We all know the joke about distributed systems right? There are only two hard problems in distributed systems: 2. Exactly-once delivery 1. Guaranteed order of messages 2. Exactly-once delivery — Mathias Verraes (@mathiasverraes) August 14, 2015 (btw you should go follow Mathias if you don’t already) 1 Exactly-once delivery defined First of all: Exactly-once delivery guarantee is the guarantee that a message can be delivered to a recipient once, and only once.