Merge pull request #70 from openzipkin/exposes_binary_samplers

[#68] Exposes alwaysSample and NeverSample to easier usage.

noop_test.go

@@ -17,7 +17,7 @@ func TestNoopContext(t *testing.T) {
 		tr, _    = NewTracer(
 			reporter.NewNoopReporter(),
 			WithNoopSpan(true),
-			WithSampler(neverSample),
+			WithSampler(NeverSample),
 			WithSharedSpans(true),
 		)
 	)

sample.go

@@ -12,14 +12,20 @@ import (
 // traceID.
 type Sampler func(id uint64) bool
 
-func neverSample(_ uint64) bool { return false }
+// NeverSample will always return false. If used by a service it will not allow
+// the service to start traces but will still allow the service to participate 
+// in traces started upstream.
+func NeverSample(_ uint64) bool { return false }
 
-func alwaysSample(_ uint64) bool { return true }
+// AlwaysSample will always return true. If used by a service it will always start
+// traces if no upstream trace has been propagated. If an incoming upstream trace 
+// is not sampled the service will adhere to this and only propagate the context.
+func AlwaysSample(_ uint64) bool { return true }
 
 // NewModuloSampler provides a generic type Sampler.
 func NewModuloSampler(mod uint64) Sampler {
 	if mod < 2 {
-		return alwaysSample
+		return AlwaysSample
 	}
 	return func(id uint64) bool {
 		return (id % mod) == 0
@@ -31,10 +37,10 @@ func NewModuloSampler(mod uint64) Sampler {
 // It defends against nodes in the cluster selecting exactly the same ids.
 func NewBoundarySampler(rate float64, salt int64) (Sampler, error) {
 	if rate == 0.0 {
-		return neverSample, nil
+		return NeverSample, nil
 	}
 	if rate == 1.0 {
-		return alwaysSample, nil
+		return AlwaysSample, nil
 	}
 	if rate < 0.0001 || rate > 1 {
 		return nil, fmt.Errorf("rate should be 0.0 or between 0.0001 and 1: was %f", rate)
@@ -55,10 +61,10 @@ func NewBoundarySampler(rate float64, salt int64) (Sampler, error) {
 // on trace id).
 func NewCountingSampler(rate float64) (Sampler, error) {
 	if rate == 0.0 {
-		return neverSample, nil
+		return NeverSample, nil
 	}
 	if rate == 1.0 {
-		return alwaysSample, nil
+		return AlwaysSample, nil
 	}
 	if rate < 0.01 || rate > 1 {
 		return nil, fmt.Errorf("rate should be 0.0 or between 0.01 and 1: was %f", rate)

tracer.go

@@ -31,7 +31,7 @@ func NewTracer(rep reporter.Reporter, opts ...TracerOption) (*Tracer, error) {
 	t := &Tracer{
 		defaultTags:          make(map[string]string),
 		extractFailurePolicy: ExtractFailurePolicyRestart,
-		sampler:              alwaysSample,
+		sampler:              AlwaysSample,
 		generate:             idgenerator.NewRandom64(),
 		reporter:             rep,
 		localEndpoint:        nil,