func createDB(dirname string, opts *db.Options) (retErr error) {
const manifestFileNum = 1
ve := versionEdit{
comparatorName: opts.GetComparer().Name(),
nextFileNumber: manifestFileNum + 1,
}
manifestFilename := dbFilename(dirname, fileTypeManifest, manifestFileNum)
f, err := opts.GetFileSystem().Create(manifestFilename)
if err != nil {
return fmt.Errorf("leveldb: could not create %q: %v", manifestFilename, err)
}
defer func() {
if retErr != nil {
opts.GetFileSystem().Remove(manifestFilename)
}
}()
defer f.Close()
recWriter := record.NewWriter(f)
w, err := recWriter.Next()
if err != nil {
return err
}
err = ve.encode(w)
if err != nil {
return err
}
err = recWriter.Close()
if err != nil {
return err
}
return setCurrentFile(dirname, opts.GetFileSystem(), manifestFileNum)
}
// 创建leveldb存储目录时，需要传入db.Options, 那么db.Options有哪些属性呢？
func createDB(dirname string, opts *db.Options) (retErr error) {
const manifestFileNum = 1
// versionEdit保存版本之间的差异，oldVersion _+ versionEdit = newVersion
ve := versionEdit{
// 原始key比较器的名称
comparatorName: opts.GetComparer().Name(),
// ?
nextFileNumber: manifestFileNum + 1,
}
// 先创建manifest文件，往里面写一些元信息，如上面versionEdit的comparatorName、nextFileNumber字段
manifestFilename := dbFilename(dirname, fileTypeManifest, manifestFileNum)
f, err := opts.GetFileSystem().Create(manifestFilename)
if err != nil {
return fmt.Errorf("leveldb: could not create %q: %v", manifestFilename, err)
}
defer func() {
if retErr != nil {
opts.GetFileSystem().Remove(manifestFilename)
}
}()
defer f.Close()
// 生成一个record.Writer对象
recWriter := record.NewWriter(f)
// 返回一个record.SingleWriter对象w
w, err := recWriter.Next()
if err != nil {
return err
}
// 将ve中的数据写入manifest文件中
err = ve.encode(w)
if err != nil {
return err
}
// 加入重启点, 压缩，crc校验？
err = recWriter.Close()
if err != nil {
return err
}
// 创建current文件，其中写入了当前的manifest文件名
return setCurrentFile(dirname, opts.GetFileSystem(), manifestFileNum)
}
// Open opens a LevelDB whose files live in the given directory.
func Open(dirname string, opts *db.Options) (*DB, error) {
d := &DB{
dirname: dirname,
opts: opts,
icmp: internalKeyComparer{opts.GetComparer()},
pendingOutputs: make(map[uint64]struct{}),
}
if opts != nil {
d.icmpOpts = *opts
}
d.icmpOpts.Comparer = d.icmp
tableCacheSize := opts.GetMaxOpenFiles() - numNonTableCacheFiles
if tableCacheSize < minTableCacheSize {
tableCacheSize = minTableCacheSize
}
// tableCache初始化
d.tableCache.init(dirname, opts.GetFileSystem(), &d.icmpOpts, tableCacheSize)
// 初始化一个MemDB
d.mem = memdb.New(&d.icmpOpts)
// sync.Cond在Locker的基础上增加的一个消息通知的功能。
// Cond有三个方法：Wait，Signal，Broadcast。
// Wait添加一个计数，也就是添加一个阻塞的goroutine。
// Signal解除一个goroutine的阻塞，计数减一。
// Broadcast接触所有wait goroutine的阻塞。
d.compactionCond = sync.Cond{L: &d.mu}
fs := opts.GetFileSystem()
d.mu.Lock()
defer d.mu.Unlock()
// Lock the database directory.
// If the directory already exists, MkdirAll does nothing and returns nil.
