From da2bdb824920ff45e1ead8b4bc74cfb682e992fb Mon Sep 17 00:00:00 2001
From: jmacd <jmacd@lightstep.com>
Date: Tue, 9 Jun 2020 22:41:51 -0700
Subject: [PATCH 1/7] Update aggregation.go/aggregator.go

---
 .../aggregation.go}                           | 54 +++++++++----------
 sdk/metric/aggregator/aggregator.go           | 51 ++++++++++++++++++
 .../metric/aggregator/aggregator_test.go      | 11 ++--
 3 files changed, 82 insertions(+), 34 deletions(-)
 rename sdk/export/metric/{aggregator/aggregator.go => aggregation/aggregation.go} (73%)
 create mode 100644 sdk/metric/aggregator/aggregator.go
 rename sdk/{export => }/metric/aggregator/aggregator_test.go (89%)

diff --git a/sdk/export/metric/aggregator/aggregator.go b/sdk/export/metric/aggregation/aggregation.go
similarity index 73%
rename from sdk/export/metric/aggregator/aggregator.go
rename to sdk/export/metric/aggregation/aggregation.go
index 58176041281..bec6449b766 100644
--- a/sdk/export/metric/aggregator/aggregator.go
+++ b/sdk/export/metric/aggregation/aggregation.go
@@ -12,21 +12,25 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-package aggregator // import "go.opentelemetry.io/otel/sdk/export/metric/aggregator"
+package aggregation // import "go.opentelemetry.io/otel/sdk/export/metric/aggregation"
 
 import (
 	"fmt"
-	"math"
 	"time"
 
 	"go.opentelemetry.io/otel/api/metric"
-	export "go.opentelemetry.io/otel/sdk/export/metric"
 )
 
 // These interfaces describe the various ways to access state from an
-// Aggregator.
+// Aggregation.
 
 type (
+	// Aggregation is one of the interfaces below.
+	Aggregation interface {
+		// Kind returns the kind of aggregation used.
+		Kind() Kind
+	}
+
 	// Sum returns an aggregated sum.
 	Sum interface {
 		Sum() (metric.Number, error)
@@ -100,11 +104,25 @@ type (
 	}
 )
 
+type (
+	Kind string
+)
+
+const (
+	SumKind            Kind = "sum"
+	MinMaxSumCountKind Kind = "minmaxsumcount"
+	HistogramKind      Kind = "histogram"
+	LastValueKind      Kind = "lastvalue"
+	SketchKind         Kind = "sketch"
+	ExactKind          Kind = "exact"
+)
+
 var (
 	ErrInvalidQuantile  = fmt.Errorf("the requested quantile is out of range")
 	ErrNegativeInput    = fmt.Errorf("negative value is out of range for this instrument")
 	ErrNaNInput         = fmt.Errorf("NaN value is an invalid input")
 	ErrInconsistentType = fmt.Errorf("inconsistent aggregator types")
+	ErrNoSubtraction    = fmt.Errorf("aggregator does not subtract")
 
 	// ErrNoData is returned when (due to a race with collection)
 	// the Aggregator is check-pointed before the first value is set.
@@ -112,29 +130,7 @@ var (
 	ErrNoData = fmt.Errorf("no data collected by this aggregator")
 )
 
-// NewInconsistentMergeError formats an error describing an attempt to
-// merge different-type aggregators.  The result can be unwrapped as
-// an ErrInconsistentType.
-func NewInconsistentMergeError(a1, a2 export.Aggregator) error {
-	return fmt.Errorf("cannot merge %T with %T: %w", a1, a2, ErrInconsistentType)
-}
-
-// RangeTest is a commmon routine for testing for valid input values.
-// This rejects NaN values.  This rejects negative values when the
-// metric instrument does not support negative values, including
-// monotonic counter metrics and absolute ValueRecorder metrics.
-func RangeTest(number metric.Number, descriptor *metric.Descriptor) error {
-	numberKind := descriptor.NumberKind()
-
-	if numberKind == metric.Float64NumberKind && math.IsNaN(number.AsFloat64()) {
-		return ErrNaNInput
-	}
-
-	switch descriptor.MetricKind() {
-	case metric.CounterKind, metric.SumObserverKind:
-		if number.IsNegative(numberKind) {
-			return ErrNegativeInput
-		}
-	}
-	return nil
+// String returns a string representation of the aggregation kind.
+func (k Kind) String() string {
+	return string(k)
 }
diff --git a/sdk/metric/aggregator/aggregator.go b/sdk/metric/aggregator/aggregator.go
new file mode 100644
index 00000000000..f401347389c
--- /dev/null
+++ b/sdk/metric/aggregator/aggregator.go
@@ -0,0 +1,51 @@
+// Copyright The 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 aggregator // import "go.opentelemetry.io/otel/sdk/metric/aggregator"
+
+import (
+	"fmt"
+	"math"
+
+	"go.opentelemetry.io/otel/api/metric"
+	export "go.opentelemetry.io/otel/sdk/export/metric"
+	"go.opentelemetry.io/otel/sdk/export/metric/aggregation"
+)
+
+// NewInconsistentMergeError formats an error describing an attempt to
+// merge different-type aggregators.  The result can be unwrapped as
+// an ErrInconsistentType.
+func NewInconsistentMergeError(a1, a2 export.Aggregator) error {
+	return fmt.Errorf("cannot merge %T with %T: %w", a1, a2, aggregation.ErrInconsistentType)
+}
+
+// RangeTest is a commmon routine for testing for valid input values.
+// This rejects NaN values.  This rejects negative values when the
+// metric instrument does not support negative values, including
+// monotonic counter metrics and absolute ValueRecorder metrics.
+func RangeTest(number metric.Number, descriptor *metric.Descriptor) error {
+	numberKind := descriptor.NumberKind()
+
+	if numberKind == metric.Float64NumberKind && math.IsNaN(number.AsFloat64()) {
+		return aggregation.ErrNaNInput
+	}
+
+	switch descriptor.MetricKind() {
+	case metric.CounterKind, metric.SumObserverKind:
+		if number.IsNegative(numberKind) {
+			return aggregation.ErrNegativeInput
+		}
+	}
+	return nil
+}
diff --git a/sdk/export/metric/aggregator/aggregator_test.go b/sdk/metric/aggregator/aggregator_test.go
similarity index 89%
rename from sdk/export/metric/aggregator/aggregator_test.go
rename to sdk/metric/aggregator/aggregator_test.go
index ce7624949a2..350a261f119 100644
--- a/sdk/export/metric/aggregator/aggregator_test.go
+++ b/sdk/metric/aggregator/aggregator_test.go
@@ -12,7 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-package aggregator_test // import "go.opentelemetry.io/otel/sdk/export/metric/aggregator"
+package aggregator_test // import "go.opentelemetry.io/otel/sdk/metric/aggregator"
 
 import (
 	"errors"
@@ -22,7 +22,8 @@ import (
 	"github.com/stretchr/testify/require"
 
 	"go.opentelemetry.io/otel/api/metric"
-	"go.opentelemetry.io/otel/sdk/export/metric/aggregator"
+	"go.opentelemetry.io/otel/sdk/export/metric/aggregation"
+	"go.opentelemetry.io/otel/sdk/metric/aggregator"
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/lastvalue"
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/sum"
 )
@@ -34,7 +35,7 @@ func TestInconsistentMergeErr(t *testing.T) {
 		"cannot merge *sum.Aggregator with *lastvalue.Aggregator: inconsistent aggregator types",
 		err.Error(),
 	)
-	require.True(t, errors.Is(err, aggregator.ErrInconsistentType))
+	require.True(t, errors.Is(err, aggregation.ErrInconsistentType))
 }
 
 func testRangeNaN(t *testing.T, desc *metric.Descriptor) {
@@ -43,7 +44,7 @@ func testRangeNaN(t *testing.T, desc *metric.Descriptor) {
 	err := aggregator.RangeTest(nan, desc)
 
 	if desc.NumberKind() == metric.Float64NumberKind {
-		require.Equal(t, aggregator.ErrNaNInput, err)
+		require.Equal(t, aggregation.ErrNaNInput, err)
 	} else {
 		require.Nil(t, err)
 	}
@@ -64,7 +65,7 @@ func testRangeNegative(t *testing.T, desc *metric.Descriptor) {
 	negErr := aggregator.RangeTest(neg, desc)
 
 	require.Nil(t, posErr)
-	require.Equal(t, negErr, aggregator.ErrNegativeInput)
+	require.Equal(t, negErr, aggregation.ErrNegativeInput)
 }
 
 func TestRangeTest(t *testing.T) {

From 6f0c6b46834db644cc93f0314cca99d37c0d3f87 Mon Sep 17 00:00:00 2001
From: jmacd <jmacd@lightstep.com>
Date: Tue, 9 Jun 2020 22:53:30 -0700
Subject: [PATCH 2/7] Split aggregation.go

