With DynamoDB, say a write takes 1ms then for that period another write with the same key can't happen, so if a second write comes along one microsecond after the first, what happens? Are they serialised up? Or does the second bomb out? The specific reason for asking is this - when performing an updateItem with an ADD action to, say, increment a count attribute, if multiple were executed concurrently would all the updates (including the maths) get carried out sequentially, resulting in the correct final count? Or could the maths get carried out in parallel, so for example if the count started out a 9, could two concurrent calls to increment it result in 10 (because both calls did the sum on the same starting value).

Dynamo allows you to write to the same item at the same time, they process in order received. So it will always be incremented properly.