Sequence generates duplicate values

What version of Go are you using (go version)?

$ go version
go version go1.14.1 windows/amd64

What version of Badger are you using?

e029e93

Does this issue reproduce with the latest master?

yes

What are the hardware specifications of the machine (RAM, OS, Disk)?

mem:32g
os:windows 10
disk:Samsung SSD 970 EVO Plus

What did you do?

package main

import (
	"fmt"
	"log"
	"os"

	"github.com/dgraph-io/badger"
)

func main() {
	opts := badger.DefaultOptions("./tmp")
	os.RemoveAll(opts.Dir)
	os.MkdirAll(opts.Dir, 0700)
	db, err := badger.Open(opts)
	if err != nil {
		log.Fatal(err)
	}
	defer db.Close()

	key := []byte("key")
	seq1, err := db.GetSequence(key, 2)
	if err != nil {
		panic(err)
	}

	seq2, err := db.GetSequence(key, 2)
	if err != nil {
		panic(err)
	}
	num, err := seq2.Next()
	if err != nil {
		panic(err)
	}

	fmt.Printf("num:%v\r\n", num)

	err = seq2.Release()
	if err != nil {
		panic(err)
	}
	err = seq1.Release()
	if err != nil {
		panic(err)
	}

	seq3, err := db.GetSequence(key, 2)
	if err != nil {
		panic(err)
	}

	for i := 0; i < 5; i++ {
		num, err := seq3.Next()
		if err != nil {
			panic(err)
		}

		fmt.Printf("num:%v\r\n", num)
	}

	seq3.Release()
}

What did you expect to see?

Unique numbers

What did you see instead?

num:2
num:0
num:1
num:2
num:3
num:4

[jarifibrahim]

Hey @GameXG, I made a small modification to you code and it works as expected.

        key := []byte("key")
	seq1, err := db.GetSequence(key, 2)
	if err != nil {
		panic(err)
	}

	seq2, err := db.GetSequence(key, 2)
	if err != nil {
		panic(err)
	}
	num, err := seq2.Next()
	if err != nil {
		panic(err)
	}

	fmt.Printf("num:%v\r\n", num)
      
       // Release seq1 before releasing seq2. 
	err = seq1.Release()
	if err != nil {
		panic(err)
	}

	err = seq2.Release()
	if err != nil {
		panic(err)
	}

	seq3, err := db.GetSequence(key, 2)
	if err != nil {
		panic(err)
	}

	for i := 0; i < 5; i++ {
		num, err := seq3.Next()
		if err != nil {
			panic(err)
		}

		fmt.Printf("num:%v\r\n", num)
	}

	seq3.Release()

and here's the output

num:2
num:3
num:4
num:5
num:6
num:7

It is important that the sequences are released in the order they're fetched. It should be okay if you release only the last sequence object.
Here's what seq.Release() does
https://github.com/dgraph-io/badger/blob/4b539b98851fd2fb32683098cfe23f172c437a3f/db.go#L1232-L1245
So the last sequence object on which release is called will be stored in the DB and so the next call to seq.Next() will return the following numbers.

Hi,
Thank you for your reply.

There is no control over the release order when multithreading.
I feel the documentation needs to add warnings or fix this.

[jarifibrahim]

I agree. This issue should be fixed by #1281.

Thank you for fixing it @GameXG

hi, does this question involve #601?
In some cases, the auto-incrementing id will be used as the key. Duplicate keys will overwrite old entries by mistake.

simulate http concurrent upload data . Add dataList1, dataList2, dataList3 to the database.
dataList2 is completely missing.

package main

import (
	"encoding/binary"
	"encoding/json"
	"fmt"
	"log"
	"os"
	"sync"
	"time"

	"github.com/dgraph-io/badger"
)

type Data struct {
	V string
	// ...
}

func main() {
	opts := badger.DefaultOptions("./tmp/badger")
	_ = os.RemoveAll(opts.Dir)
	_ = os.MkdirAll(opts.Dir, 0700)
	db, err := badger.Open(opts)
	if err != nil {
		log.Fatal(err)
	}
	defer db.Close()

	////////////////////////////////////////////////////
	// simulate http concurrent upload data
	// Add dataList1, dataList2, dataList3 to the database
	////////////////////////////////////////////////////

	dataList1 := make([]*Data, 5)
	dataList2 := make([]*Data, 5)
	dataList3 := make([]*Data, 20)

	for k, _ := range dataList1 {
		dataList1[k] = &Data{V: fmt.Sprintf("dataList1-%v", k)}
	}
	for k, _ := range dataList2 {
		dataList2[k] = &Data{V: fmt.Sprintf("dataList2-%v", k)}
	}
	for k, _ := range dataList3 {
		dataList3[k] = &Data{V: fmt.Sprintf("dataList3-%v", k)}
	}

	wg := sync.WaitGroup{}
	wg.Add(2)

	go func() {
		defer wg.Done()

		// Increased latency makes it easier to reproduce
		err := addDataList(db, dataList1, 100*time.Millisecond)
		if err != nil {
			panic(err)
		}
	}()

	go func() {
		defer wg.Done()

		// Increased latency makes it easier to reproduce
		time.Sleep(10 * time.Millisecond)

		err := addDataList(db, dataList2, 0)
		if err != nil {
			panic(err)
		}
	}()

	wg.Wait()

	err = addDataList(db, dataList3, 0)
	if err != nil {
		panic(err)
	}

	// Output dataList1, dataList2, dataList3
	printDataList(dataList1)
	printDataList(dataList2)
	printDataList(dataList3)

	// Output database data
	err = db.View(func(txn *badger.Txn) error {
		opts := badger.DefaultIteratorOptions
		opts.PrefetchSize = 10
		it := txn.NewIterator(opts)
		defer it.Close()
		for it.Rewind(); it.Valid(); it.Next() {
			item := it.Item()
			//k := item.Key()
			err := item.Value(func(v []byte) error {
				fmt.Printf("%s\n", v)
				return nil
			})
			if err != nil {
				return err
			}
		}
		return nil
	})
	if err != nil {
		panic(err)
	}
}

// http Request
// add data in bulk
func addDataList(db *badger.DB, dataList []*Data, delay time.Duration) error {
	seq, err := db.GetSequence([]byte("dataId"), 10)
	if err != nil {
		return err
	}
	defer seq.Release()

	err = db.Update(func(txn *badger.Txn) error {
		for _, v := range dataList {
			id, err := seq.Next()
			if err != nil {
				return err
			}
			// Use incrementing id as primary key
			key := append([]byte("data-"), uint64ToBytes(id)...)

			value, err := json.Marshal(v)
			if err != nil {
				return err
			}
			err = txn.Set(key, value)
			if err != nil {
				return err
			}
		}

		return nil
	})

	// Increased latency makes it easier to reproduce
	time.Sleep(delay)

	if err != nil {
		return err
	}

	return nil
}

func uint64ToBytes(i uint64) []byte {
	var buf [8]byte
	binary.BigEndian.PutUint64(buf[:], i)
	return buf[:]
}

func bytesToUint64(b []byte) uint64 {
	return binary.BigEndian.Uint64(b)
}

func printDataList(dataList []*Data) {
	for _, v := range dataList {
		fmt.Printf("v:%v\r\n", v.V)
	}
}

[jarifibrahim]

Hey @GameXG, since the sequences were released in a different order, different values were stored in badger. This doesn't mean badger lost any data. All the data that was inserted is stored in badger and can be retrieved.

This isn't a data loss issue but more like an incorrect API use issue (or missing documentation issue). Thank you for looking into the issue and sending a PR for it. We appreciate the effort. :)