有条件地更新交易中的值

Question

I have a totalAmount and purchasedItems of items column in my table items . I want to atomically update purchasedItems if: totalAmount >= purchasedItems + 1 , else I want to throw an error. I tried doing something like this but it fails. How can I achieve this atomically (I am doing this with java jdbc)?

SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
BEGIN TRANSACTION
    DECLARE @purchased AS INT
    DECLARE @total AS INT
    SELECT @purchased = SELECT (purchasedItems + 1) FROM events WHERE id=1
    SELECT @total = SELECT totalAmount FROM items WHERE id=1
    IF @purchased > @total
    BEGIN
        SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Max value passed'
    END 
    # ELSE DO UPDATE 
    SELECT @remaining
COMMIT TRANSACTION

The error message is never thrown

Answer 1

If I start a transaction and do a get to check my condition passes and then update the table, another process cant update in between the get and update, right?

That is correct, but even with SERIALIZABLE transaction isolation you can still encounter deadlocks if multiple processes try to use your "check then update and commit" strategy. Consider a simplified example where the code simply wants to increment purchasedItems to a maximum of 10:

try (Connection conn = DriverManager.getConnection(connectionUrl, myUid, myPwd)) {
    conn.setAutoCommit(false);
    conn.setTransactionIsolation(Connection.TRANSACTION_SERIALIZABLE);
    final int maxPurchasedItems = 10;
    Statement st = conn.createStatement();
    System.out.println("Initial SELECT ...");
    Long t0 = System.nanoTime();
    ResultSet rs = st.executeQuery("SELECT purchasedItems FROM items WHERE id = 1");
    rs.next();
    int n = rs.getInt(1);
    System.out.printf("Original value: %d (%d ms)%n",
            n, (System.nanoTime() - t0) / 1000000);
    if (n >= maxPurchasedItems) {
        System.out.printf("Increment would exceed limit of %d. Cancelled.%n", maxPurchasedItems);
        conn.rollback();
    } else {
        Thread.sleep(5000);
        t0 = System.nanoTime();
        System.out.println("Attempting UPDATE ...");
        st.executeUpdate("UPDATE items SET purchasedItems = purchasedItems+1 WHERE id = 1");
        rs = st.executeQuery("SELECT purchasedItems FROM items WHERE id = 1");
        rs.next();
        n = rs.getInt(1);
        System.out.printf("Updated value: %d (%d ms)%n",
                n, (System.nanoTime() - t0) / 1000000);
        Thread.sleep(5000);
        conn.commit();
    }
} catch (Throwable ex) {
    ex.printStackTrace(System.err);
}

If we try to run that code simultaneously under two independent processes we see

Process_A:

Initial SELECT ...
Original value: 6 (142 ms)
Attempting UPDATE ...
Updated value: 7 (1910 ms)

Process_B:

Initial SELECT ...
Original value: 6 (144 ms)
Attempting UPDATE ...
com.mysql.cj.jdbc.exceptions.MySQLTransactionRollbackException: Deadlock found when trying to get lock; try restarting transaction

That's because Process_A's initial SELECT places a read lock (aka shared lock) on the row. It's only a read lock, so Process_B's initial SELECT is allowed to proceed. However, it also places a read lock on that same row so the two transactions are deadlocked when it comes to writes. MySQL has to pick a transaction to kill, and Process_B is the unlucky one.

Instead, you should use an "update then check and rollback if necessary" strategy:

try (Connection conn = DriverManager.getConnection(connectionUrl, myUid, myPwd)) {
    conn.setAutoCommit(false);
    final int maxPurchasedItems = 10;
    Statement st = conn.createStatement();
    System.out.println("Initial UPDATE ...");
    Long t0 = System.nanoTime();
    st.executeUpdate("UPDATE items SET purchasedItems = purchasedItems+1 WHERE id = 1");
    ResultSet rs = st.executeQuery("SELECT purchasedItems FROM items WHERE id = 1");
    rs.next();
    int n = rs.getInt(1);
    System.out.printf("Updated value: %d (%d ms)%n",
            n, (System.nanoTime() - t0) / 1000000);
    Thread.sleep(5000);
    if (n > maxPurchasedItems) {
        System.out.printf("Increment exceeds limit of %d. Rolling back.%n", maxPurchasedItems);
        conn.rollback();
    } else {
        conn.commit();
    }
} catch (Throwable ex) {
    ex.printStackTrace(System.err);
}