[Day 11] How to avoid deadlock in DB transaction?

15th鐵人賽 postgresql golang transaction deadlock

kcih4518

2023-09-26 11:21:56

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Potential Deadlock Scenarios

store.go

func (store *Store) TransferTx(ctx context.Context, arg TransferTxParams) (TransferTxResult, error) {
	var result TransferTxResult
	err := store.execTx(ctx, func(q *Queries) error {
		var err error

		txName := ctx.Value(txKey)
		fmt.Println(txName, "create transfer")

		// should convert arg (type TransferTxParams) to CreateTransferParams instead of using struct literal (gosimple)
		// Because CreateTransferParams has same fields as TransferTxParams.
		result.Transfer, err = q.CreateTransfer(ctx, CreateTransferParams(arg))

		if err != nil {
			return err
		}

		fmt.Println(txName, "create entry 1")
		result.FromEntry, err = q.CreateEntry(ctx, CreateEntryParams{
			AccountID: arg.FromAccountID,
			Amount:    -arg.Amount,
		})

		if err != nil {
			return err
		}

		fmt.Println(txName, "create entry 2")
		result.ToEntry, err = q.CreateEntry(ctx, CreateEntryParams{
			AccountID: arg.ToAccountID,
			Amount:    arg.Amount,
		})

		if err != nil {
			return err
		}

		// Implement update accounts' balance
		// move money out of account1
		fmt.Println(txName, "update account 1")
		result.FromAccount, err = q.AddAccountBalance(ctx, AddAccountBalanceParams{
			ID:     arg.FromAccountID,
			Amount: -arg.Amount,
		})

		if err != nil {
			return err
		}

		// move money into account2
		fmt.Println(txName, "update account 2")
		result.ToAccount, err = q.AddAccountBalance(ctx, AddAccountBalanceParams{
			ID:     arg.ToAccountID,
			Amount: arg.Amount,
		})

		if err != nil {
			return err
		}

		return nil
	})

	return result, err
}

在上一階段已經解決了foreign key所操成的deadlock,並且也將*GetAccountForUpdate 和*UpdateAccount 進行了合併，減少Transaction的執行時間，大大”降低”了 Deadlock的可能性。

-- name: GetAccountForUpdate :one
SELECT *
FROM accounts
WHERE id = $1
LIMIT 1
FOR NO KEY
UPDATE;

-- name: UpdateAccount :one
UPDATE accounts
  set balance = $2
WHERE id = $1
RETURNING *;

-- name: AddAccountBalance :one
UPDATE accounts
SET balance = balance + sqlc.arg(amount)
WHERE id = sqlc.arg(id)
RETURNING *;

store_test.go

func TestTransferTx(t *testing.T) {
	store := NewStore(testDB)

	// check results
	// init key value map
	existed := make(map[int]bool)

	// Create two accounts.
	account1 := createRandomAccount(t)
	account2 := createRandomAccount(t)
	fmt.Println(">> before:", account1.Balance, account2.Balance)

	n := 5
	amount := int64(10)

	// run n concurrent transfer transaction
	// Simulate multiple concurrent requests in real world
	errs := make(chan error)
	results := make(chan TransferTxResult)

	for i := 0; i < n; i++ {
		txName := fmt.Sprintf("tx %d", i+1)

		// 匿名函數的 goroutine
		go func() {
			ctx := context.WithValue(context.Background(), txKey, txName)
			result, err := store.TransferTx(ctx, TransferTxParams{
				FromAccountID: account1.ID,
				ToAccountID:   account2.ID,
				Amount:        amount,
			})
			errs <- err
			results <- result
		}()
	}

	// check results
	for i := 0; i < n; i++ {
		err := <-errs
		require.NoError(t, err)

		result := <-results
		require.NotEmpty(t, result)