---
 exporters/metric/prometheus/prometheus.go     | 18 +++++++--------
 exporters/metric/stdout/stdout.go             | 12 +++++-----
 exporters/metric/stdout/stdout_test.go        |  4 ++--
 exporters/metric/test/test.go                 |  4 ++--
 exporters/otlp/internal/transform/metric.go   | 12 +++++-----
 .../otlp/internal/transform/metric_test.go    |  6 ++---
 exporters/otlp/otlp_metric_test.go            |  4 ++--
 sdk/metric/aggregator/array/array.go          | 13 ++++++-----
 sdk/metric/aggregator/array/array_test.go     | 12 +++++-----
 sdk/metric/aggregator/ddsketch/ddsketch.go    | 12 +++++-----
 sdk/metric/aggregator/histogram/histogram.go  | 13 ++++++-----
 sdk/metric/aggregator/lastvalue/lastvalue.go  |  9 ++++----
 .../aggregator/lastvalue/lastvalue_test.go    |  4 ++--
 sdk/metric/aggregator/minmaxsumcount/mmsc.go  | 13 ++++++-----
 .../aggregator/minmaxsumcount/mmsc_test.go    |  4 ++--
 sdk/metric/aggregator/sum/sum.go              |  7 +++---
 sdk/metric/aggregator/test/test.go            |  2 +-
 sdk/metric/controller/push/push_test.go       |  8 +++----
 sdk/metric/correct_test.go                    | 22 +++++++++----------
 sdk/metric/integrator/simple/simple.go        |  4 ++--
 sdk/metric/integrator/test/test.go            |  6 ++---
 sdk/metric/sdk.go                             |  2 +-
 sdk/metric/stress_test.go                     |  8 +++----
 23 files changed, 102 insertions(+), 97 deletions(-)

diff --git a/exporters/metric/prometheus/prometheus.go b/exporters/metric/prometheus/prometheus.go
index a5625a2dd1e..8e082f0d19f 100644
--- a/exporters/metric/prometheus/prometheus.go
+++ b/exporters/metric/prometheus/prometheus.go
@@ -27,7 +27,7 @@ import (
 	"go.opentelemetry.io/otel/api/label"
 	"go.opentelemetry.io/otel/api/metric"
 	export "go.opentelemetry.io/otel/sdk/export/metric"
-	"go.opentelemetry.io/otel/sdk/export/metric/aggregator"
+	"go.opentelemetry.io/otel/sdk/export/metric/aggregation"
 	"go.opentelemetry.io/otel/sdk/metric/controller/pull"
 	"go.opentelemetry.io/otel/sdk/metric/selector/simple"
 )
@@ -216,11 +216,11 @@ func (c *collector) Collect(ch chan<- prometheus.Metric) {
 
 		desc := c.toDesc(record, labelKeys)
 
-		if hist, ok := agg.(aggregator.Histogram); ok {
+		if hist, ok := agg.(aggregation.Histogram); ok {
 			if err := c.exportHistogram(ch, hist, numberKind, desc, labels); err != nil {
 				return fmt.Errorf("exporting histogram: %w", err)
 			}
-		} else if dist, ok := agg.(aggregator.Distribution); ok {
+		} else if dist, ok := agg.(aggregation.Distribution); ok {
 			// TODO: summaries values are never being resetted.
 			//  As measurements are recorded, new records starts to have less impact on these summaries.
 			//  We should implement an solution that is similar to the Prometheus Clients
@@ -232,11 +232,11 @@ func (c *collector) Collect(ch chan<- prometheus.Metric) {
 			if err := c.exportSummary(ch, dist, numberKind, desc, labels); err != nil {
 				return fmt.Errorf("exporting summary: %w", err)
 			}
-		} else if sum, ok := agg.(aggregator.Sum); ok {
+		} else if sum, ok := agg.(aggregation.Sum); ok {
 			if err := c.exportCounter(ch, sum, numberKind, desc, labels); err != nil {
 				return fmt.Errorf("exporting counter: %w", err)
 			}
-		} else if lastValue, ok := agg.(aggregator.LastValue); ok {
+		} else if lastValue, ok := agg.(aggregation.LastValue); ok {
 			if err := c.exportLastValue(ch, lastValue, numberKind, desc, labels); err != nil {
 				return fmt.Errorf("exporting last value: %w", err)
 			}
@@ -248,7 +248,7 @@ func (c *collector) Collect(ch chan<- prometheus.Metric) {
 	}
 }
 
-func (c *collector) exportLastValue(ch chan<- prometheus.Metric, lvagg aggregator.LastValue, kind metric.NumberKind, desc *prometheus.Desc, labels []string) error {
+func (c *collector) exportLastValue(ch chan<- prometheus.Metric, lvagg aggregation.LastValue, kind metric.NumberKind, desc *prometheus.Desc, labels []string) error {
 	lv, _, err := lvagg.LastValue()
 	if err != nil {
 		return fmt.Errorf("error retrieving last value: %w", err)
@@ -263,7 +263,7 @@ func (c *collector) exportLastValue(ch chan<- prometheus.Metric, lvagg aggregato
 	return nil
 }
 
-func (c *collector) exportCounter(ch chan<- prometheus.Metric, sum aggregator.Sum, kind metric.NumberKind, desc *prometheus.Desc, labels []string) error {
+func (c *collector) exportCounter(ch chan<- prometheus.Metric, sum aggregation.Sum, kind metric.NumberKind, desc *prometheus.Desc, labels []string) error {
 	v, err := sum.Sum()
 	if err != nil {
 		return fmt.Errorf("error retrieving counter: %w", err)
@@ -278,7 +278,7 @@ func (c *collector) exportCounter(ch chan<- prometheus.Metric, sum aggregator.Su
 	return nil
 }
 
-func (c *collector) exportSummary(ch chan<- prometheus.Metric, dist aggregator.Distribution, kind metric.NumberKind, desc *prometheus.Desc, labels []string) error {
+func (c *collector) exportSummary(ch chan<- prometheus.Metric, dist aggregation.Distribution, kind metric.NumberKind, desc *prometheus.Desc, labels []string) error {
 	count, err := dist.Count()
 	if err != nil {
 		return fmt.Errorf("error retrieving count: %w", err)
@@ -305,7 +305,7 @@ func (c *collector) exportSummary(ch chan<- prometheus.Metric, dist aggregator.D
 	return nil
 }
 
-func (c *collector) exportHistogram(ch chan<- prometheus.Metric, hist aggregator.Histogram, kind metric.NumberKind, desc *prometheus.Desc, labels []string) error {
+func (c *collector) exportHistogram(ch chan<- prometheus.Metric, hist aggregation.Histogram, kind metric.NumberKind, desc *prometheus.Desc, labels []string) error {
 	buckets, err := hist.Histogram()
 	if err != nil {
 		return fmt.Errorf("error retrieving histogram: %w", err)
diff --git a/exporters/metric/stdout/stdout.go b/exporters/metric/stdout/stdout.go
index 270edaa0245..110d2271c70 100644
--- a/exporters/metric/stdout/stdout.go
+++ b/exporters/metric/stdout/stdout.go
@@ -27,7 +27,7 @@ import (
 	"go.opentelemetry.io/otel/api/label"
 
 	export "go.opentelemetry.io/otel/sdk/export/metric"
-	"go.opentelemetry.io/otel/sdk/export/metric/aggregator"
+	"go.opentelemetry.io/otel/sdk/export/metric/aggregation"
 	"go.opentelemetry.io/otel/sdk/metric/controller/push"
 	"go.opentelemetry.io/otel/sdk/metric/selector/simple"
 )
@@ -98,7 +98,7 @@ func NewRawExporter(config Config) (*Exporter, error) {
 	} else {
 		for _, q := range config.Quantiles {
 			if q < 0 || q > 1 {
-				return nil, aggregator.ErrInvalidQuantile
+				return nil, aggregation.ErrInvalidQuantile
 			}
 		}
 	}
@@ -164,7 +164,7 @@ func (e *Exporter) Export(_ context.Context, checkpointSet export.CheckpointSet)
 
 		var expose expoLine
 
-		if sum, ok := agg.(aggregator.Sum); ok {
+		if sum, ok := agg.(aggregation.Sum); ok {
 			value, err := sum.Sum()
 			if err != nil {
 				return err
@@ -172,7 +172,7 @@ func (e *Exporter) Export(_ context.Context, checkpointSet export.CheckpointSet)
 			expose.Sum = value.AsInterface(kind)
 		}
 