// 如果目录已经存在，则MkdirAll啥都不干
err := fs.MkdirAll(dirname, 0755)
if err != nil {
return nil, err
}
// 创建LOCK文件，并加文件锁
fileLock, err := fs.Lock(dbFilename(dirname, fileTypeLock, 0))
if err != nil {
return nil, err
}
defer func() {
if fileLock != nil {
fileLock.Close()
}
}()
// 若CURRENT文件不存在，则调用createDB
if _, err := fs.Stat(dbFilename(dirname, fileTypeCurrent, 0)); os.IsNotExist(err) {
// Create the DB if it did not already exist.
if err := createDB(dirname, opts); err != nil {
return nil, err
}
} else if err != nil {
return nil, fmt.Errorf("leveldb: database %q: %v", dirname, err)
} else if opts.GetErrorIfDBExists() {
return nil, fmt.Errorf("leveldb: database %q already exists", dirname)
}
// Load the version set.
// 先读取CURRENT文件内容，获取manifest文件名，然后逐条记录读取manifest文件的内容，根据内容生成一个新version，放入d.versions中
err = d.versions.load(dirname, opts)
if err != nil {
return nil, err
}
// Replay any newer log files than the ones named in the manifest.
var ve versionEdit
ls, err := fs.List(dirname)
if err != nil {
return nil, err
}
var logFiles fileNumAndNameSlice
for _, filename := range ls {
ft, fn, ok := parseDBFilename(filename)
if ok && ft == fileTypeLog && (fn >= d.versions.logNumber || fn == d.versions.prevLogNumber) {
logFiles = append(logFiles, fileNumAndName{fn, filename})
}
}
sort.Sort(logFiles)
for _, lf := range logFiles {
// 根据日志文件重做日志中记录的操作，先将这些操作记录存入一个临时的memtable中，然后转存入磁盘上level0存储文件中
maxSeqNum, err := d.replayLogFile(&ve, fs, filepath.Join(dirname, lf.name))
if err != nil {
return nil, err
}
d.versions.markFileNumUsed(lf.num)
// 设置最新的操作序列号
if d.versions.lastSequence < maxSeqNum {
d.versions.lastSequence = maxSeqNum
}
}
// Create an empty .log file.
// 创建一个新的空log文件
ve.logNumber = d.versions.nextFileNum()
d.logNumber = ve.logNumber
logFile, err := fs.Create(dbFilename(dirname, fileTypeLog, ve.logNumber))
if err != nil {
return nil, err
}
defer func() {
if logFile != nil {
logFile.Close()
}
}()
d.log = record.NewWriter(logFile)
// Write a new manifest to disk.
// 根据前面重做日志得到的ve的信息创建一个新的manifest文件
// 并在CURRENT文件中指向这个新manifest文件
if err := d.versions.logAndApply(dirname, &ve); err != nil {
return nil, err
}
d.deleteObsoleteFiles()
// 尝试调度compaction
d.maybeScheduleCompaction()
d.logFile, logFile = logFile, nil
d.fileLock, fileLock = fileLock, nil
return d, nil
}
// load loads the version set from the manifest file.
func (vs *versionSet) load(dirname string, opts *db.Options) error {
vs.dirname = dirname
vs.opts = opts
vs.fs = opts.GetFileSystem()
vs.ucmp = opts.GetComparer()
vs.icmp = internalKeyComparer{vs.ucmp}
vs.dummyVersion.prev = &vs.dummyVersion
vs.dummyVersion.next = &vs.dummyVersion
// For historical reasons, the next file number is initialized to 2.
vs.nextFileNumber = 2
// Read the CURRENT file to find the current manifest file.
