Add Min() interface - rename MaxSumCount to MinMaxSumCount (#352)

* Add Min() interface - rename MaxSumCount to MinMaxSumCount

Fixes https://github.com/open-telemetry/opentelemetry-go/issues/319

* update stdout exporter to collect and output the minimum value
* update min and max atomically in Aggregator Update
* changed all references to maxsumcount to minmaxsumcount

* Address PR comments

api/core/number.go

@@ -49,6 +49,21 @@ func (k NumberKind) Minimum() Number {
 	}
 }
 
+// Maximum returns the maximum representable value
+// for a given NumberKind
+func (k NumberKind) Maximum() Number {
+	switch k {
+	case Int64NumberKind:
+		return NewInt64Number(math.MaxInt64)
+	case Float64NumberKind:
+		return NewFloat64Number(math.MaxFloat64)
+	case Uint64NumberKind:
+		return NewUint64Number(math.MaxUint64)
+	default:
+		return Number(0)
+	}
+}
+
 // Number represents either an integral or a floating point value. It
 // needs to be accompanied with a source of NumberKind that describes
 // the actual type of the value stored within Number.

exporter/metric/stdout/stdout.go

@@ -63,6 +63,7 @@ type expoBatch struct {
 
 type expoLine struct {
 	Name      string      `json:"name"`
+	Min       interface{} `json:"min,omitempty"`
 	Max       interface{} `json:"max,omitempty"`
 	Sum       interface{} `json:"sum,omitempty"`
 	Count     interface{} `json:"count,omitempty"`
@@ -122,15 +123,15 @@ func (e *Exporter) Export(_ context.Context, checkpointSet export.CheckpointSet)
 			}
 		}
 
-		if msc, ok := agg.(aggregator.MaxSumCount); ok {
-			if count, err := msc.Count(); err != nil {
+		if mmsc, ok := agg.(aggregator.MinMaxSumCount); ok {
+			if count, err := mmsc.Count(); err != nil {
 				aggError = err
 				expose.Count = "NaN"
 			} else {
 				expose.Count = count
 			}
 
-			if max, err := msc.Max(); err != nil {
+			if max, err := mmsc.Max(); err != nil {
 				if err == aggregator.ErrEmptyDataSet {
 					// This is a special case, indicates an aggregator that
 					// was checkpointed before its first value was set.
@@ -143,6 +144,19 @@ func (e *Exporter) Export(_ context.Context, checkpointSet export.CheckpointSet)
 				expose.Max = max.AsInterface(kind)
 			}
 
+			if min, err := mmsc.Min(); err != nil {
+				if err == aggregator.ErrEmptyDataSet {
+					// This is a special case, indicates an aggregator that
+					// was checkpointed before its first value was set.
+					return
+				}
+
+				aggError = err
+				expose.Min = "NaN"
+			} else {
+				expose.Min = min.AsInterface(kind)
+			}
+
 			if dist, ok := agg.(aggregator.Distribution); ok && len(e.options.Quantiles) != 0 {
 				summary := make([]expoQuantile, len(e.options.Quantiles))
 				expose.Quantiles = summary

exporter/metric/stdout/stdout_test.go

@@ -21,7 +21,7 @@ import (
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/counter"
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/ddsketch"
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/gauge"
-	"go.opentelemetry.io/otel/sdk/metric/aggregator/maxsumcount"
+	"go.opentelemetry.io/otel/sdk/metric/aggregator/minmaxsumcount"
 	aggtest "go.opentelemetry.io/otel/sdk/metric/aggregator/test"
 )
 
@@ -156,13 +156,13 @@ func TestStdoutGaugeFormat(t *testing.T) {
 	require.Equal(t, `{"updates":[{"name":"test.name{A=B,C=D}","last":123.456}]}`, fix.Output())
 }
 
-func TestStdoutMaxSumCount(t *testing.T) {
+func TestStdoutMinMaxSumCount(t *testing.T) {
 	fix := newFixture(t, stdout.Options{})
 
 	checkpointSet := test.NewCheckpointSet(sdk.NewDefaultLabelEncoder())
 
 	desc := export.NewDescriptor("test.name", export.MeasureKind, nil, "", "", core.Float64NumberKind, false)
-	magg := maxsumcount.New(desc)
+	magg := minmaxsumcount.New(desc)
 	aggtest.CheckedUpdate(fix.t, magg, core.NewFloat64Number(123.456), desc)
 	aggtest.CheckedUpdate(fix.t, magg, core.NewFloat64Number(876.543), desc)
 	magg.Checkpoint(fix.ctx, desc)
@@ -171,7 +171,7 @@ func TestStdoutMaxSumCount(t *testing.T) {
 
 	fix.Export(checkpointSet)
 
