Question

A developer is building a user-registration microservice that stores user profiles in a DynamoDB table with 'userId' as the partition key. During registration, the service must create a new item only if a profile with the same userId does not already exist; if one exists, the operation must fail without overwriting the existing data. Which approach implements this requirement most efficiently?

Accepted Answer

Call PutItem with a ConditionExpression of 'attribute_not_exists(userId)' so the write fails if the item already exists.

Answer

Call GetItem first to check whether the userId exists, then call PutItem only if the GetItem response is empty.

Answer

Call UpdateItem with the SET action, which automatically refuses to modify items that already have all attributes populated.

Answer

Enable DynamoDB Streams on the table and use a Lambda function to reject duplicate userId values after the write completes.