Can we achieve strong write consistency in DynamoDB?

While all writes to DynamoDB are strongly consistent, I believe what you want is the item as it was directly after the update. One such solution is what you mention, a strongly consistent read directly after the update, which will cost you an additional read request.

I believe the best way to do this is to use the ReturnValues parameter of the UpdateItem API:

Use ReturnValues if you want to get the item attributes as they appear before or after they are successfully updated. For UpdateItem, the valid values are:

• NONE - If ReturnValues is not specified, or if its value is NONE, then nothing is returned. (This setting is the default for ReturnValues.)
• ALL_OLD - Returns all of the attributes of the item, as they appeared before the UpdateItem operation.
• UPDATED_OLD - Returns only the updated attributes, as they appeared before the UpdateItem operation.
• ALL_NEW - Returns all of the attributes of the item, as they appear after the UpdateItem operation.
• UPDATED_NEW - Returns only the updated attributes, as they appear after the UpdateItem operation.

There is no additional cost associated with requesting a return value aside from the small network and processing overhead of receiving a larger response. No read capacity units are consumed.