added WAL Clean and Replay

db/db.go

@@ -22,13 +22,22 @@ type LoremDB struct {
 func NewLoremDB(useBloom bool) *LoremDB {
 
 	os.MkdirAll("sstable", 0755)
-	wal, _ := wal.NewWal("wal.log")
+	writeAheadLog, _ := wal.NewWal("wal.log")
 	memTable := memtable.NewMemTable()
+
+	writeAheadLog.Recover(func(walRow *wal.WalRow) {
+		if !walRow.IsDeleted {
+			memTable.Put(walRow.Key, walRow.Value)
+		} else {
+			memTable.Delete(walRow.Key)
+		}
+	})
+
 	ssTables := []*sstable.SSTable{}
 	ssTablePath := "sstable"
 
 	return &LoremDB{
-		wal:          wal,
+		wal:          writeAheadLog,
 		memTable:     memTable,
 		ssTables:     ssTables,
 		ssTablePath:  ssTablePath,
@@ -64,6 +73,7 @@ func (db *LoremDB) Put(key string, value string) error {
 
 		}
 		db.memTable = memtable.NewMemTable()
+		db.wal.Clear()
 	}
 	return nil
 }

db/db_test.go

@@ -0,0 +1,32 @@
+package db
+
+import (
+	"os"
+	"testing"
+)
+
+func TestCrashRecovery(t *testing.T) {
+	os.RemoveAll("sstable")
+	os.Remove("wal.log")
+	os.MkdirAll("sstable", 0755)
+
+	// write some keys
+	store := NewLoremDB(true)
+	store.Put("name", "sakar")
+	store.Put("lang", "go")
+
+	// simulate crash — don't flush, just create a new instance
+	store2 := NewLoremDB(true)
+
+	val, ok := store2.Get("name")
+	if !ok || val != "sakar" {
+		t.Errorf("expected sakar, got %s", val)
+	}
+	val, ok = store2.Get("lang")
+	if !ok || val != "go" {
+		t.Errorf("expected go, got %s", val)
+	}
+
+	os.RemoveAll("sstable")
+	os.Remove("wal.log")
+}

wal/wal.go

@@ -46,6 +46,16 @@ func (wal *Wal) Append(walRow WalRow) {
 	wal.file.WriteString(wal.lineDelimiter)
 }
 
+func (wal *Wal) Clear() error {
+
+	err := wal.file.Truncate(0)
+	if err != nil {
+		return err
+	}
+	_, err = wal.file.Seek(0, 0)
+	return err
+}
+
 func (wal *Wal) Recover(callback func(*WalRow)) error {
 	file, err := os.Open(wal.file.Name())