-	require.Equal(t, `{"updates":[{"name":"test.name{A=B,C=D}","max":876.543,"sum":999.999,"count":2}]}`, fix.Output())
+	require.Equal(t, `{"updates":[{"name":"test.name{A=B,C=D}","min":123.456,"max":876.543,"sum":999.999,"count":2}]}`, fix.Output())
 }
 
 func TestStdoutMeasureFormat(t *testing.T) {
@@ -198,6 +198,7 @@ func TestStdoutMeasureFormat(t *testing.T) {
 	"updates": [
 		{
 			"name": "test.name{A=B,C=D}",
+			"min": 0.5,
 			"max": 999.5,
 			"sum": 500000,
 			"count": 1000,
@@ -223,8 +224,8 @@ func TestStdoutMeasureFormat(t *testing.T) {
 func TestStdoutEmptyDataSet(t *testing.T) {
 	desc := export.NewDescriptor("test.name", export.MeasureKind, nil, "", "", core.Float64NumberKind, false)
 	for name, tc := range map[string]export.Aggregator{
-		"ddsketch":    ddsketch.New(ddsketch.NewDefaultConfig(), desc),
-		"maxsumcount": maxsumcount.New(desc),
+		"ddsketch":       ddsketch.New(ddsketch.NewDefaultConfig(), desc),
+		"minmaxsumcount": minmaxsumcount.New(desc),
 	} {
 		tc := tc
 		t.Run(name, func(t *testing.T) {

sdk/export/metric/aggregator/aggregator.go

@@ -37,6 +37,11 @@ type (
 		Count() (int64, error)
 	}
 
+	// Min returns the minimum value over the set of values that were aggregated.
+	Min interface {
+		Min() (core.Number, error)
+	}
+
 	// Max returns the maximum value over the set of values that were aggregated.
 	Max interface {
 		Max() (core.Number, error)
@@ -58,17 +63,18 @@ type (
 		Points() ([]core.Number, error)
 	}
 
-	// MaxSumCount supports the Max, Sum, and Count interfaces.
-	MaxSumCount interface {
+	// MinMaxSumCount supports the Min, Max, Sum, and Count interfaces.
+	MinMaxSumCount interface {
+		Min
+		Max
 		Sum
 		Count
-		Max
 	}
 
-	// MaxSumCount supports the Max, Sum, Count, and Quantile
+	// Distribution supports the Min, Max, Sum, Count, and Quantile
 	// interfaces.
 	Distribution interface {
-		MaxSumCount
+		MinMaxSumCount
 		Quantile
 	}
 )

sdk/metric/aggregator/array/array.go

@@ -38,7 +38,7 @@ type (
 )
 
 var _ export.Aggregator = &Aggregator{}
-var _ aggregator.MaxSumCount = &Aggregator{}
+var _ aggregator.MinMaxSumCount = &Aggregator{}
 var _ aggregator.Distribution = &Aggregator{}
 var _ aggregator.Points = &Aggregator{}
 

sdk/metric/aggregator/ddsketch/ddsketch.go

@@ -39,7 +39,7 @@ type Aggregator struct {
 }
 
 var _ export.Aggregator = &Aggregator{}
-var _ aggregator.MaxSumCount = &Aggregator{}
+var _ aggregator.MinMaxSumCount = &Aggregator{}
 var _ aggregator.Distribution = &Aggregator{}
 
 // New returns a new DDSketch aggregator.

sdk/metric/aggregator/minmaxsumcount/mmsc.go

@@ -12,7 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-package maxsumcount // import "go.opentelemetry.io/otel/sdk/metric/aggregator/maxsumcount"
+package minmaxsumcount // import "go.opentelemetry.io/otel/sdk/metric/aggregator/minmaxsumcount"
 
 import (
 	"context"
@@ -34,17 +34,15 @@ type (
 	state struct {
 		count core.Number
 		sum   core.Number
+		min   core.Number
 		max   core.Number
 	}
 )
 