-		if mmsc, ok := agg.(aggregator.MinMaxSumCount); ok {
+		if mmsc, ok := agg.(aggregation.MinMaxSumCount); ok {
 			count, err := mmsc.Count()
 			if err != nil {
 				return err
@@ -191,7 +191,7 @@ func (e *Exporter) Export(_ context.Context, checkpointSet export.CheckpointSet)
 			}
 			expose.Min = min.AsInterface(kind)
 
-			if dist, ok := agg.(aggregator.Distribution); ok && len(e.config.Quantiles) != 0 {
+			if dist, ok := agg.(aggregation.Distribution); ok && len(e.config.Quantiles) != 0 {
 				summary := make([]expoQuantile, len(e.config.Quantiles))
 				expose.Quantiles = summary
 
@@ -208,7 +208,7 @@ func (e *Exporter) Export(_ context.Context, checkpointSet export.CheckpointSet)
 					}
 				}
 			}
-		} else if lv, ok := agg.(aggregator.LastValue); ok {
+		} else if lv, ok := agg.(aggregation.LastValue); ok {
 			value, timestamp, err := lv.LastValue()
 			if err != nil {
 				return err
diff --git a/exporters/metric/stdout/stdout_test.go b/exporters/metric/stdout/stdout_test.go
index 9f7e2a62495..e08af2d7e4f 100644
--- a/exporters/metric/stdout/stdout_test.go
+++ b/exporters/metric/stdout/stdout_test.go
@@ -29,7 +29,7 @@ import (
 	"go.opentelemetry.io/otel/exporters/metric/stdout"
 	"go.opentelemetry.io/otel/exporters/metric/test"
 	export "go.opentelemetry.io/otel/sdk/export/metric"
-	"go.opentelemetry.io/otel/sdk/export/metric/aggregator"
+	"go.opentelemetry.io/otel/sdk/export/metric/aggregation"
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/array"
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/ddsketch"
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/lastvalue"
@@ -80,7 +80,7 @@ func TestStdoutInvalidQuantile(t *testing.T) {
 		Quantiles: []float64{1.1, 0.9},
 	})
 	require.Error(t, err, "Invalid quantile error expected")
-	require.Equal(t, aggregator.ErrInvalidQuantile, err)
+	require.Equal(t, aggregation.ErrInvalidQuantile, err)
 }
 
 func TestStdoutTimestamp(t *testing.T) {
diff --git a/exporters/metric/test/test.go b/exporters/metric/test/test.go
index 396c8284c12..2185de49bff 100644
--- a/exporters/metric/test/test.go
+++ b/exporters/metric/test/test.go
@@ -23,7 +23,7 @@ import (
 	"go.opentelemetry.io/otel/api/label"
 	"go.opentelemetry.io/otel/api/metric"
 	export "go.opentelemetry.io/otel/sdk/export/metric"
-	"go.opentelemetry.io/otel/sdk/export/metric/aggregator"
+	"go.opentelemetry.io/otel/sdk/export/metric/aggregation"
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/array"
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/histogram"
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/lastvalue"
@@ -117,7 +117,7 @@ func (p *CheckpointSet) updateAggregator(desc *metric.Descriptor, newAgg export.
 
 func (p *CheckpointSet) ForEach(f func(export.Record) error) error {
 	for _, r := range p.updates {
-		if err := f(r); err != nil && !errors.Is(err, aggregator.ErrNoData) {
+		if err := f(r); err != nil && !errors.Is(err, aggregation.ErrNoData) {
 			return err
 		}
 	}
diff --git a/exporters/otlp/internal/transform/metric.go b/exporters/otlp/internal/transform/metric.go
index 318378961bc..4875cce5bac 100644
--- a/exporters/otlp/internal/transform/metric.go
+++ b/exporters/otlp/internal/transform/metric.go
@@ -30,7 +30,7 @@ import (
 	"go.opentelemetry.io/otel/api/label"
 	"go.opentelemetry.io/otel/api/metric"
 	export "go.opentelemetry.io/otel/sdk/export/metric"
-	"go.opentelemetry.io/otel/sdk/export/metric/aggregator"
+	"go.opentelemetry.io/otel/sdk/export/metric/aggregation"
 	"go.opentelemetry.io/otel/sdk/resource"
 )
 
@@ -230,9 +230,9 @@ func Record(r export.Record) (*metricpb.Metric, error) {
 	d := r.Descriptor()
 	l := r.Labels()
 	switch a := r.Aggregator().(type) {
-	case aggregator.MinMaxSumCount:
+	case aggregation.MinMaxSumCount:
 		return minMaxSumCount(d, l, a)
-	case aggregator.Sum:
+	case aggregation.Sum:
 		return sum(d, l, a)
 	default:
 		return nil, fmt.Errorf("%w: %v", ErrUnimplementedAgg, a)
@@ -240,7 +240,7 @@ func Record(r export.Record) (*metricpb.Metric, error) {
 }
 
 // sum transforms a Sum Aggregator into an OTLP Metric.
-func sum(desc *metric.Descriptor, labels *label.Set, a aggregator.Sum) (*metricpb.Metric, error) {
+func sum(desc *metric.Descriptor, labels *label.Set, a aggregation.Sum) (*metricpb.Metric, error) {
 	sum, err := a.Sum()
 	if err != nil {
 		return nil, err
@@ -275,7 +275,7 @@ func sum(desc *metric.Descriptor, labels *label.Set, a aggregator.Sum) (*metricp
 
 // minMaxSumCountValue returns the values of the MinMaxSumCount Aggregator
 // as discret values.
-func minMaxSumCountValues(a aggregator.MinMaxSumCount) (min, max, sum metric.Number, count int64, err error) {
+func minMaxSumCountValues(a aggregation.MinMaxSumCount) (min, max, sum metric.Number, count int64, err error) {
 	if min, err = a.Min(); err != nil {
 		return
 	}
@@ -292,7 +292,7 @@ func minMaxSumCountValues(a aggregator.MinMaxSumCount) (min, max, sum metric.Num
 }
 
 // minMaxSumCount transforms a MinMaxSumCount Aggregator into an OTLP Metric.
-func minMaxSumCount(desc *metric.Descriptor, labels *label.Set, a aggregator.MinMaxSumCount) (*metricpb.Metric, error) {
+func minMaxSumCount(desc *metric.Descriptor, labels *label.Set, a aggregation.MinMaxSumCount) (*metricpb.Metric, error) {
 	min, max, sum, count, err := minMaxSumCountValues(a)
 	if err != nil {
 		return nil, err
diff --git a/exporters/otlp/internal/transform/metric_test.go b/exporters/otlp/internal/transform/metric_test.go
index 930915d2cb4..55c76c50225 100644
--- a/exporters/otlp/internal/transform/metric_test.go
+++ b/exporters/otlp/internal/transform/metric_test.go
@@ -27,7 +27,7 @@ import (
 	"go.opentelemetry.io/otel/api/label"
 	"go.opentelemetry.io/otel/api/metric"
 	"go.opentelemetry.io/otel/api/unit"
-	"go.opentelemetry.io/otel/sdk/export/metric/aggregator"
+	"go.opentelemetry.io/otel/sdk/export/metric/aggregation"
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/minmaxsumcount"
 	sumAgg "go.opentelemetry.io/otel/sdk/metric/aggregator/sum"
 )
@@ -86,7 +86,7 @@ func TestMinMaxSumCountValue(t *testing.T) {
 
 	// Prior to checkpointing ErrNoData should be returned.
 	_, _, _, _, err := minMaxSumCountValues(mmsc)
-	assert.EqualError(t, err, aggregator.ErrNoData.Error())
+	assert.EqualError(t, err, aggregation.ErrNoData.Error())
 
 	// Checkpoint to set non-zero values
 	mmsc.Checkpoint(&metric.Descriptor{})
@@ -198,7 +198,7 @@ func TestMinMaxSumCountPropagatesErrors(t *testing.T) {
 	mmsc := minmaxsumcount.New(&metric.Descriptor{})
 	_, _, _, _, err := minMaxSumCountValues(mmsc)
 	assert.Error(t, err)
-	assert.Equal(t, aggregator.ErrNoData, err)
+	assert.Equal(t, aggregation.ErrNoData, err)
 }
 
 func TestSumMetricDescriptor(t *testing.T) {
diff --git a/exporters/otlp/otlp_metric_test.go b/exporters/otlp/otlp_metric_test.go
index 4d90c5f024b..1afcfb44640 100644
--- a/exporters/otlp/otlp_metric_test.go
+++ b/exporters/otlp/otlp_metric_test.go
@@ -30,7 +30,7 @@ import (
 	"go.opentelemetry.io/otel/api/label"
 	"go.opentelemetry.io/otel/api/metric"
 	metricsdk "go.opentelemetry.io/otel/sdk/export/metric"
-	"go.opentelemetry.io/otel/sdk/export/metric/aggregator"
+	"go.opentelemetry.io/otel/sdk/export/metric/aggregation"
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/minmaxsumcount"
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/sum"
 	"go.opentelemetry.io/otel/sdk/resource"
@@ -67,7 +67,7 @@ type checkpointSet struct {
 
 func (m *checkpointSet) ForEach(fn func(metricsdk.Record) error) error {
 	for _, r := range m.records {
-		if err := fn(r); err != nil && err != aggregator.ErrNoData {
+		if err := fn(r); err != nil && err != aggregation.ErrNoData {
 			return err
 		}
 	}
diff --git a/sdk/metric/aggregator/array/array.go b/sdk/metric/aggregator/array/array.go
index f3b15c564bc..0284000523a 100644
--- a/sdk/metric/aggregator/array/array.go
+++ b/sdk/metric/aggregator/array/array.go
@@ -23,7 +23,8 @@ import (
 
