README.md

Honeycomb Beeline Golang - Field Sampler

Usage:

// Configure it with a default sample rate
hook, err := fieldsampler.NewFieldSamplerHook(1)
if err != nil {
	log.Fatal(err)
}
beeline.Init(beeline.Config{
	WriteKey:    honeycombWriteKey,
	Dataset:     "nomics",
	ServiceName: honeycombServiceName,
	SamplerHook: hook,
	STDOUT:      honeycombWriteKey == "",
})

// When creating a span, add a trace field to optionally override the sample rate on a per-trace basis
ctx, span := beeline.StartSpan(ctx, "HighVolumeSpan")
span.AddTraceField("CustomSampleRate", 5)		

field_sampler.go

package fieldsampler

import (
	"github.com/honeycombio/beeline-go/sample"
)

const (
	// CustomSampleRateFieldName is the field to use to override the default Sample Rate
	CustomSampleRateFieldName = "CustomSampleRate"

	// DeterministicSampleFieldName is the field used as a determinant for the sampler
	DeterministicSampleFieldName = "trace.trace_id"
)

// NewFieldSamplerHook creates a new SamplerHook that respects CustomSampleRateFieldName if present,
// otherwise it uses the given defaultSampleRate. Then it delegates to the beeline deterministic sampler.
func NewFieldSamplerHook(defaultSampleRate uint) (func(map[string]interface{}) (bool, int), error) {
	defaultSampler, err := sample.NewDeterministicSampler(defaultSampleRate)
	if err != nil {
		return nil, err
	}

	return func(fields map[string]interface{}) (bool, int) {
		sampler := defaultSampler

		rate := int(defaultSampleRate)
		if custom, ok := fields[CustomSampleRateFieldName].(int); ok {
			rate = custom
			newSampler, err := sample.NewDeterministicSampler(uint(rate))
			if err == nil && rate > 0 {
				sampler = newSampler
			}
		}

		determinant := ""
		if traceID, ok := fields[DeterministicSampleFieldName].(string); ok {
			determinant = traceID
		}

		shouldSample := sampler.Sample(determinant)
		return shouldSample, rate
	}, nil
}

field_sampler_test.go

package fieldsampler

import (
	"context"
	"testing"

	beeline "github.com/honeycombio/beeline-go"
	"github.com/honeycombio/beeline-go/trace"
	libhoney "github.com/honeycombio/libhoney-go"
)

func TestOverrideSampleRate(t *testing.T) {
	hook, err := NewFieldSamplerHook(5)
	if err != nil {
		t.Fatal(err)
	}

	mo := &libhoney.MockOutput{}
	libhoney.Init(
		libhoney.Config{
			APIHost:  "placeholder",
			WriteKey: "placeholder",
			Dataset:  "placeholder",
			Output:   mo,
		},
	)
	trace.GlobalConfig.SamplerHook = hook

	_, span := beeline.StartSpan(context.Background(), "start")
	span.AddTraceField("CustomSampleRate", 1)
	span.Send()

	events := mo.Events()
	if len(events) != 1 {
		t.Fatal(len(events))
	}

	ev := events[0]

	if ev.SampleRate != 1 {
		t.Fatal("Expected SampleRate to be 1:", ev.SampleRate)
	}
	if custom, ok := ev.Fields()["CustomSampleRate"]; !ok {
		t.Fatal("Expected CustomSampleRate to be set", ev.Fields())
	} else if custom != 1 {
		t.Fatal("Expected CustomSampleRate to be 1:", custom)
	}
}