Update DynamoDB Atomic Counter with Python / Boto

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.

For those looking for the answer I have found it. First IMPORTANT NOTE, I am currently unaware of what is going on BUT for the moment, to get a layer1 instance I have had to do the following:

import boto
AWS_ACCESS_KEY=XXXXX
AWS_SECRET_KEY=YYYYY
dynoConn = boto.connect_dynamodb(AWS_ACCESS_KEY, AWS_SECRET_KEY)
dynoConnLayer1 = boto.dynamodb.layer1.Layer1(AWS_ACCESS_KEY, AWS_SECRET_KEY) 

Essentially instantiating a layer2 FIRST and THEN a layer 1. Maybe Im doing something stupid but at this point Im just happy to have it working.... I'll sort the details later. THEN...to actually do the atomic update call:

dynoConnLayer1.update_item("influencer_data", 
                    {"HashKeyElement":{"S":"9f08b4f5-d25a-4950-a948-0381c34aed1c"}},
                    {"direct_influence":
                        {"Action":"ADD","Value":{"N":"20"}}
                    }
                );

Note in the example above Dynamo will ADD 20 to what ever the current value is and this operation will be atomic meaning other operations happening at the "same time" will be correctly "scheduled" to happen after the new value has been established as +20 OR before this operation is executed. Either way the desired effect will be accomplished.

Be certain to do this on the instance of the layer1 connection as the layer2 will throw errors given it expects a different set of parameter types.

Thats all there is to it!!!! Just so folks know, I figured this out using the PHP SDK. Takes a very short time to install and set up AND THEN when you do a call, the debug data will actually show you the format of the HTTP request body so you will be able to copy/model your layer1 parameters after the example. Here is the code I used to do the atomic update in PHP:

<?php 
    // Instantiate the class
    $dynamodb = new AmazonDynamoDB();

    $update_response = $dynamodb->update_item(array(
        'TableName' => 'influencer_data',
            'Key' => array(
                'HashKeyElement' => array(
                    AmazonDynamoDB::TYPE_STRING=> '9f08b4f5-d25a-4950-a948-0381c34aed1c'
                )
            ),
            'AttributeUpdates' => array(
                'direct_influence' => array(
                    'Action' => AmazonDynamoDB::ACTION_ADD,
                    'Value' => array(
                        AmazonDynamoDB::TYPE_NUMBER => '20'
                    )
                )
            )
    ));

    // status code 200 indicates success
    print_r($update_response);

?>

Hopefully this will help other up until the Boto layer2 interface catches up...or someone simply figures out how to do it in level2 :-)