Update DynamoDB Atomic Counter with Python / Boto

Question

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

Accepted Answer

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}
```

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"}}}
```

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}
```

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

Update DynamoDB Atomic Counter with Python / Boto

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