		// check transfer
		transfer := result.Transfer
		require.NotEmpty(t, transfer)
		require.Equal(t, account1.ID, transfer.FromAccountID)
		require.Equal(t, account2.ID, transfer.ToAccountID)
		require.Equal(t, amount, transfer.Amount)
		require.NotZero(t, transfer.ID)
		require.NotZero(t, transfer.CreatedAt)

		// Need to to be sure that a transfer record is really created in the database
		_, err = store.GetTransfer(context.Background(), transfer.ID)
		require.NoError(t, err)

		// check entries
		fromEntry := result.FromEntry
		require.NotEmpty(t, fromEntry)
		require.Equal(t, account1.ID, fromEntry.AccountID)
		require.Equal(t, -amount, fromEntry.Amount)
		require.NotZero(t, fromEntry.ID)
		require.NotZero(t, fromEntry.CreatedAt)

		_, err = store.GetEntry(context.Background(), fromEntry.ID)
		require.NoError(t, err)

		toEntry := result.ToEntry
		require.NotEmpty(t, toEntry)
		require.Equal(t, account2.ID, toEntry.AccountID)
		require.Equal(t, amount, toEntry.Amount)
		require.NotZero(t, toEntry.ID)
		require.NotZero(t, toEntry.CreatedAt)

		_, err = store.GetEntry(context.Background(), toEntry.ID)
		require.NoError(t, err)

		// check accounts
		fromAccount := result.FromAccount
		require.NotEmpty(t, fromAccount)
		require.Equal(t, account1.ID, fromAccount.ID)

		toAccount := result.ToAccount
		require.NotEmpty(t, toAccount)
		require.Equal(t, account2.ID, toAccount.ID)

		// check balances
		fmt.Println(">> tx:", fromAccount.Balance, toAccount.Balance)

		// check account balance
		// account1.balance and account2.Balance is original balance
		// fromAccount.balance and toAccount.Balance is after transfer
		diff1 := account1.Balance - fromAccount.Balance
		diff2 := toAccount.Balance - account2.Balance
		require.Equal(t, diff1, diff2)
		require.True(t, diff1 > 0)
		require.True(t, diff1%amount == 0)

		// 1 <= k <= n
		k := int(diff1 / amount)
		require.True(t, k >= 1 && k <= n)

		require.NotContains(t, existed, k)
		existed[k] = true
	}

	// check the final updated balance
	updatedAccount1, err := store.GetAccountForUpdate(context.Background(), account1.ID)
	require.NoError(t, err)

	updatedAccount2, err := store.GetAccountForUpdate(context.Background(), account2.ID)
	require.NoError(t, err)

	fmt.Println(">> after:", updatedAccount1.Balance, updatedAccount2.Balance)

	require.Equal(t, account1.Balance-int64(n)*amount, updatedAccount1.Balance)
	require.Equal(t, account2.Balance+int64(n)*amount, updatedAccount2.Balance)
}

為何會說只是降低了可能呢? 雖然我們在unit test中模擬了多個並行的轉帳交易，但這些交易的操作順序都是一致的從account 1轉到account 2（先從account 1減少餘額，再向account 2增加餘額)，所以較不容易出現DeadLock的情況。
但現實中會有更復雜的Transaction ，例如，同時有一個交易是從account 1轉到account 2，而另一個交易剛好是從account 2轉到account 1，這樣的情況下，如果這兩個交易的操作順序不一致，就有可能出現死鎖。(transaction1先鎖定了account 1，同時transaction2也鎖定了account 2，這兩個交易都會等待對方釋放Lock的資源，這就會導致DeadLock的情況。)

我們可以用以下SQL並搭配平行處理來進行模擬與確認:

第一筆交易將從account 1轉移10美元到account 2，首先從account 1的餘額中減去10美元，然後再將10美元加到account 2的餘額中。
第二筆交易將執行相反的操作：從account 2轉移10美元到account 1。首先從account 2的餘額中減去10美元，然後再將10美元加到account 1的餘額中。

-- Tx1: transfer $10 from account 1 to account 2
BEGIN;

