我正在查看 Java 中的 Sharded Counters 示例: http://code.google.com/appengine/articles/sharding_counters.html

我对增量方法的实现有疑问。在 python 中，它显式地包装了 get() 和事务中的增量。在 Java 示例中，它只是检索并设置它。我不确定我是否完全理解数据存储和事务，但似乎关键更新部分应该包含在数据存储事务中。我错过了什么吗？

原代码:

  public void increment() {
    PersistenceManager pm = PMF.get().getPersistenceManager();

    Random generator = new Random();
    int shardNum = generator.nextInt(NUM_SHARDS);

    try {
      Query shardQuery = pm.newQuery(SimpleCounterShard.class);
      shardQuery.setFilter("shardNumber == numParam");
      shardQuery.declareParameters("int numParam");

      List<SimpleCounterShard> shards =
          (List<SimpleCounterShard>) shardQuery.execute(shardNum);
      SimpleCounterShard shard;

      // If the shard with the passed shard number exists, increment its count
      // by 1. Otherwise, create a new shard object, set its count to 1, and
      // persist it.
      if (shards != null && !shards.isEmpty()) {
        shard = shards.get(0);
        shard.setCount(shard.getCount() + 1);
      } else {
        shard = new SimpleCounterShard();
        shard.setShardNumber(shardNum);
        shard.setCount(1);
      }

      pm.makePersistent(shard);
    } finally {
      pm.close();
    }
  }
}

事务代码(我相信你需要在事务中运行它以保证并发事务下的正确性？):

public void increment() { 
    PersistenceManager pm = PMF.get().getPersistenceManager(); 
    Random generator = new Random(); 
    int shardNum = generator.nextInt(NUM_SHARDS); 
    try { 
      Query shardQuery = pm.newQuery(SimpleCounterShard.class); 
      shardQuery.setFilter("shardNumber == numParam"); 
      shardQuery.declareParameters("int numParam"); 
      List<SimpleCounterShard> shards = 
          (List<SimpleCounterShard>) shardQuery.execute(shardNum); 
      SimpleCounterShard shard; 
      // If the shard with the passed shard number exists, increment its count 
      // by 1. Otherwise, create a new shard object, set its count to 1, and 
      // persist it. 
      if (shards != null && !shards.isEmpty()) { 
            Transaction tx = pm.currentTransaction(); 
        try { 
            tx.begin(); 
            //I believe in a transaction objects need to be loaded by ID (can't use the outside queried entity) 
             Key shardKey = KeyFactory.Builder(SimpleCounterShard.class.getSimpleName(), shards.get(0).getID()) 
            shard =  pm.getObjectById(SimpleCounterShard.class, shardKey); 
            shard.setCount(shard.getCount() + 1); 
            tx.commit(); 
        } finally { 
            if (tx.isActive()) { 
                tx.rollback(); 
            } 
        } 
      } else { 
        shard = new SimpleCounterShard(); 
        shard.setShardNumber(shardNum); 
        shard.setCount(1); 
      } 
      pm.makePersistent(shard); 
    } finally { 
      pm.close(); 
    } 
  } 

最佳答案

直接从文档中取出的这一部分表明您对需要交易的看法是完全正确的:

http://code.google.com/appengine/docs/java/datastore/transactions.html#Uses_For_Transactions

This example demonstrates one use of transactions: updating an entity with a new property value relative to its current value.

    Key k = KeyFactory.createKey("Employee", "k12345");
    Employee e = pm.getObjectById(Employee.class, k);
    e.counter += 1;
    pm.makePersistent(e);

This requires a transaction because the value may be updated by another user after this code fetches the object, but before it saves the modified object. Without a transaction, the user's request will use the value of counter prior to the other user's update, and the save will overwrite the new value. With a transaction, the application is told about the other user's update. If the entity is updated during the transaction, then the transaction fails with an exception. The application can repeat the transaction to use the new data.

它与那个分片示例所做的非常接近，和你一样，我找不到分片计数器不同的任何原因。

关于没有事务的 Java Google Appengine 分片计数器，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/3782296/