java - 集群环境中的 Hibernate session 缓存行为？

Question

在我参加的一次采访中，我被问到了以下问题 -

Let's say we have multi-clustered application servers, where each has created its own SessionFactory (pointing to same single DB). One server retrieves a record [Person with ID = 1] which caches the same in the session (s1), which is still open.

In the meantime, another server, opens another session (s2) and updates the Person record with ID = 1. Now what happens to the Person record cached within s1? Will it be a stale record?

我创建了一个小程序来模拟单个 JVM 上的场景。令我惊讶的是，尽管从同一个 SessionFactory 创建了 2 个 session ， session 缓存仍保留了过时的数据。甚至 s1.flush() 也没有帮助。

    Session s1 = HibernateManager.getInstance().openNewSession();
    System.out.println("Opened session s1");
    Person p1 = s1.get(Person.class, 1);
    System.out.println("Retreived person with ID = 1 using s1, Name is");
    System.out.println(p1.getName()); //prints Test1
    
    Session s2 = HibernateManager.getInstance().openNewSession();
    System.out.println("Opened session s2");
    Person p2 = s2.get(Person.class, 1);
    System.out.println("Retreived person with ID = 1 using s2, Name is");
    System.out.println(p2.getName()); //prints Test1
    p2.setName("Test2");
    Transaction tx = s2.beginTransaction();
    s2.saveOrUpdate(p2);
    tx.commit();
    HibernateManager.getInstance().close(s2);
    System.out.println("Changed name and updated - closed s2");     
    
    p1 = s1.get(Person.class, 1);
    System.out.println("Retreived person with ID = 1 using s1 AGAIN, Name is");
    System.out.println(p1.getName()); //prints Test1
    
    System.out.println("Flushed s1");
    s1.flush();
    
    p1 = s1.get(Person.class, 1);
    System.out.println("Retreived person with ID = 1 using s1 AGAIN, Name is");
    System.out.println(p1.getName()); //still prints Test1. I was expecting Test2 because I flushed s1 (synced the session with DB)
    
    HibernateManager.getInstance().close(s1);

我的问题是，如果使用另一个 session 更新相同的记录， session 会使缓存失效吗？

Answer 1

是的，p1 将是一个过时的记录。如果您更新 p1，则在 session s1 的 session 刷新或事务提交时，p1 将覆盖 p2 的更新。

为了防止这种情况，您可以使用乐观锁定g。指定版本号(java源代码中的@Version)(如果有这样的列，则指定最后更新日期)并指示hibernate使用此列进行版本更新。在这种情况下，如果我们更新 p1，在 session 刷新时，hibernate 将在更新之前执行选择，查看版本信息不匹配(有人进入并更新记录)并抛出 StaleObject 异常