A developer is writing an application that stores data in an Amazon DynamoDB table by...

The correct condition expression is attribute_not_exists(streamID) AND attribute_not_exists(seqID). This prevents PutItem from overwriting an item that already has the same composite primary key. In DynamoDB, PutItem normally replaces an existing item with the same primary key unless a condition expression blocks the operation. The option must reference the actual partition key and sort key attribute names, not generic words like PARTITION or SORT. The attribute_exists function would do the opposite: it would only allow the operation when those attributes already exist, which does not preserve uniqueness. AWS DynamoDB documentation explains that condition expressions with attribute_not_exists are used to prevent replacing existing items, and for composite keys the partition and sort key checks are evaluated together. ( AWS Documentation )

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