Right on, great question. Some examples:
Example Option 1
You give up on the guarantees across partition keys (bank accounts), and you accept that balances will not reflect a causally consistent state of the past.
E.g., Bob deposits 100, Bob sends 50 to Alice.
Balances:
Bob 0 Alice 50 # the source system was never in this state
Bob 100 Alice 50 # the source system was never in this state
Bob 50 Alice 50 # eventually consistent final state
Example Option 2
You give up on parallelism, and consume in total order (i.e., one single partition / unit of parallelism - e.g., in Kafka set a partitioner that always hashes to the same value).
Example Option 3
In the consumer you "wait" whenever you get a message that violates causal order.
E.g.,
Bob deposits 100
Bob sends 50 to Alice (Bob-MoneyOut 50 -> Alice-MoneyIn 50).
If we attempt to consume Alice-MoneyIn before Bob-MoneyOut, we exponentially back off from the partition containing Alice-MoneyIn.
(Option 3 is terrible because of O(n^2) processing times in the worst case and the possibility for deadlocks (two partitions are waiting for one another))
Example Option 1
You give up on the guarantees across partition keys (bank accounts), and you accept that balances will not reflect a causally consistent state of the past.
E.g., Bob deposits 100, Bob sends 50 to Alice.
Balances: Bob 0 Alice 50 # the source system was never in this state Bob 100 Alice 50 # the source system was never in this state Bob 50 Alice 50 # eventually consistent final state
Example Option 2
You give up on parallelism, and consume in total order (i.e., one single partition / unit of parallelism - e.g., in Kafka set a partitioner that always hashes to the same value).
Example Option 3
In the consumer you "wait" whenever you get a message that violates causal order.
E.g., Bob deposits 100 Bob sends 50 to Alice (Bob-MoneyOut 50 -> Alice-MoneyIn 50).
If we attempt to consume Alice-MoneyIn before Bob-MoneyOut, we exponentially back off from the partition containing Alice-MoneyIn.
(Option 3 is terrible because of O(n^2) processing times in the worst case and the possibility for deadlocks (two partitions are waiting for one another))