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Comment by tsegratis | Hacker News Reader

Im no expert but my recent conclusion from writing an ordering system is customers can instruct their banks to roll back many payments a month or even a year after the fact

So actual ACID transactional consistency requires a year. So after basic checks just say you'll give them what they want and accept payment -- since you'll need failure recovery anyway for lack of delivery, refunds, acts of God etc

So accept and plan for failure

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The idea I've seen repeated here a lot is to aim for strong consistency internal to your system, even though the real world/things external to your system will be eventually consistent.

So the inventory <-> create order interactions are internal and can be strongly consistent, while the realities of the banking system are external and will be eventually consistent.

Just because you can't make everything strongly consistent doesn't mean you shouldn't bother at all; it can still eliminate entire classes of errors (in this case, errors around inventory being over-consumed leading to cancelled orders are completely avoided with ACID)

edit: I'm also no expert when it comes to distributed systems. Most of my experience is with transactional systems which I know quite well, and distributed is still a mystery to me. I am very open to new ideas around it and don't have a ton of confidence in my comments here.

A lot of things have improved in distributed databases as of late. There are distributed databases today (my co. provides one) that can have strict serializability (the highest data integrity level) for hyper scale applications. Tolerating "eventual consistency" is largely a choice, oftentimes forced because your DB application or implementation (NoSQL and traditional RDBMS, depending on their implementation) does not provide you strict transaction guarantees. There are third parties (https://jepsen.io/analyses is a great source) that can evaluate and validate such claims. A lot of the anecdotal information in this thread here is no longer accurate or applicable. Yes, there are databases (like Fauna) that don't compromise on the highest transactional levels, and yes, there are more widely used databases (MongoDB, DynamoDB, and many others) that either cannot make those guarantees, or their implementation by their service provider cannot demonstrate that guarantee. Happy to provide more info if interested.

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