-// TODO: The SDK specification says this type should support Min
-// values, see #319.
-
 var _ export.Aggregator = &Aggregator{}
-var _ aggregator.MaxSumCount = &Aggregator{}
+var _ aggregator.MinMaxSumCount = &Aggregator{}
 
-// New returns a new measure aggregator for computing max, sum, and
+// New returns a new measure aggregator for computing min, max, sum, and
 // count.  It does not compute quantile information other than Max.
 //
 // Note that this aggregator maintains each value using independent
@@ -54,12 +52,12 @@ var _ aggregator.MaxSumCount = &Aggregator{}
 func New(desc *export.Descriptor) *Aggregator {
 	return &Aggregator{
 		kind:    desc.NumberKind(),
-		current: unsetMaxSumCount(desc.NumberKind()),
+		current: unsetMinMaxSumCount(desc.NumberKind()),
 	}
 }
 
-func unsetMaxSumCount(kind core.NumberKind) state {
-	return state{max: kind.Minimum()}
+func unsetMinMaxSumCount(kind core.NumberKind) state {
+	return state{min: kind.Maximum(), max: kind.Minimum()}
 }
 
 // Sum returns the sum of values in the checkpoint.
@@ -72,9 +70,23 @@ func (c *Aggregator) Count() (int64, error) {
 	return int64(c.checkpoint.count.AsUint64()), nil
 }
 
+// Min returns the minimum value in the checkpoint.
+// The error value aggregator.ErrEmptyDataSet will be returned if
+// (due to a race condition) the checkpoint was set prior to
+// current.min being computed in Update().
+//
+// Note: If a measure's recorded values for a given checkpoint are
+// all equal to NumberKind.Maximum(), Min() will return ErrEmptyDataSet
+func (c *Aggregator) Min() (core.Number, error) {
+	if c.checkpoint.min == c.kind.Maximum() {
+		return core.Number(0), aggregator.ErrEmptyDataSet
+	}
+	return c.checkpoint.min, nil
+}
+
 // Max returns the maximum value in the checkpoint.
 // The error value aggregator.ErrEmptyDataSet will be returned if
-// (due to a race condition) the checkpoint was set prior to the
+// (due to a race condition) the checkpoint was set prior to
 // current.max being computed in Update().
 //
 // Note: If a measure's recorded values for a given checkpoint are
@@ -88,7 +100,7 @@ func (c *Aggregator) Max() (core.Number, error) {
 
 // Checkpoint saves the current state and resets the current state to
 // the empty set.  Since no locks are taken, there is a chance that
-// the independent Max, Sum, and Count are not consistent with each
+// the independent Min, Max, Sum, and Count are not consistent with each
 // other.
 func (c *Aggregator) Checkpoint(ctx context.Context, desc *export.Descriptor) {
 	// N.B. There is no atomic operation that can update all three
@@ -104,6 +116,7 @@ func (c *Aggregator) Checkpoint(ctx context.Context, desc *export.Descriptor) {
 	c.checkpoint.count.SetUint64(c.current.count.SwapUint64Atomic(0))
 	c.checkpoint.sum = c.current.sum.SwapNumberAtomic(core.Number(0))
 	c.checkpoint.max = c.current.max.SwapNumberAtomic(c.kind.Minimum())
+	c.checkpoint.min = c.current.min.SwapNumberAtomic(c.kind.Maximum())
 }
 
 // Update adds the recorded measurement to the current data set.
@@ -113,6 +126,16 @@ func (c *Aggregator) Update(_ context.Context, number core.Number, desc *export.
 	c.current.count.AddUint64Atomic(1)
 	c.current.sum.AddNumberAtomic(kind, number)
 
+	for {
+		current := c.current.min.AsNumberAtomic()
+
+		if number.CompareNumber(kind, current) >= 0 {
+			break
+		}
+		if c.current.min.CompareAndSwapNumber(current, number) {
+			break
+		}
+	}
 	for {
 		current := c.current.max.AsNumberAtomic()
 
@@ -136,6 +159,9 @@ func (c *Aggregator) Merge(oa export.Aggregator, desc *export.Descriptor) error
 	c.checkpoint.sum.AddNumber(desc.NumberKind(), o.checkpoint.sum)
 	c.checkpoint.count.AddNumber(core.Uint64NumberKind, o.checkpoint.count)
 
