Add Rollback() and docs to Appender interface

db.go

@@ -38,19 +38,24 @@ type Options struct {
 	WALFlushInterval time.Duration
 }
 
-// Appender allows committing batches of samples to a database.
-// The data held by the appender is reset after Commit returndb.
+// Appender allows appending a batch of data. It must be completed with a
+// call to Commit or Rollback and must not be reused afterwards.
 type Appender interface {
-	// SetSeries registers a new known series label set with the appender
-	// and returns a reference number used to add samples to it over the
-	// life time of the Appender.
+	// SetSeries ensures that a series with the given label set exists and
+	// returns a unique reference number identifying it. Returned reference
+	// numbers are ephemeral and may be rejected in calls to Add() at any point.
+	// A new reference number can then be requested with another call to
+	// SetSeries.
 	SetSeries(labels.Labels) (uint64, error)
 
-	// Add adds a sample pair for the referenced seriedb.
+	// Add adds a sample pair for the referenced serie.
 	Add(ref uint64, t int64, v float64) error
 
 	// Commit submits the collected samples and purges the batch.
 	Commit() error
+
+	// Rollback rolls back all modifications made in the appender so far.
+	Rollback() error
 }
 
 type hashedSample struct {
@@ -382,6 +387,12 @@ func (a *dbAppender) Commit() error {
 	return err
 }
 
+func (a *dbAppender) Rollback() error {
+	err := a.head.Rollback()
+	a.db.mtx.RUnlock()
+	return err
+}
+
 func (db *DB) headForDir(dir string) (int, bool) {
 	for i, b := range db.heads {
 		if b.Dir() == dir {
@@ -646,13 +657,21 @@ func (a *partitionedAppender) Add(ref uint64, t int64, v float64) error {
 func (a *partitionedAppender) Commit() error {
 	var merr MultiError
 
-	// Spill buckets into partitiondb.
 	for _, p := range a.partitions {
 		merr.Add(p.Commit())
 	}
 	return merr.Err()
 }
 
+func (a *partitionedAppender) Rollback() error {
+	var merr MultiError
+
+	for _, p := range a.partitions {
+		merr.Add(p.Rollback())
+	}
+	return merr.Err()
+}
+
 // The MultiError type implements the error interface, and contains the
 // Errors used to construct it.
 type MultiError []error

head.go

@@ -266,6 +266,7 @@ func (a *headAppender) Commit() error {
 }
 
 func (a *headAppender) Rollback() error {
+	putHeadAppendBuffer(a.samples)
 	a.mtx.RUnlock()
 	return nil
 }