 	"go.opentelemetry.io/otel/api/metric"
 	export "go.opentelemetry.io/otel/sdk/export/metric"
-	"go.opentelemetry.io/otel/sdk/export/metric/aggregator"
+	"go.opentelemetry.io/otel/sdk/export/metric/aggregation"
+	"go.opentelemetry.io/otel/sdk/metric/aggregator"
 )
 
 type (
@@ -41,9 +42,9 @@ type (
 )
 
 var _ export.Aggregator = &Aggregator{}
-var _ aggregator.MinMaxSumCount = &Aggregator{}
-var _ aggregator.Distribution = &Aggregator{}
-var _ aggregator.Points = &Aggregator{}
+var _ aggregation.MinMaxSumCount = &Aggregator{}
+var _ aggregation.Distribution = &Aggregator{}
+var _ aggregation.Points = &Aggregator{}
 
 // New returns a new array aggregator, which aggregates recorded
 // measurements by storing them in an array.  This type uses a mutex
@@ -179,11 +180,11 @@ func (p *points) Swap(i, j int) {
 // of a quantile.
 func (p *points) Quantile(q float64) (metric.Number, error) {
 	if len(*p) == 0 {
-		return metric.Number(0), aggregator.ErrNoData
+		return metric.Number(0), aggregation.ErrNoData
 	}
 
 	if q < 0 || q > 1 {
-		return metric.Number(0), aggregator.ErrInvalidQuantile
+		return metric.Number(0), aggregation.ErrInvalidQuantile
 	}
 
 	if q == 0 || len(*p) == 1 {
diff --git a/sdk/metric/aggregator/array/array_test.go b/sdk/metric/aggregator/array/array_test.go
index e6ce683bf25..3678530f5d3 100644
--- a/sdk/metric/aggregator/array/array_test.go
+++ b/sdk/metric/aggregator/array/array_test.go
@@ -25,7 +25,7 @@ import (
 
 	"go.opentelemetry.io/otel/api/metric"
 	ottest "go.opentelemetry.io/otel/internal/testing"
-	"go.opentelemetry.io/otel/sdk/export/metric/aggregator"
+	"go.opentelemetry.io/otel/sdk/export/metric/aggregation"
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/test"
 )
 
@@ -199,15 +199,15 @@ func TestArrayErrors(t *testing.T) {
 
 		_, err := agg.Max()
 		require.Error(t, err)
-		require.Equal(t, err, aggregator.ErrNoData)
+		require.Equal(t, err, aggregation.ErrNoData)
 
 		_, err = agg.Min()
 		require.Error(t, err)
-		require.Equal(t, err, aggregator.ErrNoData)
+		require.Equal(t, err, aggregation.ErrNoData)
 
 		_, err = agg.Quantile(0.1)
 		require.Error(t, err)
-		require.Equal(t, err, aggregator.ErrNoData)
+		require.Equal(t, err, aggregation.ErrNoData)
 
 		descriptor := test.NewAggregatorTest(metric.ValueRecorderKind, profile.NumberKind)
 
@@ -228,11 +228,11 @@ func TestArrayErrors(t *testing.T) {
 
 		_, err = agg.Quantile(-0.0001)
 		require.Error(t, err)
-		require.Equal(t, err, aggregator.ErrInvalidQuantile)
+		require.Equal(t, err, aggregation.ErrInvalidQuantile)
 
 		_, err = agg.Quantile(1.0001)
 		require.Error(t, err)
-		require.Equal(t, err, aggregator.ErrInvalidQuantile)
+		require.Equal(t, err, aggregation.ErrInvalidQuantile)
 	})
 }
 
diff --git a/sdk/metric/aggregator/ddsketch/ddsketch.go b/sdk/metric/aggregator/ddsketch/ddsketch.go
index 32069c03136..19047c4d9fa 100644
--- a/sdk/metric/aggregator/ddsketch/ddsketch.go
+++ b/sdk/metric/aggregator/ddsketch/ddsketch.go
@@ -21,9 +21,9 @@ import (
 	sdk "github.com/DataDog/sketches-go/ddsketch"
 
 	"go.opentelemetry.io/otel/api/metric"
-
 	export "go.opentelemetry.io/otel/sdk/export/metric"
-	"go.opentelemetry.io/otel/sdk/export/metric/aggregator"
+	"go.opentelemetry.io/otel/sdk/export/metric/aggregation"
+	"go.opentelemetry.io/otel/sdk/metric/aggregator"
 )
 
 // Config is an alias for the underlying DDSketch config object.
@@ -39,8 +39,8 @@ type Aggregator struct {
 }
 
 var _ export.Aggregator = &Aggregator{}
-var _ aggregator.MinMaxSumCount = &Aggregator{}
-var _ aggregator.Distribution = &Aggregator{}
+var _ aggregation.MinMaxSumCount = &Aggregator{}
+var _ aggregation.Distribution = &Aggregator{}
 
 // New returns a new DDSketch aggregator.
 func New(desc *metric.Descriptor, cfg *Config) *Aggregator {
@@ -85,11 +85,11 @@ func (c *Aggregator) Min() (metric.Number, error) {
 // It is an error if `q` is less than 0 or greated than 1.
 func (c *Aggregator) Quantile(q float64) (metric.Number, error) {
 	if c.checkpoint.Count() == 0 {
-		return metric.Number(0), aggregator.ErrNoData
+		return metric.Number(0), aggregation.ErrNoData
 	}
 	f := c.checkpoint.Quantile(q)
 	if math.IsNaN(f) {
-		return metric.Number(0), aggregator.ErrInvalidQuantile
+		return metric.Number(0), aggregation.ErrInvalidQuantile
 	}
 	return c.toNumber(f), nil
 }
diff --git a/sdk/metric/aggregator/histogram/histogram.go b/sdk/metric/aggregator/histogram/histogram.go
index 11aaaf229a0..e72e8038fcf 100644
--- a/sdk/metric/aggregator/histogram/histogram.go
+++ b/sdk/metric/aggregator/histogram/histogram.go
@@ -21,7 +21,8 @@ import (
 
 	"go.opentelemetry.io/otel/api/metric"
 	export "go.opentelemetry.io/otel/sdk/export/metric"
-	"go.opentelemetry.io/otel/sdk/export/metric/aggregator"
+	"go.opentelemetry.io/otel/sdk/export/metric/aggregation"
+	"go.opentelemetry.io/otel/sdk/metric/aggregator"
 )
 
 // Note: This code uses a Mutex to govern access to the exclusive
@@ -51,9 +52,9 @@ type (
 )
 
 var _ export.Aggregator = &Aggregator{}
-var _ aggregator.Sum = &Aggregator{}
-var _ aggregator.Count = &Aggregator{}
-var _ aggregator.Histogram = &Aggregator{}
+var _ aggregation.Sum = &Aggregator{}
+var _ aggregation.Count = &Aggregator{}
+var _ aggregation.Histogram = &Aggregator{}
 
 // New returns a new aggregator for computing Histograms.
 //
@@ -94,10 +95,10 @@ func (c *Aggregator) Count() (int64, error) {
 }
 
 // Histogram returns the count of events in pre-determined buckets.
-func (c *Aggregator) Histogram() (aggregator.Buckets, error) {
+func (c *Aggregator) Histogram() (aggregation.Buckets, error) {
 	c.lock.Lock()
 	defer c.lock.Unlock()
-	return aggregator.Buckets{
+	return aggregation.Buckets{
 		Boundaries: c.boundaries,
 		Counts:     c.checkpoint.bucketCounts,
 	}, nil
diff --git a/sdk/metric/aggregator/lastvalue/lastvalue.go b/sdk/metric/aggregator/lastvalue/lastvalue.go
index 8c2f74cb31c..a5837f48e6c 100644
--- a/sdk/metric/aggregator/lastvalue/lastvalue.go
+++ b/sdk/metric/aggregator/lastvalue/lastvalue.go
@@ -22,7 +22,8 @@ import (
 
