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DynamoDB does not do any reads when executing a write operation. Furthermore, it provides serializable isolation, so when you execute 2 requests concurrently, one of those items will get executed before the other and the second request will always see the most up to date data (the results of the first request).
I suspect that perhaps there is an issue in your logic when executing the requests, as both values should exist when all processes are complete. Ensure you are not overwriting data by calling PutItem etc...
I think you misunderstood. I am not doing two operations. I do only a single operation, which is
UpdateItem. Which, I was thinking does reading by itself since it needs to resolveif_not_existsinupdateExpression.Understood, but my answer remains relevant. DynamoDB does not read, there's no consistency at play. All write operations go to the leader node of a partition, meaning it will only see the most up-to-date values for that item.
if_not_existsis resolved at the underlying storage level, on the SSD disk, in a serializable way. All single operation writes (PutItem, UpdateItem, DeleteItem) provide full ACID guarantees.