google-app-engine - 如何在 AppEngine (GAE) 中进行数据库锁定？

Question

在 GAE 中，我有一个充满“一次性”的表格——诸如“最后使用的序列号”之类的东西实际上并不属于其他表格。这是一个带有字符串值对的简单字符串键。

我有一些代码可以获取一个命名整数并递增它，如下所示:

@PersistenceCapable(detachable="true")
public class OneOff
{
    @PrimaryKey
    @Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
    private Key key;

    @Persistent
    private String dataKey;

    @Persistent
    private String value;


    public OneOff(String kk, String vv)
    {
        this.dataKey = kk;
        this.value = vv;
    }


    public static OneOff persistOneOff(String kk, String vv)
    {
        OneOff oneoff= new OneOff(kk, vv);
        PersistenceManager pm = PMF.get().getPersistenceManager();
        try
        {
            pm.makePersistent(oneoff);
        }
        finally
        {
            pm.close();
        }

        return oneoff;
    }


    // snip...
    @SuppressWarnings("unchecked")
    synchronized
    public static int getIntValueForKeyAndIncrement(String kk, int deFltValue)
    {
        int result = 0;
        OneOff oneOff = null;

        PersistenceManager pm = PMF.get().getPersistenceManager();
        Query query = pm.newQuery(OneOff.class);
        query.setFilter("dataKey == kkParam");
        query.declareParameters("String kkParam");
        List<OneOff> oneOffs = (List<OneOff>) query.execute(kk);

        int count = oneOffs.size();
        if (count == 1)
        {
            oneOff = oneOffs.get(0);
            result = Integer.parseInt(oneOff.value);
        }
        else if (count == 0)
        {
            oneOff = new OneOff(kk, "default");
            result = deFltValue;
        }
        else
        {
                // Log WTF error.
        }

        // update object in DB.
        oneOff.value = "" + (result+1);
        try
        {
            pm.makePersistent(oneOff);
        }
        finally
        {
            pm.close();
        }

        return result;
    }
    // etc...

但是，当我进行这些调用时:

int val1 = OneOff.getIntValueForKeyAndIncrement("someKey", 100);
int val2 = OneOff.getIntValueForKeyAndIncrement("someKey", 100);
int val3 = OneOff.getIntValueForKeyAndIncrement("someKey", 100);

有时我得到所需的增量，有时我得到相同的值。当我想为这个特定事务锁定数据库时，我的数据库访问似乎是异步运行的。

我以为

    synchronized
    public static

应该为我做那件事，但显然不是(可能是由于多个实例在运行!)

无论如何——我如何做我想做的事？ (我想在获取和更新此值时锁定我的数据库，以使整个过程是并发安全的。)

谢谢!

== 编辑 ==

我接受了罗伯特的正确答案，因为交易确实是我想要的。但是，为了完整起见，我在下面添加了更新的代码。我认为这是正确的，尽管我不确定 if(oneOff==null) 子句(try-catch 位。)

public static int getIntValueForKeyAndIncrement(String kk, int defltValue)
{
    int result = 0;
    Entity oneOff = null;
    int retries = 3;

    // Using Datastore Transactions
    DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
    while (true)
    {
        com.google.appengine.api.datastore.Transaction txn = datastore.beginTransaction();
        try
        {
            Key oneOffKey = KeyFactory.createKey("OneOff", kk);
            oneOff = datastore.get (oneOffKey);
            result = Integer.parseInt((String) oneOff.getProperty("value"));
            oneOff.setProperty("value",  "" + (result+1));
            datastore.put(oneOff);
            txn.commit();
            break;
        }
        catch (EntityNotFoundException ex)
        {
            result = defltValue;
        }
        catch (ConcurrentModificationException ex)
        {
            if (--retries < 0)
            {
                throw ex;
            }
        }

        if (oneOff == null)
        {
            try
            {
                Key oneOffKey = KeyFactory.createKey("OneOff", kk);
                oneOff = new Entity(oneOffKey);
                oneOff.setProperty("value",  "" + (defltValue+1));
                datastore.put(txn, oneOff);
                datastore.put(oneOff);
                txn.commit();
                break;
            }
            finally
            {
                if (txn.isActive())
                {
                    txn.rollback();
                }
            }
        }
        else
        {
            if (txn.isActive())
            {
                txn.rollback();
            }
        }
    }
return result;
}

Answer 1

您应该在 transaction 中更新您的值.只要您的读取和写入在单个事务中，App Engine 的事务就会防止两个更新相互覆盖。一定要关注实体组的讨论。