 	"go.opentelemetry.io/otel/api/metric"
 	export "go.opentelemetry.io/otel/sdk/export/metric"
-	"go.opentelemetry.io/otel/sdk/export/metric/aggregator"
+	"go.opentelemetry.io/otel/sdk/export/metric/aggregation"
+	"go.opentelemetry.io/otel/sdk/metric/aggregator"
 )
 
 type (
@@ -53,7 +54,7 @@ type (
 )
 
 var _ export.Aggregator = &Aggregator{}
-var _ aggregator.LastValue = &Aggregator{}
+var _ aggregation.LastValue = &Aggregator{}
 
 // An unset lastValue has zero timestamp and zero value.
 var unsetLastValue = &lastValueData{}
@@ -68,13 +69,13 @@ func New() *Aggregator {
 }
 
 // LastValue returns the last-recorded lastValue value and the
-// corresponding timestamp.  The error value aggregator.ErrNoData
+// corresponding timestamp.  The error value aggregation.ErrNoData
 // will be returned if (due to a race condition) the checkpoint was
 // computed before the first value was set.
 func (g *Aggregator) LastValue() (metric.Number, time.Time, error) {
 	gd := (*lastValueData)(g.checkpoint)
 	if gd == unsetLastValue {
-		return metric.Number(0), time.Time{}, aggregator.ErrNoData
+		return metric.Number(0), time.Time{}, aggregation.ErrNoData
 	}
 	return gd.value.AsNumber(), gd.timestamp, nil
 }
diff --git a/sdk/metric/aggregator/lastvalue/lastvalue_test.go b/sdk/metric/aggregator/lastvalue/lastvalue_test.go
index d706927dddc..2443d8542b1 100644
--- a/sdk/metric/aggregator/lastvalue/lastvalue_test.go
+++ b/sdk/metric/aggregator/lastvalue/lastvalue_test.go
@@ -25,7 +25,7 @@ import (
 	"go.opentelemetry.io/otel/api/metric"
 	ottest "go.opentelemetry.io/otel/internal/testing"
 	export "go.opentelemetry.io/otel/sdk/export/metric"
-	"go.opentelemetry.io/otel/sdk/export/metric/aggregator"
+	"go.opentelemetry.io/otel/sdk/export/metric/aggregation"
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/test"
 )
 
@@ -108,7 +108,7 @@ func TestLastValueNotSet(t *testing.T) {
 	g.Checkpoint(descriptor)
 
 	value, timestamp, err := g.LastValue()
-	require.Equal(t, aggregator.ErrNoData, err)
+	require.Equal(t, aggregation.ErrNoData, err)
 	require.True(t, timestamp.IsZero())
 	require.Equal(t, metric.Number(0), value)
 }
diff --git a/sdk/metric/aggregator/minmaxsumcount/mmsc.go b/sdk/metric/aggregator/minmaxsumcount/mmsc.go
index f72e8e3ae36..11c3c2c82a6 100644
--- a/sdk/metric/aggregator/minmaxsumcount/mmsc.go
+++ b/sdk/metric/aggregator/minmaxsumcount/mmsc.go
@@ -20,7 +20,8 @@ import (
 
 	"go.opentelemetry.io/otel/api/metric"
 	export "go.opentelemetry.io/otel/sdk/export/metric"
-	"go.opentelemetry.io/otel/sdk/export/metric/aggregator"
+	"go.opentelemetry.io/otel/sdk/export/metric/aggregation"
+	"go.opentelemetry.io/otel/sdk/metric/aggregator"
 )
 
 type (
@@ -42,7 +43,7 @@ type (
 )
 
 var _ export.Aggregator = &Aggregator{}
-var _ aggregator.MinMaxSumCount = &Aggregator{}
+var _ aggregation.MinMaxSumCount = &Aggregator{}
 
 // New returns a new aggregator for computing the min, max, sum, and
 // count.  It does not compute quantile information other than Min and
@@ -77,25 +78,25 @@ func (c *Aggregator) Count() (int64, error) {
 }
 
 // Min returns the minimum value in the checkpoint.
-// The error value aggregator.ErrNoData will be returned
+// The error value aggregation.ErrNoData will be returned
 // if there were no measurements recorded during the checkpoint.
 func (c *Aggregator) Min() (metric.Number, error) {
 	c.lock.Lock()
 	defer c.lock.Unlock()
 	if c.checkpoint.count.IsZero(metric.Uint64NumberKind) {
-		return c.kind.Zero(), aggregator.ErrNoData
+		return c.kind.Zero(), aggregation.ErrNoData
 	}
 	return c.checkpoint.min, nil
 }
 
 // Max returns the maximum value in the checkpoint.
-// The error value aggregator.ErrNoData will be returned
+// The error value aggregation.ErrNoData will be returned
 // if there were no measurements recorded during the checkpoint.
 func (c *Aggregator) Max() (metric.Number, error) {
 	c.lock.Lock()
 	defer c.lock.Unlock()
 	if c.checkpoint.count.IsZero(metric.Uint64NumberKind) {
-		return c.kind.Zero(), aggregator.ErrNoData
+		return c.kind.Zero(), aggregation.ErrNoData
 	}
 	return c.checkpoint.max, nil
 }
diff --git a/sdk/metric/aggregator/minmaxsumcount/mmsc_test.go b/sdk/metric/aggregator/minmaxsumcount/mmsc_test.go
index 7e9325b8c75..73b03f8dec8 100644
--- a/sdk/metric/aggregator/minmaxsumcount/mmsc_test.go
+++ b/sdk/metric/aggregator/minmaxsumcount/mmsc_test.go
@@ -22,7 +22,7 @@ import (
 	"github.com/stretchr/testify/require"
 
 	"go.opentelemetry.io/otel/api/metric"
-	"go.opentelemetry.io/otel/sdk/export/metric/aggregator"
+	"go.opentelemetry.io/otel/sdk/export/metric/aggregation"
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/test"
 )
 
@@ -193,7 +193,7 @@ func TestMaxSumCountNotSet(t *testing.T) {
 		require.Nil(t, err)
 
 		max, err := agg.Max()
-		require.Equal(t, aggregator.ErrNoData, err)
+		require.Equal(t, aggregation.ErrNoData, err)
 		require.Equal(t, metric.Number(0), max)
 	})
 }
diff --git a/sdk/metric/aggregator/sum/sum.go b/sdk/metric/aggregator/sum/sum.go
index 832dce33f7c..ce35ef8ae53 100644
--- a/sdk/metric/aggregator/sum/sum.go
+++ b/sdk/metric/aggregator/sum/sum.go
@@ -19,7 +19,8 @@ import (
 
 	"go.opentelemetry.io/otel/api/metric"
 	export "go.opentelemetry.io/otel/sdk/export/metric"
-	"go.opentelemetry.io/otel/sdk/export/metric/aggregator"
+	"go.opentelemetry.io/otel/sdk/export/metric/aggregation"
+	"go.opentelemetry.io/otel/sdk/metric/aggregator"
 )
 
 // Aggregator aggregates counter events.
@@ -34,10 +35,10 @@ type Aggregator struct {
 }
 
 var _ export.Aggregator = &Aggregator{}
-var _ aggregator.Sum = &Aggregator{}
+var _ aggregation.Sum = &Aggregator{}
 
 // New returns a new counter aggregator implemented by atomic
-// operations.  This aggregator implements the aggregator.Sum
+// operations.  This aggregator implements the aggregation.Sum
 // export interface.
 func New() *Aggregator {
 	return &Aggregator{}
diff --git a/sdk/metric/aggregator/test/test.go b/sdk/metric/aggregator/test/test.go
index 53cf31959c1..3e2cb8094b7 100644
--- a/sdk/metric/aggregator/test/test.go
+++ b/sdk/metric/aggregator/test/test.go
@@ -25,7 +25,7 @@ import (
 	"go.opentelemetry.io/otel/api/metric"
 	ottest "go.opentelemetry.io/otel/internal/testing"
 	export "go.opentelemetry.io/otel/sdk/export/metric"
-	"go.opentelemetry.io/otel/sdk/export/metric/aggregator"
+	"go.opentelemetry.io/otel/sdk/metric/aggregator"
 )
 
 const Magnitude = 1000
diff --git a/sdk/metric/controller/push/push_test.go b/sdk/metric/controller/push/push_test.go
index 02a910b8203..a71bc7d21e2 100644
--- a/sdk/metric/controller/push/push_test.go
+++ b/sdk/metric/controller/push/push_test.go
@@ -30,7 +30,7 @@ import (
 	"go.opentelemetry.io/otel/api/metric"
 	"go.opentelemetry.io/otel/exporters/metric/test"
 	export "go.opentelemetry.io/otel/sdk/export/metric"
-	"go.opentelemetry.io/otel/sdk/export/metric/aggregator"
+	"go.opentelemetry.io/otel/sdk/export/metric/aggregation"
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/sum"
 	"go.opentelemetry.io/otel/sdk/metric/controller/push"
 	controllerTest "go.opentelemetry.io/otel/sdk/metric/controller/test"
@@ -175,7 +175,7 @@ func TestPushTicker(t *testing.T) {
 	require.Equal(t, "counter", records[0].Descriptor().Name())
 	require.Equal(t, "R=V", records[0].Resource().Encoded(label.DefaultEncoder()))
 
