Add Sampler API (#70)

- also add implementation of AlwaysSampleSampler and NeverSampleSampler

api/trace/always_sampler.go

@@ -0,0 +1,52 @@
+// Copyright 2019, OpenTelemetry 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 trace
+
+import (
+	"go.opentelemetry.io/api/core"
+)
+
+const (
+	alwaysSamplerDescription = "AlwaysSampleSampler"
+)
+
+var alwaysSampleDecision = Decision{Sampled: true}
+
+type alwaysSampleSampler struct{}
+
+// ShouldSample implements Sampler interface.
+// It always returns a Decision with Sampled value set to true
+// and with Attributes set to an empty slice.
+func (as alwaysSampleSampler) ShouldSample(
+	_ core.SpanContext,
+	_ bool,
+	_ core.TraceID,
+	_ uint64,
+	_ string,
+) Decision {
+	return alwaysSampleDecision
+}
+
+// Description implements Sampler interface.
+// It returns the description of this sampler.
+func (as alwaysSampleSampler) Description() string {
+	return alwaysSamplerDescription
+}
+
+var _ Sampler = alwaysSampleSampler{}
+
+func AlwaysSampleSampler() alwaysSampleSampler {
+	return alwaysSampleSampler{}
+}

api/trace/always_sampler_test.go

@@ -0,0 +1,39 @@
+// Copyright 2019, OpenTelemetry 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 trace
+
+import (
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+	"go.opentelemetry.io/api/core"
+)
+
+func TestShouldSample(t *testing.T) {
+	gotD := AlwaysSampleSampler().ShouldSample(
+		core.SpanContext{}, false, core.TraceID{}, 0, "span")
+	wantD := Decision{Sampled: true}
+	if diff := cmp.Diff(wantD, gotD); diff != "" {
+		t.Errorf("Decision: +got, -want%v", diff)
+	}
+}
+
+func TestDescription(t *testing.T) {
+	gotDesc := AlwaysSampleSampler().Description()
+	wantDesc := alwaysSamplerDescription
+	if diff := cmp.Diff(wantDesc, gotDesc); diff != "" {
+		t.Errorf("Description: +got, -want%v", diff)
+	}
+}

api/trace/never_sampler.go

@@ -0,0 +1,52 @@
+// Copyright 2019, OpenTelemetry 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 trace
+
+import (
+	"go.opentelemetry.io/api/core"
+)
+
+const (
+	neverSamplerDescription = "NeverSampleSampler"
+)
+
+var neverSampledecision = Decision{Sampled: false}
+
+type neverSampleSampler struct{}
+
+// ShouldSample implements Sampler interface.
+// It always returns a Decision with Sampled value set to false
+// and with Attributes set to an empty slice.
+func (ns neverSampleSampler) ShouldSample(
+	_ core.SpanContext,
+	_ bool,
+	_ core.TraceID,
+	_ uint64,
+	_ string,
+) Decision {
+	return neverSampledecision
+}
+
+// Description implements Sampler interface.
+// It returns the description of this sampler.
+func (ns neverSampleSampler) Description() string {
+	return neverSamplerDescription
+}
+
+var _ Sampler = neverSampleSampler{}
+
+func NeverSampleSampler() neverSampleSampler {
+	return neverSampleSampler{}
+}

api/trace/never_sampler_test.go

@@ -0,0 +1,39 @@
+// Copyright 2019, OpenTelemetry 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 trace
+
+import (
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+	"go.opentelemetry.io/api/core"
+)
+
+func TestNeverSamperShouldSample(t *testing.T) {
+	gotD := NeverSampleSampler().ShouldSample(
+		core.SpanContext{}, false, core.TraceID{}, 0, "span")
+	wantD := Decision{Sampled: false}
+	if diff := cmp.Diff(wantD, gotD); diff != "" {
+		t.Errorf("Decision: +got, -want%v", diff)
+	}
+}
+
+func TestNeverSamplerDescription(t *testing.T) {
+	gotDesc := NeverSampleSampler().Description()
+	wantDesc := neverSamplerDescription
+	if diff := cmp.Diff(wantDesc, gotDesc); diff != "" {
+		t.Errorf("Description: +got, -want%v", diff)
+	}
+}

api/trace/sampler.go

@@ -0,0 +1,44 @@
+// Copyright 2019, OpenTelemetry 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 trace
+
+import "go.opentelemetry.io/api/core"
+
+type Sampler interface {
+	// ShouldSample returns a Decision that contains a decision whether to sample
+	// or not sample the span to be created. Decision is based on a Sampler specific
+	// algorithm that takes into account one or more input parameters.
+	ShouldSample(
+		sc core.SpanContext,
+		remote bool,
+		traceID core.TraceID,
+		spanID uint64,
+		spanName string,
+	) Decision
+
+	// Description returns of the sampler. It contains its name or short description
+	// and its configured properties.
+	// For example 'ProbabilitySampler:{0.00001}'
+	Description() string
+}
+
+type Decision struct {
+	// Sampled is set true if the span should be sampled.
+	Sampled bool
+
+	// Attributes provides insight into Sample	r's decision process.
+	// It could be empty slice or nil if no attributes are recorded by the sampler.
+	Attributes []core.KeyValue
+}