+	if c.checkpoint.min.CompareNumber(desc.NumberKind(), o.checkpoint.min) > 0 {
+		c.checkpoint.min.SetNumber(o.checkpoint.min)
+	}
 	if c.checkpoint.max.CompareNumber(desc.NumberKind(), o.checkpoint.max) < 0 {
 		c.checkpoint.max.SetNumber(o.checkpoint.max)
 	}

sdk/metric/aggregator/minmaxsumcount/mmsc_test.go

@@ -12,7 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-package maxsumcount
+package minmaxsumcount
 
 import (
 	"context"
@@ -59,26 +59,26 @@ var (
 	}
 )
 
-func TestMaxSumCountAbsolute(t *testing.T) {
+func TestMinMaxSumCountAbsolute(t *testing.T) {
 	test.RunProfiles(t, func(t *testing.T, profile test.Profile) {
-		maxSumCount(t, profile, positiveOnly)
+		minMaxSumCount(t, profile, positiveOnly)
 	})
 }
 
-func TestMaxSumCountNegativeOnly(t *testing.T) {
+func TestMinMaxSumCountNegativeOnly(t *testing.T) {
 	test.RunProfiles(t, func(t *testing.T, profile test.Profile) {
-		maxSumCount(t, profile, negativeOnly)
+		minMaxSumCount(t, profile, negativeOnly)
 	})
 }
 
-func TestMaxSumCountPositiveAndNegative(t *testing.T) {
+func TestMinMaxSumCountPositiveAndNegative(t *testing.T) {
 	test.RunProfiles(t, func(t *testing.T, profile test.Profile) {
-		maxSumCount(t, profile, positiveAndNegative)
+		minMaxSumCount(t, profile, positiveAndNegative)
 	})
 }
 
-// Validates max, sum and count for a given profile and policy
-func maxSumCount(t *testing.T, profile test.Profile, policy policy) {
+// Validates min, max, sum and count for a given profile and policy
+func minMaxSumCount(t *testing.T, profile test.Profile, policy policy) {
 	ctx := context.Background()
 	descriptor := test.NewAggregatorTest(export.MeasureKind, profile.NumberKind, !policy.absolute)
 
@@ -108,6 +108,13 @@ func maxSumCount(t *testing.T, profile test.Profile, policy policy) {
 	require.Equal(t, all.Count(), count, "Same count -"+policy.name)
 	require.Nil(t, err)
 
+	min, err := agg.Min()
+	require.Nil(t, err)
+	require.Equal(t,
+		all.Min(),
+		min,
+		"Same min -"+policy.name)
+
 	max, err := agg.Max()
 	require.Nil(t, err)
 	require.Equal(t,
@@ -116,7 +123,7 @@ func maxSumCount(t *testing.T, profile test.Profile, policy policy) {
 		"Same max -"+policy.name)
 }
 
-func TestMaxSumCountMerge(t *testing.T) {
+func TestMinMaxSumCountMerge(t *testing.T) {
 	ctx := context.Background()
 
 	test.RunProfiles(t, func(t *testing.T, profile test.Profile) {
@@ -157,6 +164,13 @@ func TestMaxSumCountMerge(t *testing.T) {
 		require.Equal(t, all.Count(), count, "Same count - absolute")
 		require.Nil(t, err)
 
+		min, err := agg1.Min()
+		require.Nil(t, err)
+		require.Equal(t,
+			all.Min(),
+			min,
+			"Same min - absolute")
+
 		max, err := agg1.Max()
 		require.Nil(t, err)
 		require.Equal(t,

sdk/metric/benchmark_test.go

@@ -29,7 +29,7 @@ import (
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/counter"
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/ddsketch"
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/gauge"
-	"go.opentelemetry.io/otel/sdk/metric/aggregator/maxsumcount"
+	"go.opentelemetry.io/otel/sdk/metric/aggregator/minmaxsumcount"
 )
 
