DynamoDB Expected condition - amazon-dynamodb

Question

What will happen if I create a PutItem request like this:
{
"Expected":
{
"testAttribute" :
{
"Exists": "false",
}
},
"Item":
{
"testAttribute" :
{
"S": "testValue"
}
},
"TableName": "TableName"
}
where "testAttribute" is not part of the primary key.
Will DynamoDB scan the table to see if there is an item with attribute "testAttribute" == "testValue" ?
If not, how will DynamoDB determine the presence of a "testAttribute" == "testValue" ?
I can't find anything in the docs describing how this works.

Answer 1

According to the documentation of the PutItem action, you are not allowed to issue that request. It says:
Item: A map of attribute name/value pairs, one for each attribute. Only the primary key attributes are required; you can optionally provide other attribute name-value pairs for the item.
(emphasis mine)
You must provide a value for each attribute of the primary key whenever you use PutItem.
This way, as you will surely agree, it is very simple and fast for DynamoDB to check the condition you defined on the Expected clause: no scan is needed, it just has to look at the single item that could match the request. Otherwise, as you noted, DynamoDB would need to perform a full table scan (and it would possibly be very slow, and they would certainly charge you for that) or it would need to maintain a consistent index of every single item in a table, and they would charge you for the SSD space used to store it!
Also, note that the meaning of the expected clause is a little bit different than what you described in the question. Supposing you fix your request and add all the primary key attributes, the request would mean:
"If the item identified by this primary key does not exist, create it; if it does exist and does not contain an attribute named testAttribute, replace the item with the one whose attributes are described in this request; if the item does exist and does contain an attribute named testAttribute, do nothing".
Your description says that DynamoDB would check if the value of testAttribute is testValue, but it is not what happens when you use the Expected/Exists clause. To achieve the effect you described, you need to use the Expected/Value clause, and then you specify the value you are expecting in that clause -- the value specified for the attribute in the Item property of the request is just used to define the new value of the attribute, if an update (or insert) is to occur.