Add snapshot interval

This commit adds the ability to periodically perform snapshots on
an interval that is separate from retention. For example, this lets
you retain backups for 24 hours but you can snapshot your database
every six hours to improve recovery time.

replica.go

@@ -98,8 +98,11 @@ type FileReplica struct {
 	walIndexGauge      prometheus.Gauge
 	walOffsetGauge     prometheus.Gauge
 
+	// Frequency to create new snapshots.
+	SnapshotInterval time.Duration
+
 	// Time to keep snapshots and related WAL files.
-	// Database is snapshotted after interval and older WAL files are discarded.
+	// Database is snapshotted after interval, if needed, and older WAL files are discarded.
 	Retention time.Duration
 
 	// Time between checks for retention.
@@ -402,9 +405,10 @@ func (r *FileReplica) Start(ctx context.Context) {
 	ctx, r.cancel = context.WithCancel(ctx)
 
 	// Start goroutine to replicate data.
-	r.wg.Add(3)
+	r.wg.Add(4)
 	go func() { defer r.wg.Done(); r.monitor(ctx) }()
 	go func() { defer r.wg.Done(); r.retainer(ctx) }()
+	go func() { defer r.wg.Done(); r.snapshotter(ctx) }()
 	go func() { defer r.wg.Done(); r.validator(ctx) }()
 }
 
@@ -446,7 +450,18 @@ func (r *FileReplica) monitor(ctx context.Context) {
 
 // retainer runs in a separate goroutine and handles retention.
 func (r *FileReplica) retainer(ctx context.Context) {
-	ticker := time.NewTicker(r.RetentionCheckInterval)
+	// Disable retention enforcement if retention period is non-positive.
+	if r.Retention <= 0 {
+		return
+	}
+
+	// Ensure check interval is not longer than retention period.
+	checkInterval := r.RetentionCheckInterval
+	if checkInterval > r.Retention {
+		checkInterval = r.Retention
+	}
+
+	ticker := time.NewTicker(checkInterval)
 	defer ticker.Stop()
 
 	for {
@@ -462,6 +477,28 @@ func (r *FileReplica) retainer(ctx context.Context) {
 	}
 }
 
+// snapshotter runs in a separate goroutine and handles snapshotting.
+func (r *FileReplica) snapshotter(ctx context.Context) {
+	if r.SnapshotInterval <= 0 {
+		return
+	}
+
+	ticker := time.NewTicker(r.SnapshotInterval)
+	defer ticker.Stop()
+
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-ticker.C:
+			if err := r.Snapshot(ctx); err != nil && err != ErrNoGeneration {
+				log.Printf("%s(%s): snapshotter error: %s", r.db.Path(), r.Name(), err)
+				continue
+			}
+		}
+	}
+}
+
 // validator runs in a separate goroutine and handles periodic validation.
 func (r *FileReplica) validator(ctx context.Context) {
 	// Initialize counters since validation occurs infrequently.
@@ -531,6 +568,18 @@ func (r *FileReplica) CalcPos(ctx context.Context, generation string) (pos Pos,
 	return pos, nil
 }
 
+// Snapshot copies the entire database to the replica path.
+func (r *FileReplica) Snapshot(ctx context.Context) error {
+	// Find current position of database.
+	pos, err := r.db.Pos()
+	if err != nil {
+		return fmt.Errorf("cannot determine current db generation: %w", err)
+	} else if pos.IsZero() {
+		return ErrNoGeneration
+	}
+	return r.snapshot(ctx, pos.Generation, pos.Index)
+}
+
 // snapshot copies the entire database to the replica path.
 func (r *FileReplica) snapshot(ctx context.Context, generation string, index int) error {
 	// Acquire a read lock on the database during snapshot to prevent checkpoints.
@@ -557,7 +606,7 @@ func (r *FileReplica) snapshot(ctx context.Context, generation string, index int
 		return err
 	}
 
-	log.Printf("%s(%s): snapshot: creating %s/%08x t=%s", r.db.Path(), r.Name(), generation, index, time.Since(startTime))
+	log.Printf("%s(%s): snapshot: creating %s/%08x t=%s", r.db.Path(), r.Name(), generation, index, time.Since(startTime).Truncate(time.Millisecond))
 	return nil
 }