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2pc.go
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2pc.go
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// Copyright 2014 The lldb Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Two Phase Commit & Structural ACID
package lldb
import (
"bufio"
"encoding/binary"
"fmt"
"io"
"os"
"camlistore.org/third_party/github.com/cznic/fileutil"
"camlistore.org/third_party/github.com/cznic/mathutil"
)
var _ Filer = &ACIDFiler0{} // Ensure ACIDFiler0 is a Filer
type acidWrite struct {
b []byte
off int64
}
type acidWriter0 ACIDFiler0
func (a *acidWriter0) WriteAt(b []byte, off int64) (n int, err error) {
f := (*ACIDFiler0)(a)
if f.bwal == nil { // new epoch
f.data = f.data[:0]
f.bwal = bufio.NewWriter(f.wal)
if err = a.writePacket([]interface{}{wpt00Header, walTypeACIDFiler0, ""}); err != nil {
return
}
}
if err = a.writePacket([]interface{}{wpt00WriteData, b, off}); err != nil {
return
}
f.data = append(f.data, acidWrite{b, off})
return len(b), nil
}
func (a *acidWriter0) writePacket(items []interface{}) (err error) {
f := (*ACIDFiler0)(a)
b, err := EncodeScalars(items...)
if err != nil {
return
}
var b4 [4]byte
binary.BigEndian.PutUint32(b4[:], uint32(len(b)))
if _, err = f.bwal.Write(b4[:]); err != nil {
return
}
if _, err = f.bwal.Write(b); err != nil {
return
}
if m := (4 + len(b)) % 16; m != 0 {
var pad [15]byte
_, err = f.bwal.Write(pad[:16-m])
}
return
}
// WAL Packet Tags
const (
wpt00Header = iota
wpt00WriteData
wpt00Checkpoint
)
const (
walTypeACIDFiler0 = iota
)
// ACIDFiler0 is a very simple, synchronous implementation of 2PC. It uses a
// single write ahead log file to provide the structural atomicity
// (BeginUpdate/EndUpdate/Rollback) and durability (DB can be recovered from
// WAL if a crash occurred).
//
// ACIDFiler0 is a Filer.
//
// NOTE: Durable synchronous 2PC involves three fsyncs in this implementation
// (WAL, DB, zero truncated WAL). Where possible, it's recommended to collect
// transactions for, say one second before performing the two phase commit as
// the typical performance for rotational hard disks is about few tens of
// fsyncs per second atmost. For an example of such collective transaction
// approach please see the colecting FSM STT in Dbm's documentation[1].
//
// [1]: http://godoc.org/github.com/cznic/exp/dbm
type ACIDFiler0 struct {
*RollbackFiler
wal *os.File
bwal *bufio.Writer
data []acidWrite
testHook bool // keeps WAL untruncated (once)
peakWal int64 // tracks WAL maximum used size
peakBitFilerPages int // track maximum transaction memory
}
// NewACIDFiler0 returns a newly created ACIDFiler0 with WAL in wal.
//
// If the WAL is zero sized then a previous clean shutdown of db is taken for
// granted and no recovery procedure is taken.
//
// If the WAL is of non zero size then it is checked for having a
// commited/fully finished transaction not yet been reflected in db. If such
// transaction exists it's committed to db. If the recovery process finishes
// successfully, the WAL is truncated to zero size and fsync'ed prior to return
// from NewACIDFiler0.
func NewACIDFiler(db Filer, wal *os.File) (r *ACIDFiler0, err error) {
fi, err := wal.Stat()
if err != nil {
return
}
r = &ACIDFiler0{wal: wal}
if fi.Size() != 0 {
if err = r.recoverDb(db); err != nil {
return
}
}
acidWriter := (*acidWriter0)(r)
if r.RollbackFiler, err = NewRollbackFiler(
db,
func(sz int64) (err error) {
// Checkpoint
if err = acidWriter.writePacket([]interface{}{wpt00Checkpoint, sz}); err != nil {
return
}
if err = r.bwal.Flush(); err != nil {
return
}
r.bwal = nil
if err = r.wal.Sync(); err != nil {
return
}
wfi, err := r.wal.Stat()
switch err != nil {
case true:
// unexpected, but ignored
case false:
r.peakWal = mathutil.MaxInt64(wfi.Size(), r.peakWal)
}
// Phase 1 commit complete
for _, v := range r.data {
if _, err := db.WriteAt(v.b, v.off); err != nil {
return err
}
}
if err = db.Truncate(sz); err != nil {
return
}
if err = db.Sync(); err != nil {
return
}
// Phase 2 commit complete
if !r.testHook {
if err = r.wal.Truncate(0); err != nil {
return
}
if _, err = r.wal.Seek(0, 0); err != nil {
return
}
}
r.testHook = false
return r.wal.Sync()
},
acidWriter,
); err != nil {
return
}
return r, nil
}
// PeakWALSize reports the maximum size WAL has ever used.
