Support delayed sampling decisions

sampler.go

@@ -57,17 +57,23 @@ type Sampler interface {
 
 // ConstSampler is a sampler that always makes the same decision.
 type ConstSampler struct {
+	legacySamplerV1Base
 	Decision bool
 	tags     []Tag
 }
 
 // NewConstSampler creates a ConstSampler.
-func NewConstSampler(sample bool) Sampler {
+func NewConstSampler(sample bool) *ConstSampler {
 	tags := []Tag{
 		{key: SamplerTypeTagKey, value: SamplerTypeConst},
 		{key: SamplerParamTagKey, value: sample},
 	}
-	return &ConstSampler{Decision: sample, tags: tags}
+	s := &ConstSampler{
+		Decision: sample,
+		tags:     tags,
+	}
+	s.delegate = s.IsSampled
+	return s
 }
 
 // IsSampled implements IsSampled() of Sampler.

sampler_v2.go

@@ -0,0 +1,68 @@
+// Copyright (c) 2019 Uber Technologies, Inc.
+//
+// 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.
+
+package jaeger
+
+type SamplingDecision struct {
+	sample    bool
+	retryable bool
+	tags      []Tag
+}
+
+type SamplerV2 interface {
+	OnCreateSpan(span *Span) SamplingDecision
+	OnSetOperationName(span *Span, operationName string) SamplingDecision
+	OnSetTag(span *Span, key string, value interface{}) SamplingDecision
+
+	// Close does a clean shutdown of the sampler, stopping any background
+	// go-routines it may have started.
+	Close()
+}
+
+// samplerV1toV2 wraps legacy V1 sampler into an adapter that make it look like V2.
+func samplerV1toV2(s Sampler) SamplerV2 {
+	if s2, ok := s.(SamplerV2); ok {
+		return s2
+	}
+	type legacySamplerV1toV2Adapter struct {
+		legacySamplerV1Base
+	}
+	return &legacySamplerV1toV2Adapter{
+		legacySamplerV1Base: legacySamplerV1Base{
+			delegate: s.IsSampled,
+		},
+	}
+}
+
+// legacySamplerV1Base is used as a base for simple samplers that only implement
+// the legacy isSampled() function that is not sensitive to its arguments.
+type legacySamplerV1Base struct {
+	delegate func(id TraceID, operation string) (sampled bool, tags []Tag)
+}
+
+func (s *legacySamplerV1Base) OnCreateSpan(span *Span) SamplingDecision {
+	isSampled, tags := s.delegate(span.context.traceID, span.operationName)
+	return SamplingDecision{sample: isSampled, retryable: false, tags: tags}
+}
+
+func (s *legacySamplerV1Base) OnSetOperationName(span *Span, operationName string) SamplingDecision {
+	isSampled, tags := s.delegate(span.context.traceID, span.operationName)
+	return SamplingDecision{sample: isSampled, retryable: false, tags: tags}
+}
+
+func (s *legacySamplerV1Base) OnSetTag(span *Span, key string, value interface{}) SamplingDecision {
+	return SamplingDecision{sample: false, retryable: true}
+}
+
+func (s *legacySamplerV1Base) Close() {}

sampler_v2_test.go

@@ -0,0 +1,17 @@
+// Copyright (c) 2019 The Jaeger 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.
+
+package jaeger
+
+var _ SamplerV2 = new(ConstSampler)

span.go

@@ -34,6 +34,7 @@ type Span struct {
 
 	tracer *Tracer
 
+	// TODO: (major) change to use a pointer
 	context SpanContext
 
 	// The name of the "operation" this span is an instance of.
@@ -74,24 +75,36 @@ type Tag struct {
 // SetOperationName sets or changes the operation name.
 func (s *Span) SetOperationName(operationName string) opentracing.Span {
 	s.Lock()
-	defer s.Unlock()
-	if s.context.IsSampled() {
-		s.operationName = operationName
+	s.operationName = operationName
+	if !s.isSamplingFinalized() {
+		decision := s.tracer.sampler.OnSetOperationName(s, operationName)
+		s.applySamplingDecision(decision, false)
 	}
+	s.Unlock()
 	s.observer.OnSetOperationName(operationName)
 	return s
 }
 
 // SetTag implements SetTag() of opentracing.Span
 func (s *Span) SetTag(key string, value interface{}) opentracing.Span {
+	return s.setTagInternal(key, value, true)
+}
+
+func (s *Span) setTagInternal(key string, value interface{}, lock bool) opentracing.Span {
 	s.observer.OnSetTag(key, value)
 	if key == string(ext.SamplingPriority) && !setSamplingPriority(s, value) {
 		return s
 	}
-	s.Lock()
-	defer s.Unlock()
-	if s.context.IsSampled() {
-		s.setTagNoLocking(key, value)
+	if !s.isSamplingFinalized() {
+		decision := s.tracer.sampler.OnSetTag(s, key, value)
+		s.applySamplingDecision(decision, lock)
+	}
+	if s.isWriteable() {
+		if lock {
+			s.Lock()
+			defer s.Unlock()
+		}
+		s.appendTagNoLocking(key, value)
 	}
 	return s
 }
@@ -128,7 +141,7 @@ func (s *Span) Tags() opentracing.Tags {
 	return result
 }
 
