*Between two distributed systems that don't share the same transactionality domain or are not logically monotone.
It's easy to see that moving data from one row to another is doable in a clustered database, and could be interpreted as a message being delivered.
The point is that you can get exactly once delivery, if your whole system is either idempotent, or you can treat the distributed system as one single unit that can be rolled back together (i.e. side effect free wrt. some other system outside the domain).
Both are cases of some form of logical monotonicity, idempotence is easier to see, but transactionality is also based on monotonicity through the used WALs and algorithms like Raft.
The article should really mention CALM (Consistency as logical monotnicity), it's much easier to understand and a more fundamental result than CAP.
https://arxiv.org/pdf/1901.01930
> The point is that you can get exactly once delivery, if your whole system is either idempotent
If you have exactly-once delivery, there's no difference between being idempotent or not. The only effect of idempotence is to ignore extra deliveries.
If idempotence is a requirement, you don't have exactly-once delivery.
Idempotence allows you to build systems that in their internal world model allow for exactly once delivery of non-idempotent messages over an at least once medium.
It's easy to see that moving data from one row to another is doable in a clustered database, and could be interpreted as a message being delivered.
The point is that you can get exactly once delivery, if your whole system is either idempotent, or you can treat the distributed system as one single unit that can be rolled back together (i.e. side effect free wrt. some other system outside the domain).
Both are cases of some form of logical monotonicity, idempotence is easier to see, but transactionality is also based on monotonicity through the used WALs and algorithms like Raft.
The article should really mention CALM (Consistency as logical monotnicity), it's much easier to understand and a more fundamental result than CAP. https://arxiv.org/pdf/1901.01930