I'm not sure why orders/purchases/subscriptions would be seen as mutable. At some point in time, customer X decided to exchange money amount M for a subscription Y. This means the company is obliged to deliver subscription Y, because the customer engaged in a contract with the company, moved amount M to the company, and expecting to (regularly) receive Y. At renewal, the customer, again, needs to pay amount M in order to continue receiving Y.

The only mutable thing here would be the end date of said subscription, at which point the company no longer requires amount M from the customer, and the customer no longer receives Y.

Then on the accounting side, every time subscription Y renews, said customer in account 750xx needs to have its balance lowered by amount M, only to get increased again when they pay.

The only way to bridge this gap is to have the engineers know what accounting needs, and let them build the right infrastructure. In this [2018] [video] https://www.youtube.com/watch?v=KH0l8QqhzYk I recently watched, the speaker Rahul Pilani explains how Netflix organised their billing systems, and how all parts fit together. I'm not saying you should copy their infrastructure, but it doesn't hurt to look at a higher level how the business operates and what their accounting requirements are.

>I'm not sure why orders/purchases/subscriptions would be seen as mutable. At some point in time, customer X decided to exchange money amount M for a subscription Y.

Think for example about orders where a costumer bought three items and later cancelled one, the order value mutates as it is updated and at most we have a copy of the previous order state before price was updated (for some cases not even that).

If you think that’s not a good modeling for financial processes, well so do I, it’s the legacy we’re supposed to manage; moving out from that type of non ideal system to something more solid is what I’m researching.

I suppose that's a flexible way of looking at orders, but it sounds more like a shopping basket. What if the customer ordered three items, you started shipping those, then the customer cancels one item. That sounds like the wrong order (hah) of things.

Your engineers are wrong. You're not going to be able to bridge that gap.

Here's a scenario: you've partnered with a credit card provider. They charge some money each month per card, and you pass that onto your customers who use the cards.

One day the partner sends you a 'card-cancelled' message. Have you built your system to accept that message unconditionally? Or did your engineers put in defensive code ("fail fast", assertions, status checks, db constraints) so that your system can reject that message?

Because that's how we've built our system at work. Our engineers are proud of something called "data integrity" that our almost-ACID (READ_COMMITTED) DB supposedly has. We won't move to events (listening to what actually happened) because we'd be giving up on pretending that our DB guarantees somehow correspond to what's going on in the real world.