Remove metric aggregator Subtract interface

CHANGELOG.md

@@ -8,6 +8,10 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 
 ## [Unreleased]
 
+### Removed
+
+- Remove the metric Processor's ability to convert cumulative to delta aggregation temporality. (#2350)
+
 ## [1.2.0] - 2021-11-12
 
 ### Changed

sdk/export/metric/aggregation/aggregation.go

@@ -125,7 +125,10 @@ 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")
+
+	// ErrNoCumulativeToDelta is returned when requesting delta
+	// export kind for a precomputed sum instrument.
+	ErrNoCumulativeToDelta = fmt.Errorf("cumulative to delta not implemented")
 
 	// ErrNoData is returned when (due to a race with collection)
 	// the Aggregator is check-pointed before the first value is set.

sdk/export/metric/metric.go

@@ -193,16 +193,6 @@ type Aggregator interface {
 	Merge(aggregator Aggregator, descriptor *sdkapi.Descriptor) error
 }
 
-// Subtractor is an optional interface implemented by some
-// Aggregators.  An Aggregator must support `Subtract()` in order to
-// be configured for a Precomputed-Sum instrument (CounterObserver,
-// UpDownCounterObserver) using a DeltaExporter.
-type Subtractor interface {
-	// Subtract subtracts the `operand` from this Aggregator and
-	// outputs the value in `result`.
-	Subtract(operand, result Aggregator, descriptor *sdkapi.Descriptor) error
-}
-
 // Exporter handles presentation of the checkpoint of aggregate
 // metrics.  This is the final stage of a metrics export pipeline,
 // where metric data are formatted for a specific system.

sdk/metric/aggregator/sum/sum.go

@@ -32,7 +32,6 @@ type Aggregator struct {
 }
 
 var _ export.Aggregator = &Aggregator{}
-var _ export.Subtractor = &Aggregator{}
 var _ aggregation.Sum = &Aggregator{}
 
 // New returns a new counter aggregator implemented by atomic
@@ -88,19 +87,3 @@ func (c *Aggregator) Merge(oa export.Aggregator, desc *sdkapi.Descriptor) error
 	c.value.AddNumber(desc.NumberKind(), o.value)
 	return nil
 }
-
-func (c *Aggregator) Subtract(opAgg, resAgg export.Aggregator, descriptor *sdkapi.Descriptor) error {
-	op, _ := opAgg.(*Aggregator)
-	if op == nil {
-		return aggregator.NewInconsistentAggregatorError(c, opAgg)
-	}
-
-	res, _ := resAgg.(*Aggregator)
-	if res == nil {
-		return aggregator.NewInconsistentAggregatorError(c, resAgg)
-	}
-
-	res.value = c.value
-	res.value.AddNumber(descriptor.NumberKind(), number.NewNumberSignChange(descriptor.NumberKind(), op.value))
-	return nil
-}

sdk/metric/processor/basic/basic.go

@@ -76,11 +76,6 @@ type (
 		// values in a single collection round.
 		current export.Aggregator
 
-		// delta, if non-nil, refers to an Aggregator owned by
-		// the processor used to compute deltas between
-		// precomputed sums.
-		delta export.Aggregator
-
 		// cumulative, if non-nil, refers to an Aggregator owned
 		// by the processor used to store the last cumulative
 		// value.
@@ -94,9 +89,6 @@ type (
 		sync.RWMutex
 		values map[stateKey]*stateValue
 
