Introduce 'primary' storage in fanout, and have Add return the ref from the primary.

Also, ensure all append batches are rolled back when a commit or rollback fails.

storage/fanout.go

@@ -17,26 +17,38 @@ import (
 	"container/heap"
 	"strings"
 
+	"github.com/prometheus/common/log"
 	"github.com/prometheus/prometheus/pkg/labels"
 )
 
 type fanout struct {
-	storages []Storage
+	primary     Storage
+	secondaries []Storage
 }
 
 // NewFanout returns a new fan-out Storage, which proxies reads and writes
 // through to multiple underlying storages.
-func NewFanout(storages ...Storage) Storage {
+func NewFanout(primary Storage, secondaries ...Storage) Storage {
 	return &fanout{
-		storages: storages,
+		primary:     primary,
+		secondaries: secondaries,
 	}
 }
 
 func (f *fanout) Querier(mint, maxt int64) (Querier, error) {
 	queriers := mergeQuerier{
-		queriers: make([]Querier, 0, len(f.storages)),
+		queriers: make([]Querier, 0, 1+len(f.secondaries)),
 	}
-	for _, storage := range f.storages {
+
+	// Add primary querier
+	querier, err := f.primary.Querier(mint, maxt)
+	if err != nil {
+		return nil, err
+	}
+	queriers.queriers = append(queriers.queriers, querier)
+
+	// Add secondary queriers
+	for _, storage := range f.secondaries {
 		querier, err := storage.Querier(mint, maxt)
 		if err != nil {
 			queriers.Close()
@@ -48,24 +60,34 @@ func (f *fanout) Querier(mint, maxt int64) (Querier, error) {
 }
 
 func (f *fanout) Appender() (Appender, error) {
-	appenders := make([]Appender, 0, len(f.storages))
-	for _, storage := range f.storages {
+	primary, err := f.primary.Appender()
+	if err != nil {
+		return nil, err
+	}
+
+	secondaries := make([]Appender, 0, len(f.secondaries))
+	for _, storage := range f.secondaries {
 		appender, err := storage.Appender()
 		if err != nil {
 			return nil, err
 		}
-		appenders = append(appenders, appender)
+		secondaries = append(secondaries, appender)
 	}
 	return &fanoutAppender{
-		appenders: appenders,
+		primary:     primary,
+		secondaries: secondaries,
 	}, nil
 }
 
 // Close closes the storage and all its underlying resources.
 func (f *fanout) Close() error {
+	if err := f.primary.Close(); err != nil {
+		return err
+	}
+
 	// TODO return multiple errors?
 	var lastErr error
-	for _, storage := range f.storages {
+	for _, storage := range f.secondaries {
 		if err := storage.Close(); err != nil {
 			lastErr = err
 		}
@@ -75,40 +97,61 @@ func (f *fanout) Close() error {
 
 // fanoutAppender implements Appender.
 type fanoutAppender struct {
-	appenders []Appender
+	primary     Appender
+	secondaries []Appender
 }
 
 func (f *fanoutAppender) Add(l labels.Labels, t int64, v float64) (string, error) {
-	for _, appender := range f.appenders {
+	ref, err := f.primary.Add(l, t, v)
+	if err != nil {
+		return ref, err
+	}
+
+	for _, appender := range f.secondaries {
 		if _, err := appender.Add(l, t, v); err != nil {
 			return "", err
 		}
 	}
-	return "", nil
+	return ref, nil
 }
 
 func (f *fanoutAppender) AddFast(l labels.Labels, ref string, t int64, v float64) error {
-	for _, appender := range f.appenders {
-		if err := appender.AddFast(l, ref, t, v); err != nil {
+	if err := f.primary.AddFast(l, ref, t, v); err != nil {
+		return err
+	}
+
+	for _, appender := range f.secondaries {
+		if _, err := appender.Add(l, t, v); err != nil {
 			return err
 		}
 	}
 	return nil
 }
 
-func (f *fanoutAppender) Commit() error {
-	for _, appender := range f.appenders {
-		if err := appender.Commit(); err != nil {
-			return err
+func (f *fanoutAppender) Commit() (err error) {
+	err = f.primary.Commit()
+
+	for _, appender := range f.secondaries {
+		if err == nil {
+			err = appender.Commit()
+		} else {
+			if rollbackErr := appender.Rollback(); rollbackErr != nil {
+				log.Errorf("Squashed rollback error on commit: %v", rollbackErr)
+			}
 		}
 	}
-	return nil
+	return
 }
 
-func (f *fanoutAppender) Rollback() error {
-	for _, appender := range f.appenders {
-		if err := appender.Rollback(); err != nil {
-			return err
+func (f *fanoutAppender) Rollback() (err error) {
+	err = f.primary.Rollback()
+
+	for _, appender := range f.secondaries {
+		rollbackErr := appender.Rollback()
+		if err == nil {
+			err = rollbackErr
+		} else if rollbackErr != nil {
+			log.Errorf("Squashed rollback error on rollback: %v", rollbackErr)
 		}
 	}
 	return nil