batch.go

/*
 * Copyright 2018 Dgraph Labs, Inc. and Contributors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package badger

import (
	"sync"

	"github.com/dgraph-io/badger/v2/pb"
	"github.com/dgraph-io/badger/v2/y"
	"github.com/pkg/errors"
)

// WriteBatch holds the necessary info to perform batched writes.
type WriteBatch struct {
	sync.Mutex
	txn      *Txn
	db       *DB
	throttle *y.Throttle
	err      error

	isManaged bool
	commitTs  uint64
	finished  bool
}

// NewWriteBatch creates a new WriteBatch. This provides a way to conveniently do a lot of writes,
// batching them up as tightly as possible in a single transaction and using callbacks to avoid
// waiting for them to commit, thus achieving good performance. This API hides away the logic of
// creating and committing transactions. Due to the nature of SSI guaratees provided by Badger,
// blind writes can never encounter transaction conflicts (ErrConflict).
func (db *DB) NewWriteBatch() *WriteBatch {
	if db.opt.managedTxns {
		panic("cannot use NewWriteBatch in managed mode. Use NewWriteBatchAt instead")
	}
	return db.newWriteBatch(false)
}

func (db *DB) newWriteBatch(isManaged bool) *WriteBatch {
	return &WriteBatch{
		db:        db,
		isManaged: isManaged,
		txn:       db.newTransaction(true, isManaged),
		throttle:  y.NewThrottle(16),
	}
}

// SetMaxPendingTxns sets a limit on maximum number of pending transactions while writing batches.
// This function should be called before using WriteBatch. Default value of MaxPendingTxns is
// 16 to minimise memory usage.
func (wb *WriteBatch) SetMaxPendingTxns(max int) {
	wb.throttle = y.NewThrottle(max)
}

// Cancel function must be called if there's a chance that Flush might not get
// called. If neither Flush or Cancel is called, the transaction oracle would
// never get a chance to clear out the row commit timestamp map, thus causing an
// unbounded memory consumption. Typically, you can call Cancel as a defer
// statement right after NewWriteBatch is called.
//
// Note that any committed writes would still go through despite calling Cancel.
func (wb *WriteBatch) Cancel() {
	wb.Lock()
	defer wb.Unlock()
	wb.finished = true
	if err := wb.throttle.Finish(); err != nil {
		wb.db.opt.Errorf("WatchBatch.Cancel error while finishing: %v", err)
	}
	wb.txn.Discard()
}

func (wb *WriteBatch) callback(err error) {
	// sync.WaitGroup is thread-safe, so it doesn't need to be run inside wb.Lock.
	defer wb.throttle.Done(err)
	if err == nil {
		return
	}

	wb.Lock()
	defer wb.Unlock()
	if wb.err != nil {
		return
	}
	wb.err = err
}

func (wb *WriteBatch) Write(kvList *pb.KVList) error {
	wb.Lock()
	defer wb.Unlock()
	for _, kv := range kvList.Kv {
		e := Entry{Key: kv.Key, Value: kv.Value}
		if len(kv.UserMeta) > 0 {
			e.UserMeta = kv.UserMeta[0]
		}
		y.AssertTrue(kv.Version != 0)
		e.version = kv.Version
		if err := wb.handleEntry(&e); err != nil {
			return err
		}
	}
	return nil
}

// SetEntryAt is the equivalent of Txn.SetEntry but it also allows setting version for the entry.
// SetEntryAt can be used only in managed mode.
func (wb *WriteBatch) SetEntryAt(e *Entry, ts uint64) error {
	if !wb.db.opt.managedTxns {
		return errors.New("SetEntryAt can only be used in managed mode. Use SetEntry instead")
	}
	e.version = ts
	return wb.SetEntry(e)
}

// Should be called with lock acquired.
func (wb *WriteBatch) handleEntry(e *Entry) error {
	if err := wb.txn.SetEntry(e); err != ErrTxnTooBig {
		return err
	}
	// Txn has reached it's zenith. Commit now.
	if cerr := wb.commit(); cerr != nil {
		return cerr
	}
	// This time the error must not be ErrTxnTooBig, otherwise, we make the
	// error permanent.
	if err := wb.txn.SetEntry(e); err != nil {
		wb.err = err
		return err
	}
	return nil
}

// SetEntry is the equivalent of Txn.SetEntry.
func (wb *WriteBatch) SetEntry(e *Entry) error {
	wb.Lock()
	defer wb.Unlock()
	return wb.handleEntry(e)
}

// Set is equivalent of Txn.Set().
func (wb *WriteBatch) Set(k, v []byte) error {
	e := &Entry{Key: k, Value: v}
	return wb.SetEntry(e)
}

// DeleteAt is equivalent of Txn.Delete but accepts a delete timestamp.
func (wb *WriteBatch) DeleteAt(k []byte, ts uint64) error {
	e := Entry{Key: k, meta: bitDelete, version: ts}
	return wb.SetEntry(&e)
}

// Delete is equivalent of Txn.Delete.
func (wb *WriteBatch) Delete(k []byte) error {
	wb.Lock()
	defer wb.Unlock()

	if err := wb.txn.Delete(k); err != ErrTxnTooBig {
		return err
	}
	if err := wb.commit(); err != nil {
		return err
	}
	if err := wb.txn.Delete(k); err != nil {
		wb.err = err
		return err
	}
	return nil
}

// Caller to commit must hold a write lock.
func (wb *WriteBatch) commit() error {
	if wb.err != nil {
		return wb.err
	}
	if wb.finished {
		return y.ErrCommitAfterFinish
	}
	if err := wb.throttle.Do(); err != nil {
		wb.err = err
		return wb.err
	}
	wb.txn.CommitWith(wb.callback)
	wb.txn = wb.db.newTransaction(true, wb.isManaged)
	wb.txn.commitTs = wb.commitTs
	return wb.err
}

// Flush must be called at the end to ensure that any pending writes get committed to Badger. Flush
// returns any error stored by WriteBatch.
func (wb *WriteBatch) Flush() error {
	wb.Lock()
	err := wb.commit()
	if err != nil {
		wb.Unlock()
		return err
	}
	wb.finished = true
	wb.txn.Discard()
	wb.Unlock()

	if err := wb.throttle.Finish(); err != nil {
		if wb.err != nil {
			return errors.Errorf("wb.err: %s err: %s", wb.err, err)
		}
		return err
	}

	return wb.err
}

// Error returns any errors encountered so far. No commits would be run once an error is detected.
func (wb *WriteBatch) Error() error {
	wb.Lock()
	defer wb.Unlock()
	return wb.err
}