We saw that TCP suppresses duplicate packets at the TCP connection level, and some stream processors provide so-called exactly-once semantics at the message processing level, but that is not enough to prevent a user from submitting a duplicate request if the first one times out. By themselves, TCP, database transactions, and stream processors cannot entirely rule out these duplicates. Solving the problem requires an end-to-end solution: a transaction identifier that is passed all the way from the end-user client to the database.