-	sum, err := records[0].Aggregator().(aggregator.Sum).Sum()
+	sum, err := records[0].Aggregator().(aggregation.Sum).Sum()
 	require.Equal(t, int64(3), sum.AsInt64())
 	require.Nil(t, err)
 
@@ -192,7 +192,7 @@ func TestPushTicker(t *testing.T) {
 	require.Equal(t, "counter", records[0].Descriptor().Name())
 	require.Equal(t, "R=V", records[0].Resource().Encoded(label.DefaultEncoder()))
 
-	sum, err = records[0].Aggregator().(aggregator.Sum).Sum()
+	sum, err = records[0].Aggregator().(aggregation.Sum).Sum()
 	require.Equal(t, int64(7), sum.AsInt64())
 	require.Nil(t, err)
 
@@ -216,7 +216,7 @@ func TestPushExportError(t *testing.T) {
 		expectedError       error
 	}{
 		{"errNone", nil, []string{"counter1{R=V,X=Y}", "counter2{R=V,}"}, nil},
-		{"errNoData", aggregator.ErrNoData, []string{"counter2{R=V,}"}, nil},
+		{"errNoData", aggregation.ErrNoData, []string{"counter2{R=V,}"}, nil},
 		{"errUnexpected", errAggregator, []string{}, errAggregator},
 	}
 	for _, tt := range tests {
diff --git a/sdk/metric/correct_test.go b/sdk/metric/correct_test.go
index ac5b35c4a0d..014ff3f4429 100644
--- a/sdk/metric/correct_test.go
+++ b/sdk/metric/correct_test.go
@@ -30,7 +30,7 @@ import (
 	"go.opentelemetry.io/otel/api/label"
 	"go.opentelemetry.io/otel/api/metric"
 	export "go.opentelemetry.io/otel/sdk/export/metric"
-	"go.opentelemetry.io/otel/sdk/export/metric/aggregator"
+	"go.opentelemetry.io/otel/sdk/export/metric/aggregation"
 	metricsdk "go.opentelemetry.io/otel/sdk/metric"
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/array"
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/sum"
@@ -131,7 +131,7 @@ func TestInputRangeCounter(t *testing.T) {
 	counter := Must(meter).NewInt64Counter("name.counter")
 
 	counter.Add(ctx, -1)
-	require.Equal(t, aggregator.ErrNegativeInput, testHandler.Flush())
+	require.Equal(t, aggregation.ErrNegativeInput, testHandler.Flush())
 
 	checkpointed := sdk.Collect(ctx)
 	require.Equal(t, 0, checkpointed)
@@ -139,7 +139,7 @@ func TestInputRangeCounter(t *testing.T) {
 	integrator.records = nil
 	counter.Add(ctx, 1)
 	checkpointed = sdk.Collect(ctx)
-	sum, err := integrator.records[0].Aggregator().(aggregator.Sum).Sum()
+	sum, err := integrator.records[0].Aggregator().(aggregation.Sum).Sum()
 	require.Equal(t, int64(1), sum.AsInt64())
 	require.Equal(t, 1, checkpointed)
 	require.Nil(t, err)
@@ -158,7 +158,7 @@ func TestInputRangeUpDownCounter(t *testing.T) {
 	counter.Add(ctx, 1)
 
 	checkpointed := sdk.Collect(ctx)
-	sum, err := integrator.records[0].Aggregator().(aggregator.Sum).Sum()
+	sum, err := integrator.records[0].Aggregator().(aggregation.Sum).Sum()
 	require.Equal(t, int64(1), sum.AsInt64())
 	require.Equal(t, 1, checkpointed)
 	require.Nil(t, err)
@@ -172,7 +172,7 @@ func TestInputRangeValueRecorder(t *testing.T) {
 	valuerecorder := Must(meter).NewFloat64ValueRecorder("name.valuerecorder")
 
 	valuerecorder.Record(ctx, math.NaN())
-	require.Equal(t, aggregator.ErrNaNInput, testHandler.Flush())
+	require.Equal(t, aggregation.ErrNaNInput, testHandler.Flush())
 
 	checkpointed := sdk.Collect(ctx)
 	require.Equal(t, 0, checkpointed)
@@ -183,7 +183,7 @@ func TestInputRangeValueRecorder(t *testing.T) {
 	integrator.records = nil
 	checkpointed = sdk.Collect(ctx)
 
-	count, err := integrator.records[0].Aggregator().(aggregator.Distribution).Count()
+	count, err := integrator.records[0].Aggregator().(aggregation.Distribution).Count()
 	require.Equal(t, int64(2), count)
 	require.Equal(t, 1, checkpointed)
 	require.Nil(t, testHandler.Flush())
@@ -269,7 +269,7 @@ func TestSDKLabelsDeduplication(t *testing.T) {
 
 	var actual [][]kv.KeyValue
 	for _, rec := range integrator.records {
-		sum, _ := rec.Aggregator().(aggregator.Sum).Sum()
+		sum, _ := rec.Aggregator().(aggregation.Sum).Sum()
 		require.Equal(t, sum, metric.NewInt64Number(2))
 
 		kvs := rec.Labels().ToSlice()
@@ -392,15 +392,15 @@ func TestSumObserverInputRange(t *testing.T) {
 
 	_ = Must(meter).NewFloat64SumObserver("float.sumobserver", func(_ context.Context, result metric.Float64ObserverResult) {
 		result.Observe(-2, kv.String("A", "B"))
-		require.Equal(t, aggregator.ErrNegativeInput, testHandler.Flush())
+		require.Equal(t, aggregation.ErrNegativeInput, testHandler.Flush())
 		result.Observe(-1, kv.String("C", "D"))
-		require.Equal(t, aggregator.ErrNegativeInput, testHandler.Flush())
+		require.Equal(t, aggregation.ErrNegativeInput, testHandler.Flush())
 	})
 	_ = Must(meter).NewInt64SumObserver("int.sumobserver", func(_ context.Context, result metric.Int64ObserverResult) {
 		result.Observe(-1, kv.String("A", "B"))
-		require.Equal(t, aggregator.ErrNegativeInput, testHandler.Flush())
+		require.Equal(t, aggregation.ErrNegativeInput, testHandler.Flush())
 		result.Observe(-1)
-		require.Equal(t, aggregator.ErrNegativeInput, testHandler.Flush())
+		require.Equal(t, aggregation.ErrNegativeInput, testHandler.Flush())
 	})
 
 	collected := sdk.Collect(ctx)
diff --git a/sdk/metric/integrator/simple/simple.go b/sdk/metric/integrator/simple/simple.go
index 178c21fe043..322f2dcfc0a 100644
--- a/sdk/metric/integrator/simple/simple.go
+++ b/sdk/metric/integrator/simple/simple.go
@@ -21,7 +21,7 @@ import (
 	"go.opentelemetry.io/otel/api/label"
 	"go.opentelemetry.io/otel/api/metric"
 	export "go.opentelemetry.io/otel/sdk/export/metric"
-	"go.opentelemetry.io/otel/sdk/export/metric/aggregator"
+	"go.opentelemetry.io/otel/sdk/export/metric/aggregation"
 	"go.opentelemetry.io/otel/sdk/resource"
 )
 
@@ -119,7 +119,7 @@ func (b *batch) ForEach(f func(export.Record) error) error {
 			value.labels,
 			value.resource,
 			value.aggregator,
-		)); err != nil && !errors.Is(err, aggregator.ErrNoData) {
+		)); err != nil && !errors.Is(err, aggregation.ErrNoData) {
 			return err
 		}
 	}
diff --git a/sdk/metric/integrator/test/test.go b/sdk/metric/integrator/test/test.go
index 008cfeb1dfc..4b7e062b477 100644
--- a/sdk/metric/integrator/test/test.go
+++ b/sdk/metric/integrator/test/test.go
@@ -23,7 +23,7 @@ import (
 	"go.opentelemetry.io/otel/api/label"
 	"go.opentelemetry.io/otel/api/metric"
 	export "go.opentelemetry.io/otel/sdk/export/metric"
-	"go.opentelemetry.io/otel/sdk/export/metric/aggregator"
+	"go.opentelemetry.io/otel/sdk/export/metric/aggregation"
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/lastvalue"
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/sum"
 	"go.opentelemetry.io/otel/sdk/resource"
@@ -162,10 +162,10 @@ func (o Output) AddTo(rec export.Record) error {
 	key := fmt.Sprint(rec.Descriptor().Name(), "/", encoded, "/", rencoded)
 	var value float64
 
