Does DynamoDB have locking by default?

You are correct. DynamoDB does NOT have optimistic locking by default. There are various SDKs for DynamoDB and as far as I am aware the only one which provides optimistic locking functionality is the Java SDK.

Here's what the Java SDK optimistic locking actually supports:

• Creates an attribute in your table called version
• You must load an item from the database before updating it
• When you try and save an item the SDK tests that the client item version number matches the one in the table, and if it does, the save is completed and the version number is incremented

This is pretty simple to implement yourself if you are using a different SDK. You would create the version attribute yourself. You would create a wrapper for the putItem method (and any other required save/update operations). You would use the Condition Expression to test that the version number in the database is one less than the version you saving.

To answer the second part of your question, both updates would succeed (assuming you had put no conditions on your update). The first one would make any updates specified, and the second one would come along and overwrite them.

Dynamodb doesn't support optimistic locking by default. As you mentioned, you need to use the annotation in the Java model class in order to use the optimistic locking.

If two threads write to the same item, the Dynamodb item will have the last write data (i.e. last thread which writes the data).