-func (s *Span) setTagNoLocking(key string, value interface{}) {
+func (s *Span) appendTagNoLocking(key string, value interface{}) {
 	s.tags = append(s.tags, Tag{key: key, value: value})
 }
 
@@ -148,7 +161,7 @@ func (s *Span) logFieldsNoLocking(fields ...log.Field) {
 		Fields:    fields,
 		Timestamp: time.Now(),
 	}
-	s.appendLog(lr)
+	s.appendLogNoLocking(lr)
 }
 
 // LogKV implements opentracing.Span API
@@ -185,12 +198,12 @@ func (s *Span) Log(ld opentracing.LogData) {
 		if ld.Timestamp.IsZero() {
 			ld.Timestamp = s.tracer.timeNow()
 		}
-		s.appendLog(ld.ToLogRecord())
+		s.appendLogNoLocking(ld.ToLogRecord())
 	}
 }
 
 // this function should only be called while holding a Write lock
-func (s *Span) appendLog(lr opentracing.LogRecord) {
+func (s *Span) appendLogNoLocking(lr opentracing.LogRecord) {
 	// TODO add logic to limit number of logs per span (issue #46)
 	s.logs = append(s.logs, lr)
 }
@@ -300,6 +313,34 @@ func (s *Span) serviceName() string {
 	return s.tracer.serviceName
 }
 
+func (s *Span) applySamplingDecision(decision SamplingDecision, lock bool) {
+	if !decision.retryable {
+		s.context.samplingState.setFinal()
+	}
+	if decision.sample {
+		s.context.samplingState.setSampled()
+		if len(decision.tags) > 0 {
+			if lock {
+				s.Lock()
+				defer s.Unlock()
+			}
+			for _, tag := range decision.tags {
+				s.appendTagNoLocking(tag.key, tag.value)
+			}
+		}
+	}
+}
+
+// Span can be written to if it is sampled or the sampling decision has not been finalized.
+func (s *Span) isWriteable() bool {
+	state := s.context.samplingState
+	return !state.isFinal() || state.isSampled()
+}
+
+func (s *Span) isSamplingFinalized() bool {
+	return s.context.samplingState.isFinal()
+}
+
 // setSamplingPriority returns true if the flag was updated successfully, false otherwise.
 // The behavior of setSamplingPriority is surprising
 // If noDebugFlagOnForcedSampling is set
@@ -322,6 +363,7 @@ func setSamplingPriority(s *Span, value interface{}) bool {
 	}
 	if s.tracer.options.noDebugFlagOnForcedSampling {
 		s.context.samplingState.setSampled()
+		return true
 	} else if s.tracer.isDebugAllowed(s.operationName) {
 		s.context.samplingState.setDebugAndSampled()
 		return true

context.go → span_context.go

@@ -68,10 +68,19 @@ type SpanContext struct {
 
 	// samplingState is shared across all spans
 	samplingState *samplingState
+
+	// remote indicates that span context represents a remote parent
+	remote bool
 }
 
 type samplingState struct {
-	stateFlags atomic.Int32 // Only lower 8 bits are used. We use an int32 instead of a byte to use CAS operations
+	// Span context's state flags that are propagated across processes. Only lower 8 bits are used.
+	// We use an int32 instead of byte to be able to use CAS operations.
+	stateFlags atomic.Int32
+
+	// When state is not final, sampling will be retried on other span write operations,
+	// like SetOperationName / SetTag, and the spans will remain writable.
+	final atomic.Bool
 }
 
 func (s *samplingState) setFlag(newFlag int32) {
@@ -110,6 +119,10 @@ func (s *samplingState) setFlags(flags byte) {
 	s.stateFlags.Store(int32(flags))
 }
 
+func (s *samplingState) setFinal() {
+	s.final.Store(true)
+}
+
 func (s *samplingState) flags() byte {
 	return byte(s.stateFlags.Load())
 }
@@ -126,6 +139,10 @@ func (s *samplingState) isFirehose() bool {
 	return s.stateFlags.Load()&flagFirehose == flagFirehose
 }
 
+func (s *samplingState) isFinal() bool {
+	return s.final.Load()
+}
+
 // ForeachBaggageItem implements ForeachBaggageItem() of opentracing.SpanContext
 func (c SpanContext) ForeachBaggageItem(handler func(k, v string) bool) {
 	for k, v := range c.baggage {
@@ -252,7 +269,7 @@ func (c SpanContext) WithBaggageItem(key, value string) SpanContext {
 		newBaggage[key] = value
 	}
 	// Use positional parameters so the compiler will help catch new fields.
-	return SpanContext{c.traceID, c.spanID, c.parentID, newBaggage, "", c.samplingState}
+	return SpanContext{c.traceID, c.spanID, c.parentID, newBaggage, "", c.samplingState, c.remote}
 }
 
 // isDebugIDContainerOnly returns true when the instance of the context is only