I walked away from a job interview a few years ago on this point.
One of the technical questions was "if you have a db and a message queue, how do you get your update to alter both or neither (i.e. transactionally)"?
I thought about it for a couple of minutes, then came back with something like "I can't, and you can't either." Then I proposed the usual spiel about using a replicated-state-machine/write-ahead-log/event-sourcing (whatever it might be called at the time) and leaning into eventual consistency as the only practical solution.
He asked if I'd heard about the outbox pattern, so I let him describe it. Sure enough it sounded like this article. The secret to transacting across the database D and the message queue Q:
(D,Q)
is to split D into two parts (the State and the Outbox), transact across those instead
(S,O) Q
and then just pretend that you have a transaction across D and Q.
One of the technical questions was "if you have a db and a message queue, how do you get your update to alter both or neither (i.e. transactionally)"?
I thought about it for a couple of minutes, then came back with something like "I can't, and you can't either." Then I proposed the usual spiel about using a replicated-state-machine/write-ahead-log/event-sourcing (whatever it might be called at the time) and leaning into eventual consistency as the only practical solution.
He asked if I'd heard about the outbox pattern, so I let him describe it. Sure enough it sounded like this article. The secret to transacting across the database D and the message queue Q:
is to split D into two parts (the State and the Outbox), transact across those instead and then just pretend that you have a transaction across D and Q.