func (a ACIDFiler0) PeakWALSize() int64 {
return a.peakWal
}
func (a *ACIDFiler0) readPacket(f *bufio.Reader) (items []interface{}, err error) {
var b4 [4]byte
n, err := io.ReadAtLeast(f, b4[:], 4)
if n != 4 {
return
}
ln := int(binary.BigEndian.Uint32(b4[:]))
m := (4 + ln) % 16
padd := (16 - m) % 16
b := make([]byte, ln+padd)
if n, err = io.ReadAtLeast(f, b, len(b)); n != len(b) {
return
}
return DecodeScalars(b[:ln])
}
func (a *ACIDFiler0) recoverDb(db Filer) (err error) {
fi, err := a.wal.Stat()
if err != nil {
return &ErrILSEQ{Type: ErrInvalidWAL, Name: a.wal.Name(), More: err}
}
if sz := fi.Size(); sz%16 != 0 {
return &ErrILSEQ{Type: ErrFileSize, Name: a.wal.Name(), Arg: sz}
}
f := bufio.NewReader(a.wal)
items, err := a.readPacket(f)
if err != nil {
return
}
if len(items) != 3 || items[0] != int64(wpt00Header) || items[1] != int64(walTypeACIDFiler0) {
return &ErrILSEQ{Type: ErrInvalidWAL, Name: a.wal.Name(), More: fmt.Sprintf("invalid packet items %#v", items)}
}
tr := NewBTree(nil)
for {
items, err = a.readPacket(f)
if err != nil {
return
}
if len(items) < 2 {
return &ErrILSEQ{Type: ErrInvalidWAL, Name: a.wal.Name(), More: fmt.Sprintf("too few packet items %#v", items)}
}
switch items[0] {
case int64(wpt00WriteData):
if len(items) != 3 {
return &ErrILSEQ{Type: ErrInvalidWAL, Name: a.wal.Name(), More: fmt.Sprintf("invalid data packet items %#v", items)}
}
b, off := items[1].([]byte), items[2].(int64)
var key [8]byte
binary.BigEndian.PutUint64(key[:], uint64(off))
if err = tr.Set(key[:], b); err != nil {
return
}
case int64(wpt00Checkpoint):
var b1 [1]byte
if n, err := f.Read(b1[:]); n != 0 || err == nil {
return &ErrILSEQ{Type: ErrInvalidWAL, Name: a.wal.Name(), More: fmt.Sprintf("checkpoint n %d, err %v", n, err)}
}
if len(items) != 2 {
return &ErrILSEQ{Type: ErrInvalidWAL, Name: a.wal.Name(), More: fmt.Sprintf("checkpoint packet invalid items %#v", items)}
}
sz := items[1].(int64)
enum, err := tr.seekFirst()
if err != nil {
return err
}
for {
k, v, err := enum.current()
if err != nil {
if fileutil.IsEOF(err) {
break
}
return err
}
if _, err = db.WriteAt(v, int64(binary.BigEndian.Uint64(k))); err != nil {
return err
}
if err = enum.next(); err != nil {
if fileutil.IsEOF(err) {
break
}
return err
}
}
if err = db.Truncate(sz); err != nil {
return err
}
if err = db.Sync(); err != nil {
return err
}
// Recovery complete
if err = a.wal.Truncate(0); err != nil {
return err
}
return a.wal.Sync()
default:
return &ErrILSEQ{Type: ErrInvalidWAL, Name: a.wal.Name(), More: fmt.Sprintf("packet tag %v", items[0])}
}
}
}