracer.rb

@@ -58,7 +58,8 @@ def initialize(
         ),
         span_sampler: Sampling::Span::Sampler.new,
         tags: {},
-        writer: Writer.new
+        writer: Writer.new,
+        inherit_parent_service: false
       )
         @trace_flush = trace_flush
         @default_service = default_service
@@ -68,6 +69,7 @@ def initialize(
         @span_sampler = span_sampler
         @tags = tags
         @writer = writer
+        @inherit_parent_service = inherit_parent_service
       end
 
       # Return a {Datadog::Tracing::SpanOperation span_op} and {Datadog::Tracing::TraceOperation trace_op}
@@ -327,12 +329,16 @@ def build_trace(digest = nil)
             trace_state: digest.trace_state,
             trace_state_unknown_fields: digest.trace_state_unknown_fields,
             remote_parent: digest.span_remote,
+            default_service: @default_service,
+            inherit_parent_service: @inherit_parent_service,
           )
         else
           TraceOperation.new(
             hostname: hostname,
             profiling_enabled: profiling_enabled,
             remote_parent: false,
+            default_service: @default_service,
+            inherit_parent_service: @inherit_parent_service,
           )
         end
       end
@@ -341,12 +347,13 @@ def bind_trace_events!(trace_op)
         events = trace_op.send(:events)
 
         events.span_before_start.subscribe do |event_span_op, event_trace_op|
-          event_trace_op.service ||= @default_service
-          event_span_op.service ||= @default_service
           sample_trace(event_trace_op) if event_span_op && event_span_op.parent_id == 0
         end
 
         events.span_finished.subscribe do |event_span, event_trace_op|
+          # Fallback in case the service was never set
+          event_span.service ||= @default_service
+
           sample_span(event_trace_op, event_span)
           flush_trace(event_trace_op)
         end

spec/datadog/core/configuration/components_spec.rb

@@ -425,6 +425,7 @@
           end
         end
         let(:span_sampler) { be_a(Datadog::Tracing::Sampling::Span::Sampler) }
+        let(:inherit_parent_service) { false }
         let(:default_options) do
           {
             default_service: settings.service,
@@ -434,6 +435,7 @@
             sampler: sampler,
             span_sampler: span_sampler,
             writer: writer,
+            inherit_parent_service: inherit_parent_service,
           }
         end
 

spec/datadog/tracing/configuration/settings_spec.rb

@@ -284,6 +284,37 @@ def propagation_style_inject
       end
     end
 
+    describe '#inherit_parent_service' do
+      subject(:inherit_parent_service) { settings.tracing.inherit_parent_service }
+
+      it { is_expected.to be false }
+
+      context 'when inherit_parent_service' do
+        before { settings.tracing.inherit_parent_service = value }
+
+        context 'is set to true' do
+          let(:value) { true }
+
+          it { is_expected.to be true }
+        end
+
+        context 'is set to false' do
+          let(:value) { false }
+
+          it { is_expected.to be false }
+        end
+      end
+    end
+
+    describe '#inherit_parent_service=' do
+      it 'updates the #inherit_parent_service setting' do
+        expect { settings.tracing.inherit_parent_service = true }
+          .to change { settings.tracing.inherit_parent_service }
+          .from(false)
+          .to(true)
+      end
+    end
+
     describe '#header_tags' do
       subject(:header_tags) { settings.tracing.header_tags }
 

spec/datadog/tracing/span_operation_spec.rb

@@ -462,7 +462,7 @@
     end
 
     context 'identifying service_entry_span' do
-      context 'when service of root and child are `nil`' do
+      context 'when service of parent and child are `nil`' do
         it do
           root_span_op = described_class.new('root')
           child_span_op = described_class.new('child_1')
@@ -482,7 +482,7 @@
         end
       end
 
-      context 'when service of root and child are identical' do
+      context 'when service of parent and child are identical' do
         it do
           root_span_op = described_class.new('root', service: 'root_service')
           child_span_op = described_class.new('child_1', service: root_span_op.service)
@@ -502,7 +502,7 @@
         end
       end
 
-      context 'when service of root and child are different' do
+      context 'when service of parent and child are different' do
         it do
           root_span_op = described_class.new('root')
           child_span_op = described_class.new('child_1', service: 'child_service')
@@ -522,7 +522,7 @@
         end
       end
 
-      context 'when service of root and child are different, overriden within the measure block' do
+      context 'when service of parent and child are different, overriden within the measure block' do
         it do
           root_span_op = described_class.new('root')
           child_span_op = described_class.new('child_1')
@@ -806,6 +806,46 @@
         expect(span_op.finish).to be(original_span)
       end
     end
+
+    context 'with a parent span' do
+      let(:parent_span) { described_class.new('parent', service: 'parent_service') }
+
+      before do
+        span_op.send(:parent=, parent_span)
+      end
+
+      context 'with no service set' do
+        it 'span has no service' do
+          expect(finish.service).to eq(nil)
+        end
+      end
+
+      context 'with a service set' do
+        let(:parent_service) { 'parent_service' }
+
+        context 'without inherit_parent_service set' do
+          it 'span does not inherit the service' do
+            expect(finish.service).to eq(nil)
+          end
+        end
+
+        context 'with inherit_parent_service set' do
+          let(:options) { { inherit_parent_service: true } }
+
+          it 'span inherits the service' do
+            expect(finish.service).to eq('parent_service')
+          end
+        end
+
+        context 'and child span service set' do
+          let(:options) { { service: 'child_service' } }
+
+          it 'span does not inherit the service' do
+            expect(finish.service).to eq('child_service')
+          end
+        end
+      end
+    end
   end
 
   describe '#finished?' do