事务和ConcurrentModificationException文档含糊不清

使其具有幂等性 - 意思是说，如果您调用它两次，则无关紧要。在支付交易的情况下，您将创建支付交易实体。对于每一笔交易，您都会使用付款数据创建一个特殊实体并将其写入数据存储。这里的关键是从付款数据中生成一个自然ID：用户ID，来源账户，付款金额，目标账户，日期/小时/分钟。然后，如果交易重复，它将创建一个具有相同ID的支付交易实体，并将覆盖旧交易实体 - 这意味着如果只有一个或两个交易完成，结果将是相同的。 （然后通过一系列交易并将其添加到开户金额来计算用户账户的余额 - 这是银行在实践中实际做的AFAIK）。

As docs say - you can not be sure if transaction was applied. That is the core of the problem.

Just rollback in case of all exceptions. See this example: code.google/appengine/docs/java/datastore/transactions.html#Isolation_and_Consistency

Make it idempotent - meaning if you invoke it twice, it does not matter. In the case of payment transactions, you make "payment transaction" entity. For every transaction you make a special entity with payment data and write it to datastore. The key here is to generate a natural ID from payment data: user ID, source account, payment amount, target account, date/hour/minute. Then if transaction is repeated, it will create a payment transaction entity with the same ID and will overwrite the old one - which means the result will be the same if only one or two transactions were completed. (The balance of the user account is then calculated by going through a series of transactions and adding it to the opening amount - this is what banks actually do in practice AFAIK).