current, err := vs.fs.Open(dbFilename(dirname, fileTypeCurrent, 0))
if err != nil {
return fmt.Errorf("leveldb: could not open CURRENT file for DB %q: %v", dirname, err)
}
defer current.Close()
stat, err := current.Stat()
if err != nil {
return err
}
n := stat.Size()
if n == 0 {
return fmt.Errorf("leveldb: CURRENT file for DB %q is empty", dirname)
}
if n > 4096 {
return fmt.Errorf("leveldb: CURRENT file for DB %q is too large", dirname)
}
b := make([]byte, n)
_, err = current.ReadAt(b, 0)
if err != nil {
return err
}
if b[n-1] != '\n' {
return fmt.Errorf("leveldb: CURRENT file for DB %q is malformed", dirname)
}
b = b[:n-1]
// Read the versionEdits in the manifest file.
var bve bulkVersionEdit
manifest, err := vs.fs.Open(dirname + string(os.PathSeparator) + string(b))
if err != nil {
return fmt.Errorf("leveldb: could not open manifest file %q for DB %q: %v", b, dirname, err)
}
defer manifest.Close()
rr := record.NewReader(manifest)
for {
r, err := rr.Next()
if err == io.EOF {
break
}
if err != nil {
return err
}
var ve versionEdit
err = ve.decode(r)
if err != nil {
return err
}
if ve.comparatorName != "" {
if ve.comparatorName != vs.ucmp.Name() {
return fmt.Errorf("leveldb: manifest file %q for DB %q: "+
"comparer name from file %q != comparer name from db.Options %q",
b, dirname, ve.comparatorName, vs.ucmp.Name())
}
}
bve.accumulate(&ve)
if ve.logNumber != 0 {
vs.logNumber = ve.logNumber
}
if ve.prevLogNumber != 0 {
vs.prevLogNumber = ve.prevLogNumber
}
if ve.nextFileNumber != 0 {
vs.nextFileNumber = ve.nextFileNumber
}
if ve.lastSequence != 0 {
vs.lastSequence = ve.lastSequence
}
}
if vs.logNumber == 0 || vs.nextFileNumber == 0 {
if vs.nextFileNumber == 2 {
// We have a freshly created DB.
} else {
return fmt.Errorf("leveldb: incomplete manifest file %q for DB %q", b, dirname)
}
}
vs.markFileNumUsed(vs.logNumber)
vs.markFileNumUsed(vs.prevLogNumber)
vs.manifestFileNumber = vs.nextFileNum()
newVersion, err := bve.apply(nil, vs.icmp)
if err != nil {
return err
}
vs.append(newVersion)
return nil
}
// Open opens a LevelDB whose files live in the given directory.
func Open(dirname string, opts *db.Options) (*DB, error) {
d := &DB{
dirname: dirname,
opts: opts,
icmp: internalKeyComparer{opts.GetComparer()},
}
fs := opts.GetFileSystem()
// Lock the database directory.
err := fs.MkdirAll(dirname, 0755)
if err != nil {
return nil, err
}
fileLock, err := fs.Lock(dbFilename(dirname, fileTypeLock, 0))
if err != nil {
return nil, err
}
defer func() {
if fileLock != nil {
fileLock.Close()
}
}()
// TODO: add options for CreateIfMissing and ErrorIfExists, and check them here.
// Load the version set.
err = d.versions.load(dirname, opts)
if err != nil {
return nil, err
}
// Replay any newer log files than the ones named in the manifest.
var ve versionEdit
ls, err := fs.List(dirname)
if err != nil {
return nil, err
}
var logFiles fileNumAndNameSlice
for _, filename := range ls {
n := logFileNum(filename)
if n != 0 && (n >= d.versions.logNumber || n == d.versions.prevLogNumber) {
logFiles = append(logFiles, fileNumAndName{n, filename})
}
}
sort.Sort(logFiles)
for _, lf := range logFiles {
maxSeqNum, err := d.replayLogFile(&ve, fs, filepath.Join(dirname, lf.name))
if err != nil {
return nil, err
}
d.versions.markFileNumUsed(lf.num)
if d.versions.lastSequence < maxSeqNum {
d.versions.lastSequence = maxSeqNum
}
}
// Create an empty .log file.
ve.logNumber = d.versions.nextFileNum()
logFile, err := fs.Create(dbFilename(dirname, fileTypeLog, ve.logNumber))
if err != nil {
return nil, err
}
defer func() {
if logFile != nil {
logFile.Close()
}
}()
d.log = record.NewWriter(logFile)
// Write a new manifest to disk.
if err := d.versions.logAndApply(dirname, &ve); err != nil {
return nil, err
}
// TODO: delete obsolete files.
