title: DynamoDB concurrent write
body: <p>I have an existing DynamoDB table which has attributes say </p>
  
  <pre>
  ---------------------------------------------------------
  hk(hash-key)| rk(range-key)|   a1    |    a2   |    a3   | 
  ---------------------------------------------------------
  </pre>
  
  <p><br/>I have an existing DynamoDb client which will only update existing record for a1 only. I want to create a second writer(DDB client) which will also update an existing record, but, for a2 and a3 only. <br/>If both the ddb client tries to update same record (1 for a1 and other for a2 and a3) at the exact same time, will DynamoDb guarantee that all a1 a2 a3 are updated with correct value(all three new values)? Is using save behavior UPDATE_SKIP_NULL_ATTRIBUTES sufficient for this purpose or do I need to implement some kind of optimistic locking? If not,
  Is there something that DDB provides on the fly for this purpose?</p>