DynamoDB key uniqueness across primary and global secondary index - Stack Overflow

DynamoDB key uniqueness across primary and global secondary index - Stack Overflow

DynamoDB indexes have nothing to do with uniqueness. Global and local indexes are allowed to have duplicate hash key & range key pairs. Only the hash key and range key of the table itself are unique.

In your example, it would be possible for two different users to comment on an object at the same exact moment and produce a duplicate objectId, datePosted key. There are a couple ways to deal with this. You could use a PutItem request with a condition that the primary key is null as mentioned in the API reference. That would cause the second comment save to fail and you could report an error to the user or simply try again with an updated time-stamp. Without the condition, the second comment will overwrite the first. Alternatively, you could make the range key of the table a composite value of datePosted concatenated with userId. That way, the range keys will always be unique, but will still be sorted in date time order. This is a common practice with DynamoDB.

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