// TODO: maybe schedule compaction?
d.logFile, logFile = logFile, nil
d.fileLock, fileLock = fileLock, nil
return d, nil
}
// load loads the version set from the manifest file.
// 先读取CURRENT文件内容，获取manifest文件名，
func (vs *versionSet) load(dirname string, opts *db.Options) error {
vs.dirname = dirname
vs.opts = opts
vs.fs = opts.GetFileSystem()
vs.ucmp = opts.GetComparer()
vs.icmp = internalKeyComparer{vs.ucmp}
vs.dummyVersion.prev = &vs.dummyVersion
vs.dummyVersion.next = &vs.dummyVersion
// For historical reasons, the next file number is initialized to 2.
vs.nextFileNumber = 2
// Read the CURRENT file to find the current manifest file.
current, err := vs.fs.Open(dbFilename(dirname, fileTypeCurrent, 0))
if err != nil {
return fmt.Errorf("leveldb: could not open CURRENT file for DB %q: %v", dirname, err)
}
defer current.Close()
stat, err := current.Stat()
if err != nil {
return err
}
// 文件大小
n := stat.Size()
if n == 0 {
return fmt.Errorf("leveldb: CURRENT file for DB %q is empty", dirname)
}
if n > 4096 {
return fmt.Errorf("leveldb: CURRENT file for DB %q is too large", dirname)
}
b := make([]byte, n)
// 将文件数据读取到b中
_, err = current.ReadAt(b, 0)
if err != nil {
return err
}
if b[n-1] != '\n' {
return fmt.Errorf("leveldb: CURRENT file for DB %q is malformed", dirname)
}
// 去除掉最后的换行符
// b即为CURRENT所指向的manifest文件的文件名
b = b[:n-1]
// Read the versionEdits in the manifest file.
var bve bulkVersionEdit
manifest, err := vs.fs.Open(dirname + string(os.PathSeparator) + string(b))
if err != nil {
return fmt.Errorf("leveldb: could not open manifest file %q for DB %q: %v", b, dirname, err)
}
defer manifest.Close()
// 生成一个Reader对象
rr := record.NewReader(manifest)
for {
// Next returns a reader for the next record.
r, err := rr.Next()
if err == io.EOF {
break
}
if err != nil {
return err
}
var ve versionEdit
// 将记录读入ve
err = ve.decode(r)
if err != nil {
return err
}
if ve.comparatorName != "" {
// 读和写的key比较方法必须一致
if ve.comparatorName != vs.ucmp.Name() {
return fmt.Errorf("leveldb: manifest file %q for DB %q: "+
"comparer name from file %q != comparer name from db.Options %q",
b, dirname, ve.comparatorName, vs.ucmp.Name())
}
}
// bve.
bve.accumulate(&ve)
if ve.logNumber != 0 {
vs.logNumber = ve.logNumber
}
if ve.prevLogNumber != 0 {
vs.prevLogNumber = ve.prevLogNumber
}
if ve.nextFileNumber != 0 {
vs.nextFileNumber = ve.nextFileNumber
}
if ve.lastSequence != 0 {
vs.lastSequence = ve.lastSequence
}
}
if vs.logNumber == 0 || vs.nextFileNumber == 0 {
if vs.nextFileNumber == 2 {
// We have a freshly created DB.