UPDATE accounts SET balance = balance - 10 WHERE id = 1 RETURNING *;
UPDATE accounts SET balance = balance + 10 WHERE id = 2 RETURNING *;

ROLLBACK;

-- Tx2: transfer $10 from account 2 to account 1
BEGIN;

UPDATE accounts SET balance = balance - 10 WHERE id = 2 RETURNING *;
UPDATE accounts SET balance = balance + 10 WHERE id = 1 RETURNING *;

ROLLBACK;

我們建立兩個Terminal 1 和 Terminal 2 ,並都進入到simple_bank中:
```
docker exec -it postgres psql -U root simple_bank
```

接下來在Terminal 1 和 Terminal 2 各自執行第一步驟的UPDATE，這時Transaction 1會blocked account 1 而 Transaction 1 會blocked account2 :

Termainal 1:
simple_bank=# BEGIN;
BEGIN
simple_bank=*#
simple_bank=*# UPDATE accounts SET balance = balance - 10 WHERE id = 1 RETURNING *;
 id | owner  | balance | currency |          created_at
----+--------+---------+----------+-------------------------------
  1 | tbsyig |     838 | EUR      | 2023-08-17 01:32:51.623403+00
(1 row)

UPDATE 1

Termainal 2:
simple_bank=# BEGIN;
BEGIN
simple_bank=*#
simple_bank=*# UPDATE accounts SET balance = balance - 10 WHERE id = 2 RETURNING *;
 id | owner  | balance | currency |          created_at
----+--------+---------+----------+-------------------------------
  2 | shyufr |     676 | EUR      | 2023-08-17 01:32:51.653722+00
(1 row)

UPDATE 1

如果Transaction 1執行下一步驟的Update account2就會發生Deadlock

Termainal 1:
simple_bank=# BEGIN;
BEGIN
simple_bank=*#
simple_bank=*# UPDATE accounts SET balance = balance - 10 WHERE id = 1 RETURNING *;
 id | owner  | balance | currency |          created_at
----+--------+---------+----------+-------------------------------
  1 | tbsyig |     838 | EUR      | 2023-08-17 01:32:51.623403+00
(1 row)

UPDATE 1

UPDATE accounts SET balance = balance + 10 WHERE id = 2 RETURNING *;

Termainal 2:
simple_bank=# BEGIN;
BEGIN
simple_bank=*#
simple_bank=*# UPDATE accounts SET balance = balance - 10 WHERE id = 2 RETURNING *;
 id | owner  | balance | currency |          created_at
----+--------+---------+----------+-------------------------------
  2 | shyufr |     676 | EUR      | 2023-08-17 01:32:51.653722+00
(1 row)

UPDATE 1

接下來我們到Table Plus 執下以下SQL Query來列出目前lock的清況，可以發現transaction 1對於update account 2 的query正在嘗試獲取一個transaction ID 800的ShareLock，但它還沒有被授予。

SELECT
    a.application_name,
    l.relation::regclass,
    l.transactionid,
    l.mode,
    l.locktype,
    l.GRANTED,
    a.usename,
    a.query,
    a.pid
FROM
    pg_stat_activity a
    JOIN pg_locks l ON
    l.pid = a.pid
WHERE
    a.application_name = 'psql'
ORDER BY
    a.pid;

這是因為transaction 2已經持有了同一個transaction ID的ExclusiveLock因此，transaction 1必須等待transaction 2完成後才能繼續。
同等如果我們transaction 2 執行update account 1 的query，我們就會收到deadlock的通知，因為account 1正在被transaction 1更新，因此transaction 2也需要等待transaction 1完成才能獲得此查詢的結果。這兩個concurrent transactions都需要等待對方，因此發生了死鎖:

Replicate deadlock scenario in a test

建立一個 TestTransferTxDeadlock 用來檢測deadlock error:
- 總共會執行10次transaction : account 1 → account 2 (5 times), account 2→ account 1 (5 times)
- 這裡只會驗證deadlock的行為，所以並不需要驗證results
- 並且在經過10次的transaction 後，account 1 和 account 2 的Balance 不會改變。

func TestTransferTxDeadlock(t *testing.T) {
	store := NewStore(testDB)

