Configurable monitor-delay-interval

The `monitor-delay-interval` has been added to the DB config so that
users can change the time period between WAL checks after a file
change notification has occurred. This can be useful to batch up
changes in larger files in the shadow WAL or to reduce or eliminate
the delay in propagating changes during read replication.

Setting the interval to zero or less will disable it.

db.go

@@ -27,15 +27,13 @@ import (
 
 // Default DB settings.
 const (
-	DefaultCheckpointInterval = 1 * time.Minute
+	DefaultMonitorDelayInterval = 10 * time.Millisecond
+	DefaultCheckpointInterval   = 1 * time.Minute
+
 	DefaultMinCheckpointPageN = 1000
 	DefaultMaxCheckpointPageN = 10000
 )
 
-// MonitorDelayInterval is the time Litestream will wait after receiving a file
-// change notification before processing the WAL file for changes.
-const MonitorDelayInterval = 10 * time.Millisecond
-
 // MaxIndex is the maximum possible WAL index.
 // If this index is reached then a new generation will be started.
 const MaxIndex = 0x7FFFFFFF
@@ -98,6 +96,11 @@ type DB struct {
 	// unbounded if there are always read transactions occurring.
 	MaxCheckpointPageN int
 
+	// Time after receiving change notification before reading next WAL segment.
+	// Used for batching changes into fewer files instead of every transaction
+	// creating its own file.
+	MonitorDelayInterval time.Duration
+
 	// Time between automatic checkpoints in the WAL. This is done to allow
 	// more fine-grained WAL files so that restores can be performed with
 	// better precision.
@@ -118,9 +121,10 @@ func NewDB(path string) *DB {
 
 		itrs: make(map[*FileWALSegmentIterator]struct{}),
 
-		MinCheckpointPageN: DefaultMinCheckpointPageN,
-		MaxCheckpointPageN: DefaultMaxCheckpointPageN,
-		CheckpointInterval: DefaultCheckpointInterval,
+		MinCheckpointPageN:   DefaultMinCheckpointPageN,
+		MaxCheckpointPageN:   DefaultMaxCheckpointPageN,
+		MonitorDelayInterval: DefaultMonitorDelayInterval,
+		CheckpointInterval:   DefaultCheckpointInterval,
 
 		Logger: log.New(LogWriter, fmt.Sprintf("%s: ", logPrefixPath(path)), LogFlags),
 	}
@@ -1497,8 +1501,12 @@ func (db *DB) execCheckpoint(mode string) (err error) {
 
 // monitor runs in a separate goroutine and monitors the database & WAL.
 func (db *DB) monitor() {
-	timer := time.NewTimer(MonitorDelayInterval)
-	defer timer.Stop()
+	var timer *time.Timer
+
+	if db.MonitorDelayInterval > 0 {
+		timer := time.NewTimer(db.MonitorDelayInterval)
+		defer timer.Stop()
+	}
 
 	for {
 		// Wait for a file change notification from the file system.
@@ -1509,8 +1517,10 @@ func (db *DB) monitor() {
 		}
 
 		// Wait for small delay before processing changes.
-		timer.Reset(MonitorDelayInterval)
-		<-timer.C
+		if timer != nil {
+			timer.Reset(db.MonitorDelayInterval)
+			<-timer.C
+		}
 
 		// Clear any additional change notifications that occurred during delay.
 		select {