} else {
return fmt.Errorf("leveldb: incomplete manifest file %q for DB %q", b, dirname)
}
}
// 生成新版本version之前的准备工作
vs.markFileNumUsed(vs.logNumber)
vs.markFileNumUsed(vs.prevLogNumber)
vs.manifestFileNumber = vs.nextFileNum()
// 根据bve的added和deleted字段，生成一个新的version(newVersion)
// 并根据level0的文件数及更大level的文件所占磁盘空间大小来计算newVersion的compactionScore和compactionLevel
// 这个compactionScore和compactionLevel是判断是否需要compaction的条件
newVersion, err := bve.apply(nil, vs.icmp)
if err != nil {
return err
}
vs.append(newVersion)
return nil
}
// Open opens a LevelDB whose files live in the given directory.
func Open(dirname string, opts *db.Options) (*DB, error) {
d := &DB{
dirname: dirname,
opts: opts,
icmp: internalKeyComparer{opts.GetComparer()},
pendingOutputs: make(map[uint64]struct{}),
}
if opts != nil {
d.icmpOpts = *opts
}
d.icmpOpts.Comparer = d.icmp
tableCacheSize := opts.GetMaxOpenFiles() - numNonTableCacheFiles
if tableCacheSize < minTableCacheSize {
tableCacheSize = minTableCacheSize
}
d.tableCache.init(dirname, opts.GetFileSystem(), &d.icmpOpts, tableCacheSize)
d.mem = memdb.New(&d.icmpOpts)
d.compactionCond = sync.Cond{L: &d.mu}
fs := opts.GetFileSystem()
d.mu.Lock()
defer d.mu.Unlock()
// Lock the database directory.
err := fs.MkdirAll(dirname, 0755)
if err != nil {
return nil, err
}
fileLock, err := fs.Lock(dbFilename(dirname, fileTypeLock, 0))
if err != nil {
return nil, err
}
defer func() {
if fileLock != nil {
fileLock.Close()
}
}()
if _, err := fs.Stat(dbFilename(dirname, fileTypeCurrent, 0)); os.IsNotExist(err) {
// Create the DB if it did not already exist.
if err := createDB(dirname, opts); err != nil {
return nil, err
}
} else if err != nil {
return nil, fmt.Errorf("leveldb: database %q: %v", dirname, err)
} else if opts.GetErrorIfDBExists() {
return nil, fmt.Errorf("leveldb: database %q already exists", dirname)
}
// Load the version set.
err = d.versions.load(dirname, opts)
if err != nil {
return nil, err
}
// Replay any newer log files than the ones named in the manifest.
var ve versionEdit
ls, err := fs.List(dirname)
if err != nil {
return nil, err
}
var logFiles fileNumAndNameSlice
for _, filename := range ls {
ft, fn, ok := parseDBFilename(filename)
if ok && ft == fileTypeLog && (fn >= d.versions.logNumber || fn == d.versions.prevLogNumber) {
logFiles = append(logFiles, fileNumAndName{fn, filename})
}
}
sort.Sort(logFiles)
for _, lf := range logFiles {
maxSeqNum, err := d.replayLogFile(&ve, fs, filepath.Join(dirname, lf.name))
if err != nil {
return nil, err
}
d.versions.markFileNumUsed(lf.num)
if d.versions.lastSequence < maxSeqNum {
d.versions.lastSequence = maxSeqNum
}
}
// Create an empty .log file.
ve.logNumber = d.versions.nextFileNum()
d.logNumber = ve.logNumber
logFile, err := fs.Create(dbFilename(dirname, fileTypeLog, ve.logNumber))
if err != nil {
return nil, err
}
defer func() {
if logFile != nil {
logFile.Close()
}
}()
d.log = record.NewWriter(logFile)
// Write a new manifest to disk.
if err := d.versions.logAndApply(dirname, &ve); err != nil {
return nil, err
}
d.deleteObsoleteFiles()
d.maybeScheduleCompaction()
d.logFile, logFile = logFile, nil
d.fileLock, fileLock = fileLock, nil
return d, nil
}