	// Create two accounts.
	account1 := createRandomAccount(t)
	account2 := createRandomAccount(t)
	fmt.Println(">> before:", account1.Balance, account2.Balance)

	n := 10
	amount := int64(10)

	// run n concurrent transfer transaction
	// Simulate multiple concurrent requests in real world
	errs := make(chan error)

	for i := 0; i < n; i++ {
		txName := fmt.Sprintf("tx %d", i+1)
		fromAccountID := account1.ID
		toAccountID := account2.ID

		if i%2 == 1 {
			fromAccountID = account2.ID
			toAccountID = account1.ID
		}

		// 匿名函數的 goroutine
		go func() {
			ctx := context.WithValue(context.Background(), txKey, txName)
			_, err := store.TransferTx(ctx, TransferTxParams{
				FromAccountID: fromAccountID,
				ToAccountID:   toAccountID,
				Amount:        amount,
			})
			errs <- err
		}()
	}

	// check results
	for i := 0; i < n; i++ {
		err := <-errs
		require.NoError(t, err)
	}

	// check the final updated balance
	updatedAccount1, err := store.GetAccountForUpdate(context.Background(), account1.ID)
	require.NoError(t, err)

	updatedAccount2, err := store.GetAccountForUpdate(context.Background(), account2.ID)
	require.NoError(t, err)

	fmt.Println(">> after:", updatedAccount1.Balance, updatedAccount2.Balance)

	require.Equal(t, account1.Balance, updatedAccount1.Balance)
	require.Equal(t, account2.Balance, updatedAccount2.Balance)
}

Fix the deadlock issue

一致的`transaction` 順序

從前面我們得知transaction1先鎖定了account 1，同時transaction2也鎖定了account 2，這兩個交易都會等待對方釋放Lock的資源，這就會導致DeadLock的情況。
我們可以避免**deadlock**，只需讓兩個交易按相同的順序更新帳戶餘額。例如，在transaction2中，我們只需將更新account 1的查詢提前，其他保持不變就不會發生阻塞。

SQL 驗証

-- Tx1: transfer $10 from account 1 to account 2
BEGIN;

UPDATE accounts SET balance = balance - 10 WHERE id = 1 RETURNING *;
UPDATE accounts SET balance = balance + 10 WHERE id = 2 RETURNING *;

ROLLBACK;

-- Tx2: transfer $10 from account 2 to account 1
BEGIN;

UPDATE accounts SET balance = balance + 10 WHERE id = 1 RETURNING *; -- moved up
UPDATE accounts SET balance = balance - 10 WHERE id = 2 RETURNING *;

ROLLBACK;

Transaction2 的動作，需要等待Transaction1 操作完成，這樣就不會相互持有lock :
當Transaction1 的操作完成(COMMIT/ROLLBACK)，Transaction2 才會繼續進行:
所以我們知道防止DeadLock的最佳方式是確保應用程序總是以一致的Transaction順序獲取Lock。

Refactor the code

store.go
func (store *Store) TransferTx(ctx context.Context, arg TransferTxParams) (TransferTxResult, error) {
    var result TransferTxResult

    err := store.execTx(ctx, func(q *Queries) error {
        ...

