NO SUCH THING AS EXACTLY-ONCE DELIVERY

Summary from the TLDR Web Dev newsletter

The difference between delivery and processing in messaging systems is that delivery refers to the act of successfully transferring a message to a receiver, while processing covers the entire message lifecycle, including delivery, processing by the receiver, and acknowledgement. Sequin is a Postgres streaming library that prioritizes exactly-once processing by making sure that every message is delivered, processed, and acknowledged without duplication. Achieving exactly-once delivery is practically impossible due to the two-phase commit problem - confirming the simultaneous confirmation of message delivery across different locations.