 type benchFixture struct {
@@ -53,8 +53,8 @@ func (*benchFixture) AggregatorFor(descriptor *export.Descriptor) export.Aggrega
 	case export.GaugeKind:
 		return gauge.New()
 	case export.MeasureKind:
-		if strings.HasSuffix(descriptor.Name(), "maxsumcount") {
-			return maxsumcount.New(descriptor)
+		if strings.HasSuffix(descriptor.Name(), "minmaxsumcount") {
+			return minmaxsumcount.New(descriptor)
 		} else if strings.HasSuffix(descriptor.Name(), "ddsketch") {
 			return ddsketch.New(ddsketch.NewDefaultConfig(), descriptor)
 		} else if strings.HasSuffix(descriptor.Name(), "array") {
@@ -360,19 +360,19 @@ func benchmarkFloat64MeasureHandleAdd(b *testing.B, name string) {
 // MaxSumCount
 
 func BenchmarkInt64MaxSumCountAdd(b *testing.B) {
-	benchmarkInt64MeasureAdd(b, "int64.maxsumcount")
+	benchmarkInt64MeasureAdd(b, "int64.minmaxsumcount")
 }
 
 func BenchmarkInt64MaxSumCountHandleAdd(b *testing.B) {
-	benchmarkInt64MeasureHandleAdd(b, "int64.maxsumcount")
+	benchmarkInt64MeasureHandleAdd(b, "int64.minmaxsumcount")
 }
 
 func BenchmarkFloat64MaxSumCountAdd(b *testing.B) {
-	benchmarkFloat64MeasureAdd(b, "float64.maxsumcount")
+	benchmarkFloat64MeasureAdd(b, "float64.minmaxsumcount")
 }
 
 func BenchmarkFloat64MaxSumCountHandleAdd(b *testing.B) {
-	benchmarkFloat64MeasureHandleAdd(b, "float64.maxsumcount")
+	benchmarkFloat64MeasureHandleAdd(b, "float64.minmaxsumcount")
 }
 
 // DDSketch

sdk/metric/selector/simple/simple.go

@@ -20,7 +20,7 @@ import (
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/counter"
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/ddsketch"
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/gauge"
-	"go.opentelemetry.io/otel/sdk/metric/aggregator/maxsumcount"
+	"go.opentelemetry.io/otel/sdk/metric/aggregator/minmaxsumcount"
 )
 
 type (
@@ -38,9 +38,9 @@ var (
 )
 
 // NewWithInexpensiveMeasure returns a simple aggregation selector
-// that uses counter, gauge, and maxsumcount aggregators for the three
+// that uses counter, gauge, and minmaxsumcount aggregators for the three
 // kinds of metric.  This selector is faster and uses less memory than
-// the others because maxsumcount does not aggregate quantile
+// the others because minmaxsumcount does not aggregate quantile
 // information.
 func NewWithInexpensiveMeasure() export.AggregationSelector {
 	return selectorInexpensive{}
@@ -71,7 +71,7 @@ func (selectorInexpensive) AggregatorFor(descriptor *export.Descriptor) export.A
 	case export.GaugeKind:
 		return gauge.New()
 	case export.MeasureKind:
-		return maxsumcount.New(descriptor)
+		return minmaxsumcount.New(descriptor)
 	default:
 		return counter.New()
 	}

sdk/metric/selector/simple/simple_test.go

@@ -25,7 +25,7 @@ import (
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/counter"
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/ddsketch"
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/gauge"
-	"go.opentelemetry.io/otel/sdk/metric/aggregator/maxsumcount"
+	"go.opentelemetry.io/otel/sdk/metric/aggregator/minmaxsumcount"
 	"go.opentelemetry.io/otel/sdk/metric/selector/simple"
 )
 
@@ -39,7 +39,7 @@ func TestInexpensiveMeasure(t *testing.T) {
 	inex := simple.NewWithInexpensiveMeasure()
 	require.NotPanics(t, func() { _ = inex.AggregatorFor(testGaugeDesc).(*gauge.Aggregator) })
 	require.NotPanics(t, func() { _ = inex.AggregatorFor(testCounterDesc).(*counter.Aggregator) })
-	require.NotPanics(t, func() { _ = inex.AggregatorFor(testMeasureDesc).(*maxsumcount.Aggregator) })
+	require.NotPanics(t, func() { _ = inex.AggregatorFor(testMeasureDesc).(*minmaxsumcount.Aggregator) })
 }
 
 func TestSketchMeasure(t *testing.T) {