使用 Python/Boto 更新 DynamoDB 原子计数器

问题描述

我正在尝试使用 Python Boto 2.3.0 更新原子计数计数器，但找不到该操作的文档.

I am trying to update an atomic count counter with Python Boto 2.3.0, but can find no documentation for the operation.

似乎没有直接的接口，所以我尝试使用layer1接口进行原始"更新，但即使是简单的更新也无法完成.

It seems there is no direct interface, so I tried to go to "raw" updates using the layer1 interface, but I was unable to complete even a simple update.

我尝试了以下变体，但都没有运气

I tried the following variations but all with no luck

dynoConn.update_item(INFLUENCER_DATA_TABLE, 
                     {'HashKeyElement': "9f08b4f5-d25a-4950-a948-0381c34aed1c"}, 
                     {'new': {'Value': {'N':"1"}, 'Action': "ADD"}})    

dynoConn.update_item('influencer_data', 
                     {'HashKeyElement': "9f08b4f5-d25a-4950-a948-0381c34aed1c"}, 
                     {'new': {'S' :'hello'}})                                 

dynoConn.update_item("influencer_data", 
                     {"HashKeyElement": "9f08b4f5-d25a-4950-a948-0381c34aed1c"},
                     {"AttributesToPut" : {"new": {"S" :"hello"}}})      

它们都产生相同的错误:

They all produce the same error:

  File "/usr/local/lib/python2.6/dist-packages/boto-2.3.0-py2.6.egg/boto/dynamodb/layer1.py", line 164, in _retry_handler
    data)
boto.exception.DynamoDBResponseError: DynamoDBResponseError: 400 Bad Request
{u'Message': u'Expected null', u'__type': u'com.amazon.coral.service#SerializationException'}

我还研究了 API 文档 here，但它们非常简陋.

I also investigated the API docs here but they were pretty spartan.

我已经做了很多搜索和摆弄，我唯一剩下的就是使用 PHP API 并深入研究代码以找到它格式化"JSON 主体的位置，但这有点痛苦.请救我脱离痛苦！

I have done a lot of searching and fiddling, and the only thing I have left is to use the PHP API and dive into the code to find where it "formats" the JSON body, but that is a bit of a pain. Please save me from that pain!

推荐答案

抱歉，我误解了您要查找的内容.尽管有一个小错误需要解决，但您可以通过 layer2 完成此操作.这是一些 Layer2 代码:

Sorry, I misunderstood what you were looking for. You can accomplish this via layer2 although there is a small bug that needs to be addressed. Here's some Layer2 code:

>>> import boto
>>> c = boto.connect_dynamodb()
>>> t = c.get_table('counter')
>>> item = t.get_item('counter')
>>> item
{u'id': 'counter', u'n': 1}
>>> item.add_attribute('n', 20)
>>> item.save()
{u'ConsumedCapacityUnits': 1.0}
>>> item  # Here's the bug, local Item is not updated
{u'id': 'counter', u'n': 1}
>>> item = t.get_item('counter')  # Refetch item just to verify change occurred
>>> item
{u'id': 'counter', u'n': 21}

这会产生与您在第 1 层代码中执行的相同的在线请求，如以下调试输出所示.

This results in the same over-the-wire request as you are performing in your Layer1 code, as shown by the following debug output.

2012-04-27 04:17:59,170 foo [DEBUG]:StringToSign:
POST
/

host:dynamodb.us-east-1.amazonaws.com
x-amz-date:Fri, 27 Apr 2012 11:17:59 GMT
x-amz-security-    token:<removed> ==
x-amz-target:DynamoDB_20111205.UpdateItem

{"AttributeUpdates": {"n": {"Action": "ADD", "Value": {"N": "20"}}}, "TableName": "counter", "Key": {"HashKeyElement": {"S": "counter"}}}

如果你想避免最初的 GetItem 调用，你可以这样做:

If you want to avoid the initial GetItem call, you could do this instead:

>>> import boto
>>> c = boto.connect_dynamodb()
>>> t = c.get_table('counter')
>>> item = t.new_item('counter')
>>> item.add_attribute('n', 20)
>>> item.save()
{u'ConsumedCapacityUnits': 1.0}

如果项目已存在则更新该项目，如果项目尚不存在则创建它.

Which will update the item if it already exists or create it if it doesn't yet exist.

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