-	if s, ok := rec.Aggregator().(aggregator.Sum); ok {
+	if s, ok := rec.Aggregator().(aggregation.Sum); ok {
 		sum, _ := s.Sum()
 		value = sum.CoerceToFloat64(rec.Descriptor().NumberKind())
-	} else if l, ok := rec.Aggregator().(aggregator.LastValue); ok {
+	} else if l, ok := rec.Aggregator().(aggregation.LastValue); ok {
 		last, _, _ := l.LastValue()
 		value = last.CoerceToFloat64(rec.Descriptor().NumberKind())
 	} else {
diff --git a/sdk/metric/sdk.go b/sdk/metric/sdk.go
index 8d4acc78a68..97657970753 100644
--- a/sdk/metric/sdk.go
+++ b/sdk/metric/sdk.go
@@ -28,7 +28,7 @@ import (
 	api "go.opentelemetry.io/otel/api/metric"
 	internal "go.opentelemetry.io/otel/internal/metric"
 	export "go.opentelemetry.io/otel/sdk/export/metric"
-	"go.opentelemetry.io/otel/sdk/export/metric/aggregator"
+	"go.opentelemetry.io/otel/sdk/metric/aggregator"
 	"go.opentelemetry.io/otel/sdk/resource"
 )
 
diff --git a/sdk/metric/stress_test.go b/sdk/metric/stress_test.go
index 92094de340a..fa2bfa44563 100644
--- a/sdk/metric/stress_test.go
+++ b/sdk/metric/stress_test.go
@@ -35,7 +35,7 @@ import (
 	"go.opentelemetry.io/otel/api/metric"
 	api "go.opentelemetry.io/otel/api/metric"
 	export "go.opentelemetry.io/otel/sdk/export/metric"
-	"go.opentelemetry.io/otel/sdk/export/metric/aggregator"
+	"go.opentelemetry.io/otel/sdk/export/metric/aggregation"
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/lastvalue"
 	"go.opentelemetry.io/otel/sdk/metric/aggregator/sum"
 )
@@ -280,14 +280,14 @@ func (f *testFixture) Process(record export.Record) error {
 	agg := record.Aggregator()
 	switch record.Descriptor().MetricKind() {
 	case metric.CounterKind:
-		sum, err := agg.(aggregator.Sum).Sum()
+		sum, err := agg.(aggregation.Sum).Sum()
 		if err != nil {
 			f.T.Fatal("Sum error: ", err)
 		}
 		f.impl.storeCollect(actual, sum, time.Time{})
 	case metric.ValueRecorderKind:
-		lv, ts, err := agg.(aggregator.LastValue).LastValue()
-		if err != nil && err != aggregator.ErrNoData {
+		lv, ts, err := agg.(aggregation.LastValue).LastValue()
+		if err != nil && err != aggregation.ErrNoData {
 			f.T.Fatal("Last value error: ", err)
 		}
 		f.impl.storeCollect(actual, lv, ts)

From bb2eb8ee26c82661b17a80e752c6fd123f5497f8 Mon Sep 17 00:00:00 2001
From: jmacd <jmacd@lightstep.com>
Date: Wed, 10 Jun 2020 00:05:17 -0700
Subject: [PATCH 3/7] Comments

---
 sdk/export/metric/aggregation/aggregation.go | 22 +++++++++++++++++---
 1 file changed, 19 insertions(+), 3 deletions(-)

diff --git a/sdk/export/metric/aggregation/aggregation.go b/sdk/export/metric/aggregation/aggregation.go
index bec6449b766..f7951b63eb0 100644
--- a/sdk/export/metric/aggregation/aggregation.go
+++ b/sdk/export/metric/aggregation/aggregation.go
@@ -25,9 +25,12 @@ import (
 // Aggregation.
 
 type (
-	// Aggregation is one of the interfaces below.
+	// Aggregation is an interface returned by the Aggregator
+	// containing an interval of metric data.
 	Aggregation interface {
-		// Kind returns the kind of aggregation used.
+		// Kind returns a short identifying string to identify
+		// the Aggregator that was used to produce the
+		// Aggregation (e.g., "sum").
 		Kind() Kind
 	}
 
@@ -105,6 +108,19 @@ type (
 )
 
 type (
+	// Kind is a short name for the Aggregator that produces an
+	// Aggregation, used for descriptive purpose only.  Kind is a
+	// string to allow user-defined Aggregators.
+	//
+	// When deciding how to handle an Aggregation, Exporters are
+	// encouraged to decide based on conversion to the above
+	// interfaces based on strength, not on Kind value, when
+	// deciding how to expose metric data.  This enables
+	// user-supplied Aggregators to replace builtin Aggregators.
+	//
+	// For example, test for a Distribution before testing for a
+	// MinMaxSumCount, test for a Histogram before testing for a
+	// Sum, and so on.
 	Kind string
 )
 
@@ -130,7 +146,7 @@ var (
 	ErrNoData = fmt.Errorf("no data collected by this aggregator")
 )
 
-// String returns a string representation of the aggregation kind.
+// String returns the string value of Kind.
 func (k Kind) String() string {
 	return string(k)
 }

From 8483cc3d2347ccd930a607b5952faf6b6e2ba7c1 Mon Sep 17 00:00:00 2001
From: jmacd <jmacd@lightstep.com>
Date: Wed, 10 Jun 2020 00:32:14 -0700
Subject: [PATCH 4/7] Add Aggregation.Kind()

---
 sdk/export/metric/aggregation/aggregation.go | 30 +++++++++++++++-----
 sdk/metric/aggregator/array/array.go         |  5 ++++
 sdk/metric/aggregator/ddsketch/ddsketch.go   |  5 ++++
 sdk/metric/aggregator/histogram/histogram.go |  5 ++++
 sdk/metric/aggregator/lastvalue/lastvalue.go |  5 ++++
 sdk/metric/aggregator/minmaxsumcount/mmsc.go |  5 ++++
 sdk/metric/aggregator/sum/sum.go             |  5 ++++
 7 files changed, 53 insertions(+), 7 deletions(-)

diff --git a/sdk/export/metric/aggregation/aggregation.go b/sdk/export/metric/aggregation/aggregation.go
index f7951b63eb0..e87d1cca208 100644
--- a/sdk/export/metric/aggregation/aggregation.go
+++ b/sdk/export/metric/aggregation/aggregation.go
@@ -27,6 +27,9 @@ import (
 type (
 	// Aggregation is an interface returned by the Aggregator
 	// containing an interval of metric data.
+	//
+	// Note that the interfaces below do not embed this interface.
+	// Aggregators are expected to
 	Aggregation interface {
 		// Kind returns a short identifying string to identify
 		// the Aggregator that was used to produce the
@@ -36,37 +39,44 @@ type (
 
 	// Sum returns an aggregated sum.
 	Sum interface {
+		Aggregation
 		Sum() (metric.Number, error)
 	}
 
 	// Sum returns the number of values that were aggregated.
 	Count interface {
+		Aggregation
 		Count() (int64, error)
 	}
 
 	// Min returns the minimum value over the set of values that were aggregated.
 	Min interface {
+		Aggregation
 		Min() (metric.Number, error)
 	}
 
 	// Max returns the maximum value over the set of values that were aggregated.
 	Max interface {
+		Aggregation
 		Max() (metric.Number, error)
 	}
 
 	// Quantile returns an exact or estimated quantile over the
 	// set of values that were aggregated.
 	Quantile interface {
+		Aggregation
 		Quantile(float64) (metric.Number, error)
 	}
 
 	// LastValue returns the latest value that was aggregated.
 	LastValue interface {
+		Aggregation
 		LastValue() (metric.Number, time.Time, error)
 	}
 
 	// Points returns the raw set of values that were aggregated.
 	Points interface {
+		Aggregation
 		Points() ([]metric.Number, error)
 	}
 
@@ -87,23 +97,29 @@ type (
 
 	// Histogram returns the count of events in pre-determined buckets.
 	Histogram interface {
-		Sum
+		Aggregation
+		Sum() (metric.Number, error)
 		Histogram() (Buckets, error)
 	}
 
 	// MinMaxSumCount supports the Min, Max, Sum, and Count interfaces.
 	MinMaxSumCount interface {
-		Min
-		Max
-		Sum
-		Count
+		Aggregation
+		Min() (metric.Number, error)
+		Max() (metric.Number, error)
+		Sum() (metric.Number, error)
+		Count() (int64, error)
 	}
 
