Question

我尝试在两个帐户之间实现转移，以便它在并发事务中正常工作。但由于某种原因，第二个交易正在等待第一个交易进行所有更新，但它没有看到更改。因此，余额未正确更新。

行为是否正确？
如果是对的，为什么呢？ H2 docs明确表示提交事务所做的更改对其他事务可见。
如果这种转移方式基本上是错误的，哪一个是正确的？但请记住，我想依靠数据库设备，而不是Java锁等。

H2数据库设置

版本1.4.196
MVCC引擎已开启：MVCC mode is enabled by default in version 1.4.x
忽略事务隔离级别：If MVCC is enabled, changing the lock mode (LOCK_MODE) has no effect)
已启用多线程模式：MULTI_THREADED=1

测试用例

初始余额：
1. acc1 = 300
2. acc2 = 400
进行两次并行传输（第二次传输延迟200ms）：
1. 50从acc1到acc2
2. 200从acc2到acc1
预计期末余额：
1. acc1 = 450（减去50，加上200）
2. acc2 = 250（加50，减去200）
实际期末余额：
1. acc1 = 500
2. acc2 = 200

SQL架构

CREATE TABLE ACCOUNT (
  ID BIGINT PRIMARY KEY AUTO_INCREMENT,
  NUMBER VARCHAR(25) NOT NULL,
  BALANCE DECIMAL(19,2) DEFAULT 0 CHECK (BALANCE >= 0)
);

INSERT INTO ACCOUNT(NUMBER, BALANCE) VALUES('acc1', 300);
INSERT INTO ACCOUNT(NUMBER, BALANCE) VALUES('acc2', 400);

单元测试

public class ConcurrentTransfersTest {
    @Test
    void testForStackOverflow() throws Exception {
        final String url = "jdbc:h2:mem:test;DB_CLOSE_DELAY=-1;MULTI_THREADED=1;" +
                "INIT=RUNSCRIPT FROM 'classpath:/h2/schema.sql'\\;RUNSCRIPT FROM 'classpath:/h2/test-data.sql'";

        JdbcConnectionPool pool = JdbcConnectionPool.create(url, "sa", "");

        assertEquals(BigDecimal.valueOf(30000, 2), readData(pool, 1));
        assertEquals(BigDecimal.valueOf(40000, 2), readData(pool, 2));

        Thread t1 = new Thread(() -> updateData("t1", pool, 1, 2, BigDecimal.valueOf(50)));
        Thread t2 = new Thread(() -> updateData("t2", pool, 2, 1, BigDecimal.valueOf(200)));

        t1.start();
        Thread.sleep(200);
        t2.start();

        t1.join();
        t2.join();

        assertEquals(BigDecimal.valueOf(40000, 2), readData(pool, 1));
        assertEquals(BigDecimal.valueOf(30000, 2), readData(pool, 2));

        try (Connection c = pool.getConnection(); Statement st = c.createStatement()) {
            st.execute("SHUTDOWN IMMEDIATELY");
        }
    }

    private BigDecimal readData(DataSource ds, long id) {
        try (Connection c = ds.getConnection();
             PreparedStatement select = c.prepareStatement("SELECT BALANCE FROM ACCOUNT WHERE ID = ?")) {
            return readFromPreparedStatment(select, id);
        } catch (Exception e) {
            e.printStackTrace();
        }

        return null;
    }

    private BigDecimal readFromPreparedStatment(PreparedStatement select, long id) throws Exception {
        select.setLong(1, id);
        ResultSet rs = select.executeQuery();
        rs.next();
        BigDecimal b = rs.getBigDecimal(1);
        rs.close();

        return b;
    }

    private void updateData(String thread, DataSource ds, long from, long to, BigDecimal amount) {
        try {
            try (Connection c = ds.getConnection();
                 PreparedStatement minus = c.prepareStatement("UPDATE ACCOUNT SET BALANCE = BALANCE - ? WHERE ID = ?");
                 PreparedStatement plus = c.prepareStatement("UPDATE ACCOUNT SET BALANCE = BALANCE + ? WHERE ID = ?");
                 PreparedStatement select = c.prepareStatement("SELECT BALANCE FROM ACCOUNT WHERE ID = ?")
            ) {
                c.setAutoCommit(false);

                System.out.println(System.currentTimeMillis() + " " + thread + ": transfer " + amount + " " +
                                "from = " + readFromPreparedStatment(select, from) + " to = " + readFromPreparedStatment(select, to));

                minus.setBigDecimal(1, amount);
                minus.setLong(2, from);
                System.out.println(System.currentTimeMillis() + " " + thread + ": from minus " + amount);
                minus.executeUpdate();

                Thread.sleep(2000);

                plus.setBigDecimal(1, amount);
                plus.setLong(2, to);
                System.out.println(System.currentTimeMillis() + " " + thread + ": to plus " + amount);
                plus.executeUpdate();

                System.out.println(System.currentTimeMillis() + " " + thread + ": from = " + readFromPreparedStatment(select, from));
                System.out.println(System.currentTimeMillis() + " " + thread + ": to = " + readFromPreparedStatment(select, to));

                c.commit();
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

输出

1508149269895 t1: transfer 50 from = 300.00 to = 400.00
1508149269895 t1: from minus 50
1508149271899 t1: to plus 50
1508149271900 t1: before commit from = 250.00
1508149271900 t1: before commit to = 450.00
1508149271913 t2: transfer 200 from = 400.00 to = 300.00
1508149271913 t2: from minus 200
1508149273918 t2: to plus 200
1508149273918 t2: before commit from = 200.00
1508149273918 t2: before commit to = 500.00

在H2数据库中的并行事务中，一个事务中提交的更改不可用

0 个答案: