《互联网产品的服务端架构·交易、增长与平台化》13.3 进入卷四

// file: internal/growthbase/metrics.go
// tier: L1
// verify: go test ./internal/growthbase/

package growthbase

import "fmt"

// Sample 是一个统计窗口内的交易原始数据。
type Sample struct {
	GMV            int64
	PaidOrders     int64
	RefundedOrders int64
	Visits         int64
}

// Summary 是派生后的质量指标。
type Summary struct {
	GMV        int64
	PaidRate   float64
	RefundRate float64
}

// Aggregate 计算支付成功率与退款率。
//
// 支付率的分母是访问量，用来看“来了多少人、多少人付了钱”；
// 退款率的分母是支付订单数，用来看“付了钱的人里有多少退掉”。
// 两个比率的分母不同，不能混用，也不能用一个 GMV 代替。
func Aggregate(s Sample) (Summary, error) {
	if s.GMV < 0 {
		return Summary{}, fmt.Errorf("growthbase: GMV %d must not be negative", s.GMV)
	}
	if s.PaidOrders < 0 {
		return Summary{}, fmt.Errorf("growthbase: paid orders %d must not be negative", s.PaidOrders)
	}
	if s.RefundedOrders < 0 {
		return Summary{}, fmt.Errorf("growthbase: refunded orders %d must not be negative", s.RefundedOrders)
	}
	if s.Visits < 0 {
		return Summary{}, fmt.Errorf("growthbase: visits %d must not be negative", s.Visits)
	}

	out := Summary{GMV: s.GMV}
	if s.Visits > 0 {
		out.PaidRate = float64(s.PaidOrders) / float64(s.Visits)
	}
	if s.PaidOrders > 0 {
		out.RefundRate = float64(s.RefundedOrders) / float64(s.PaidOrders)
	}
	return out, nil
}

// Meets 用严格不等式判断指标是否越过阈值。
//
// 等于阈值不算达标：阈值是底线而不是目标，刚好踩线通常意味着
// 下一分钟就会跌破。调用方应把阈值设成业务可接受的下限。
func Meets(s Summary, minPaidRate, maxRefundRate float64) bool {
	return s.PaidRate > minPaidRate && s.RefundRate < maxRefundRate
}

// file: internal/growthbase/metrics_test.go
// tier: L1
// verify: go test ./internal/growthbase/

package growthbase

import (
	"math"
	"strings"
	"testing"
)

func almostEqual(a, b float64) bool {
	return math.Abs(a-b) < 1e-9
}

func TestAggregateRates(t *testing.T) {
	s := Sample{GMV: 1250000, PaidOrders: 420, RefundedOrders: 21, Visits: 10000}
	got, err := Aggregate(s)
	if err != nil {
		t.Fatal(err)
	}
	if got.GMV != 1250000 {
		t.Fatalf("gmv = %d, want 1250000", got.GMV)
	}
	if !almostEqual(got.PaidRate, 0.042) {
		t.Fatalf("paid rate = %v, want 0.042", got.PaidRate)
	}
	if !almostEqual(got.RefundRate, 0.05) {
		t.Fatalf("refund rate = %v, want 0.05", got.RefundRate)
	}
}

func TestAggregateZeroDenominators(t *testing.T) {
	got, err := Aggregate(Sample{GMV: 0, PaidOrders: 0, RefundedOrders: 0, Visits: 0})
	if err != nil {
		t.Fatal(err)
	}
	if got.PaidRate != 0 || got.RefundRate != 0 {
		t.Fatalf("rates = %v/%v, want 0/0", got.PaidRate, got.RefundRate)
	}
}

func TestAggregatePaidWithoutVisits(t *testing.T) {
	got, err := Aggregate(Sample{GMV: 100, PaidOrders: 4, RefundedOrders: 2, Visits: 0})
	if err != nil {
		t.Fatal(err)
	}
	if got.PaidRate != 0 {
		t.Fatalf("paid rate = %v, want 0", got.PaidRate)
	}
	if !almostEqual(got.RefundRate, 0.5) {
		t.Fatalf("refund rate = %v, want 0.5", got.RefundRate)
	}
}

func TestAggregateRejectsNegative(t *testing.T) {
	cases := []struct {
		name string
		s    Sample
		want string
	}{
		{"gmv", Sample{GMV: -1}, "GMV -1 must not be negative"},
		{"paid", Sample{PaidOrders: -1}, "paid orders -1 must not be negative"},
		{"refunded", Sample{RefundedOrders: -1}, "refunded orders -1 must not be negative"},
		{"visits", Sample{Visits: -1}, "visits -1 must not be negative"},
	}
	for _, c := range cases {
		t.Run(c.name, func(t *testing.T) {
			_, err := Aggregate(c.s)
			if err == nil {
				t.Fatal("want error, got nil")
			}
			if !strings.Contains(err.Error(), c.want) {
				t.Fatalf("error %q does not contain %q", err, c.want)
			}
		})
	}
}

func TestMeetsStrictThresholds(t *testing.T) {
	s := Summary{GMV: 1250000, PaidRate: 0.042, RefundRate: 0.05}
	if !Meets(s, 0.04, 0.06) {
		t.Fatal("expected metrics above/below thresholds to pass")
	}
	if Meets(s, 0.042, 0.06) {
		t.Fatal("paid rate equal to threshold must not pass")
	}
	if Meets(s, 0.04, 0.05) {
		t.Fatal("refund rate equal to threshold must not pass")
	}
	if Meets(s, 0.05, 0.06) {
		t.Fatal("paid rate below threshold must not pass")
	}
}

cd .local/checks/webapp-lab && go test ./internal/growthbase/ -run 'TestAggregate|TestMeets' -count=1 -v

=== RUN   TestAggregateRates
--- PASS: TestAggregateRates (0.00s)
=== RUN   TestAggregateZeroDenominators
--- PASS: TestAggregateZeroDenominators (0.00s)
=== RUN   TestAggregatePaidWithoutVisits
--- PASS: TestAggregatePaidWithoutVisits (0.00s)
=== RUN   TestAggregateRejectsNegative
=== RUN   TestAggregateRejectsNegative/gmv
=== RUN   TestAggregateRejectsNegative/paid
=== RUN   TestAggregateRejectsNegative/refunded
=== RUN   TestAggregateRejectsNegative/visits
--- PASS: TestAggregateRejectsNegative (0.00s)
    --- PASS: TestAggregateRejectsNegative/gmv (0.00s)
    --- PASS: TestAggregateRejectsNegative/paid (0.00s)
    --- PASS: TestAggregateRejectsNegative/refunded (0.00s)
    --- PASS: TestAggregateRejectsNegative/visits (0.00s)
=== RUN   TestMeetsStrictThresholds
--- PASS: TestMeetsStrictThresholds (0.00s)
PASS
ok  	webapp/internal/growthbase	0.262s