 	// Distribution supports the Min, Max, Sum, Count, and Quantile
 	// interfaces.
 	Distribution interface {
-		MinMaxSumCount
-		Quantile
+		Aggregation
+		Min() (metric.Number, error)
+		Max() (metric.Number, error)
+		Sum() (metric.Number, error)
+		Count() (int64, error)
+		Quantile(float64) (metric.Number, error)
 	}
 )
 
diff --git a/sdk/metric/aggregator/array/array.go b/sdk/metric/aggregator/array/array.go
index 0284000523a..6557f693b35 100644
--- a/sdk/metric/aggregator/array/array.go
+++ b/sdk/metric/aggregator/array/array.go
@@ -53,6 +53,11 @@ func New() *Aggregator {
 	return &Aggregator{}
 }
 
+// Kind returns aggregation.ExactKind.
+func (c *Aggregator) Kind() aggregation.Kind {
+	return aggregation.ExactKind
+}
+
 // Sum returns the sum of values in the checkpoint.
 func (c *Aggregator) Sum() (metric.Number, error) {
 	return c.ckptSum, nil
diff --git a/sdk/metric/aggregator/ddsketch/ddsketch.go b/sdk/metric/aggregator/ddsketch/ddsketch.go
index 19047c4d9fa..01f30a57e20 100644
--- a/sdk/metric/aggregator/ddsketch/ddsketch.go
+++ b/sdk/metric/aggregator/ddsketch/ddsketch.go
@@ -52,6 +52,11 @@ func New(desc *metric.Descriptor, cfg *Config) *Aggregator {
 	}
 }
 
+// Kind returns aggregation.SketchKind.
+func (c *Aggregator) Kind() aggregation.Kind {
+	return aggregation.SketchKind
+}
+
 // NewDefaultConfig returns a new, default DDSketch config.
 //
 // TODO: Should the Config constructor set minValue to -Inf to
diff --git a/sdk/metric/aggregator/histogram/histogram.go b/sdk/metric/aggregator/histogram/histogram.go
index e72e8038fcf..8319b2e621d 100644
--- a/sdk/metric/aggregator/histogram/histogram.go
+++ b/sdk/metric/aggregator/histogram/histogram.go
@@ -80,6 +80,11 @@ func New(desc *metric.Descriptor, boundaries []float64) *Aggregator {
 	}
 }
 
+// Kind returns aggregation.HistogramKind.
+func (c *Aggregator) Kind() aggregation.Kind {
+	return aggregation.HistogramKind
+}
+
 // Sum returns the sum of all values in the checkpoint.
 func (c *Aggregator) Sum() (metric.Number, error) {
 	c.lock.Lock()
diff --git a/sdk/metric/aggregator/lastvalue/lastvalue.go b/sdk/metric/aggregator/lastvalue/lastvalue.go
index a5837f48e6c..cd7c5b926c0 100644
--- a/sdk/metric/aggregator/lastvalue/lastvalue.go
+++ b/sdk/metric/aggregator/lastvalue/lastvalue.go
@@ -68,6 +68,11 @@ func New() *Aggregator {
 	}
 }
 
+// Kind returns aggregation.LastValueKind.
+func (g *Aggregator) Kind() aggregation.Kind {
+	return aggregation.LastValueKind
+}
+
 // LastValue returns the last-recorded lastValue value and the
 // corresponding timestamp.  The error value aggregation.ErrNoData
 // will be returned if (due to a race condition) the checkpoint was
diff --git a/sdk/metric/aggregator/minmaxsumcount/mmsc.go b/sdk/metric/aggregator/minmaxsumcount/mmsc.go
index 11c3c2c82a6..44bdd3402bc 100644
--- a/sdk/metric/aggregator/minmaxsumcount/mmsc.go
+++ b/sdk/metric/aggregator/minmaxsumcount/mmsc.go
@@ -63,6 +63,11 @@ func New(desc *metric.Descriptor) *Aggregator {
 	}
 }
 
+// Kind returns aggregation.MinMaxSumCountKind.
+func (c *Aggregator) Kind() aggregation.Kind {
+	return aggregation.MinMaxSumCountKind
+}
+
 // Sum returns the sum of values in the checkpoint.
 func (c *Aggregator) Sum() (metric.Number, error) {
 	c.lock.Lock()
diff --git a/sdk/metric/aggregator/sum/sum.go b/sdk/metric/aggregator/sum/sum.go
index ce35ef8ae53..9fcedc848f4 100644
--- a/sdk/metric/aggregator/sum/sum.go
+++ b/sdk/metric/aggregator/sum/sum.go
@@ -44,6 +44,11 @@ func New() *Aggregator {
 	return &Aggregator{}
 }
 
+// Kind returns aggregation.SumKind.
+func (c *Aggregator) Kind() aggregation.Kind {
+	return aggregation.SumKind
+}
+
 // Sum returns the last-checkpointed sum.  This will never return an
 // error.
 func (c *Aggregator) Sum() (metric.Number, error) {

From a877fbbb85ad1afb6cb228da557f3ea16e132c4f Mon Sep 17 00:00:00 2001
From: jmacd <jmacd@lightstep.com>
Date: Wed, 10 Jun 2020 00:42:57 -0700
Subject: [PATCH 5/7] Comments

---
 sdk/export/metric/aggregation/aggregation.go | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/sdk/export/metric/aggregation/aggregation.go b/sdk/export/metric/aggregation/aggregation.go
index e87d1cca208..fe97f61e0c0 100644
--- a/sdk/export/metric/aggregation/aggregation.go
+++ b/sdk/export/metric/aggregation/aggregation.go
@@ -27,9 +27,6 @@ import (
 type (
 	// Aggregation is an interface returned by the Aggregator
 	// containing an interval of metric data.
-	//
-	// Note that the interfaces below do not embed this interface.
-	// Aggregators are expected to
 	Aggregation interface {
 		// Kind returns a short identifying string to identify
 		// the Aggregator that was used to produce the

From e3c992931372dbf1ede637f4b8f6e1dacc5348d7 Mon Sep 17 00:00:00 2001
From: jmacd <jmacd@lightstep.com>
Date: Wed, 10 Jun 2020 00:52:41 -0700
Subject: [PATCH 6/7] Remove dead code

---
 sdk/export/metric/aggregation/aggregation.go | 1 -
 1 file changed, 1 deletion(-)

diff --git a/sdk/export/metric/aggregation/aggregation.go b/sdk/export/metric/aggregation/aggregation.go
index fe97f61e0c0..c2ca93d3337 100644
--- a/sdk/export/metric/aggregation/aggregation.go
+++ b/sdk/export/metric/aggregation/aggregation.go
@@ -151,7 +151,6 @@ var (
 	ErrNegativeInput    = fmt.Errorf("negative value is out of range for this instrument")
 	ErrNaNInput         = fmt.Errorf("NaN value is an invalid input")
 	ErrInconsistentType = fmt.Errorf("inconsistent aggregator types")
-	ErrNoSubtraction    = fmt.Errorf("aggregator does not subtract")
 
 	// ErrNoData is returned when (due to a race with collection)
 	// the Aggregator is check-pointed before the first value is set.

From 8b58f4f48e330532623175d6eacc579e172d9bcf Mon Sep 17 00:00:00 2001
From: jmacd <jmacd@lightstep.com>
Date: Wed, 10 Jun 2020 00:53:31 -0700
Subject: [PATCH 7/7] Uppercase

---
 sdk/export/metric/aggregation/aggregation.go | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/sdk/export/metric/aggregation/aggregation.go b/sdk/export/metric/aggregation/aggregation.go
index c2ca93d3337..b42a4fc13b3 100644
--- a/sdk/export/metric/aggregation/aggregation.go
+++ b/sdk/export/metric/aggregation/aggregation.go
@@ -30,7 +30,7 @@ type (
 	Aggregation interface {
 		// Kind returns a short identifying string to identify
 		// the Aggregator that was used to produce the
-		// Aggregation (e.g., "sum").
+		// Aggregation (e.g., "Sum").
 		Kind() Kind
 	}
 
@@ -138,12 +138,12 @@ type (
 )
 
 const (
-	SumKind            Kind = "sum"
-	MinMaxSumCountKind Kind = "minmaxsumcount"
-	HistogramKind      Kind = "histogram"
-	LastValueKind      Kind = "lastvalue"
-	SketchKind         Kind = "sketch"
-	ExactKind          Kind = "exact"
+	SumKind            Kind = "Sum"
+	MinMaxSumCountKind Kind = "MinMaxSumCount"
+	HistogramKind      Kind = "Histogram"
+	LastValueKind      Kind = "Lastvalue"
+	SketchKind         Kind = "Sketch"
+	ExactKind          Kind = "Exact"
 )
 
 var (