-		// Note: the timestamp logic currently assumes all
-		// exports are deltas.
-
 		processStart  time.Time
 		intervalStart time.Time
 		intervalEnd   time.Time
@@ -124,8 +116,8 @@ var ErrInvalidTemporality = fmt.Errorf("invalid aggregation temporality")
 // New returns a basic Processor that is also a Checkpointer using the provided
 // AggregatorSelector to select Aggregators.  The TemporalitySelector
 // is consulted to determine the kind(s) of exporter that will consume
-// data, so that this Processor can prepare to compute Delta or
-// Cumulative Aggregations as needed.
+// data, so that this Processor can prepare to compute Cumulative Aggregations
+// as needed.
 func New(aselector export.AggregatorSelector, tselector aggregation.TemporalitySelector, opts ...Option) *Processor {
 	return NewFactory(aselector, tselector, opts...).NewCheckpointer().(*Processor)
 }
@@ -191,13 +183,17 @@ func (b *Processor) Process(accum export.Accumulation) error {
 		}
 		if stateful {
 			if desc.InstrumentKind().PrecomputedSum() {
-				// If we know we need to compute deltas, allocate two aggregators.
-				b.AggregatorFor(desc, &newValue.cumulative, &newValue.delta)
-			} else {
-				// In this case we are certain not to need a delta, only allocate
-				// a cumulative aggregator.
-				b.AggregatorFor(desc, &newValue.cumulative)
+				// To convert precomputed sums to
+				// deltas requires two aggregators to
+				// be allocated, one for the prior
+				// value and one for the output delta.
+				// This functionality was removed from
+				// the basic processor in PR #2350.
+				return aggregation.ErrNoCumulativeToDelta
 			}
+			// In this case allocate one aggregator to
+			// save the current state.
+			b.AggregatorFor(desc, &newValue.cumulative)
 		}
 		b.state.values[key] = newValue
 		return nil
@@ -310,28 +306,15 @@ func (b *Processor) FinishCollection() error {
 			continue
 		}
 
-		// Update Aggregator state to support exporting either a
-		// delta or a cumulative aggregation.
-		var err error
-		if mkind.PrecomputedSum() {
-			if currentSubtractor, ok := value.current.(export.Subtractor); ok {
-				// This line is equivalent to:
-				// value.delta = currentSubtractor - value.cumulative
-				err = currentSubtractor.Subtract(value.cumulative, value.delta, key.descriptor)
-
-				if err == nil {
-					err = value.current.SynchronizedMove(value.cumulative, key.descriptor)
-				}
-			} else {
-				err = aggregation.ErrNoSubtraction
-			}
-		} else {
+		// The only kind of aggregators that are not stateless
+		// are the ones needing delta to cumulative
+		// conversion.  Merge aggregator state in this case.
+		if !mkind.PrecomputedSum() {
 			// This line is equivalent to:
-			// value.cumulative = value.cumulative + value.delta
-			err = value.cumulative.Merge(value.current, key.descriptor)
-		}
-		if err != nil {
-			return err
+			// value.cumulative = value.cumulative + value.current
+			if err := value.cumulative.Merge(value.current, key.descriptor); err != nil {
+				return err
+			}
 		}
 	}
 	return nil
@@ -350,13 +333,8 @@ func (b *state) ForEach(exporter aggregation.TemporalitySelector, f func(export.
 		var agg aggregation.Aggregation
 		var start time.Time
 
-		// If the processor does not have Config.Memory and it was not updated
-		// in the prior round, do not visit this value.
-		if !b.config.Memory && value.updated != (b.finishedCollection-1) {
-			continue
-		}
-
 		aggTemp := exporter.TemporalityFor(key.descriptor, value.current.Aggregation().Kind())
+
 		switch aggTemp {
 		case aggregation.CumulativeTemporality:
 			// If stateful, the sum has been computed.  If stateless, the
@@ -372,16 +350,23 @@ func (b *state) ForEach(exporter aggregation.TemporalitySelector, f func(export.
 		case aggregation.DeltaTemporality:
 			// Precomputed sums are a special case.
 			if mkind.PrecomputedSum() {
-				agg = value.delta.Aggregation()
-			} else {
-				agg = value.current.Aggregation()
+				// This functionality was removed from
+				// the basic processor in PR #2350.
+				return aggregation.ErrNoCumulativeToDelta
 			}
+			agg = value.current.Aggregation()
 			start = b.intervalStart
 
 		default:
 			return fmt.Errorf("%v: %w", aggTemp, ErrInvalidTemporality)
 		}
 
+		// If the processor does not have Config.Memory and it was not updated
+		// in the prior round, do not visit this value.
+		if !b.config.Memory && value.updated != (b.finishedCollection-1) {
+			continue
+		}
+
 		if err := f(export.NewRecord(
 			key.descriptor,
 			value.labels,