        if arg.FromAccountID < arg.ToAccountID {
            result.FromAccount, err = q.AddAccountBalance(ctx, AddAccountBalanceParams{
                ID:     arg.FromAccountID,
                Amount: -arg.Amount,
            })
            if err != nil {
                return err
            }

            result.ToAccount, err = q.AddAccountBalance(ctx, AddAccountBalanceParams{
                ID:     arg.ToAccountID,
                Amount: arg.Amount,
            })
            if err != nil {
                return err
            }
        } else {
            result.ToAccount, err = q.AddAccountBalance(ctx, AddAccountBalanceParams{
                ID:     arg.ToAccountID,
                Amount: arg.Amount,
            })
            if err != nil {
                return err
            }

            result.FromAccount, err = q.AddAccountBalance(ctx, AddAccountBalanceParams{
                ID:     arg.FromAccountID,
                Amount: -arg.Amount,
            })
            if err != nil {
                return err
            }
        }

        return nil
    })

    return result, err
}

建了保持Transaction的動作一致，我們透過AccountID來判斷執行順序，但現在的function太過冗長，我們可以將整體Update Account Balance的動作抽換成單一的function來處理(addMoney)，再配合原先的判斷式來帶入不同的參數:

func addMoney(
    ctx context.Context,
    q *Queries,
    accountID1 int64,
    amount1 int64,
    accountID2 int64,
    amount2 int64,
) (account1 Account, account2 Account, err error) {
    account1, err = q.AddAccountBalance(ctx, AddAccountBalanceParams{
        ID:     accountID1,
        Amount: amount1,
    })
    if err != nil {
        return
    }

    account2, err = q.AddAccountBalance(ctx, AddAccountBalanceParams{
        ID:     accountID2,
        Amount: amount2,
    })
    return
}

func (store *Store) TransferTx(ctx context.Context, arg TransferTxParams) (TransferTxResult, error) {
    var result TransferTxResult

    err := store.execTx(ctx, func(q *Queries) error {
        ...

        if arg.FromAccountID < arg.ToAccountID {
            result.FromAccount, result.ToAccount, err = addMoney(ctx, q, arg.FromAccountID, -arg.Amount, arg.ToAccountID, arg.Amount)
        } else {
            result.ToAccount, result.FromAccount, err = addMoney(ctx, q, arg.ToAccountID, arg.Amount, arg.FromAccountID, -arg.Amount)
        }

        return err
    })

    return result, err
}

這樣的改動帶來以下幾個好處：
1. 提高代碼重用性：通過將轉帳的邏輯抽取到一個單獨的函數addMoney中，可以在其他地方更容易地重用這段代碼，減少代碼重複。
2. 提高代碼可讀性：原先的代碼在TransferTx函數中直接執行轉帳操作，這使得這個函數變得相對冗長。通過將部分邏輯抽取到外部函數中，可以使TransferTx函數更簡潔、易讀。
3. 減少代碼複雜性：原先的代碼中有兩段非常類似的代碼，這增加了代碼的複雜性。通過使用addMoney函數，我們可以將這些重複的邏輯合併起來，減少代碼的複雜性。
4. 降低維護成本：未來如果需要修改轉帳的邏輯，只需要修改addMoney函數即可，無需在多個地方進行修改，降低了維護成本。

Q&A:

為何相同的順序下就能解決deadlock ?
1. 當兩個交易按相同的順序更新帳戶餘額時，它們不會同時試圖鎖定對方已經鎖定的資源。這樣，就不會有交易等待另一個交易釋放資源的情況，因此也就不會出現死鎖。當所有交易都按照一致的順序訪問資源時，可以確保每一個交易都能順利完成，不會互相阻塞。

什麼是 naked return ?

函式簽名宣告了有名稱的返回變量 (account1 Account, account2 Account, err error)。因此，當在函式體內遇到 return 語句時，該函式將返回 account1、account2 和 err 變量的當前值。

func addMoney(ctx context.Context,
	q *Queries,
	accountID1 int64,
	amount1 int64,
	accountID2 int64,
	amount2 int64,
) (account1 Account, account2 Account, err error) {
	account1, err = q.AddAccountBalance(ctx, AddAccountBalanceParams{
		ID:     accountID1,
		Amount: amount1,
	})
	if err != nil {
		return
	}
	account2, err = q.AddAccountBalance(ctx, AddAccountBalanceParams{
		ID:     accountID2,
		Amount: amount2,
	})
	return
}