[joaomlneto]

The idea of the "just right consistency" is that it brings the best of both worlds, without any drawbacks.

Your application works as well as if it was executed fully in strong consistency, but with improved scalability for the set of operations that can execute in an eventual consistent model.

https://www.youtube.com/watch?v=HJjWqNDh-GA

[erik_seaberg]

The biggest drawback is that only some operations can be supported. E.g., without strong consistency you can detect double-spending from an account but you can't prevent it, because the validity of an operation can't depend on operations a datacenter hasn't seen.

[KirinDave]

Financial examples are bad because in fact the financial world IS eventually consistent. It's quite possible to withdraw the same $100 from an account via multiple ATM machines.

[nine_k]

With ATMs and debit cards, I thinks it's generally not true, they seem to use the online mode and update the balance of a checking account within seconds.

With credit cards, you can indeed start more transactions against the same balance, and you're never sure in which order they will complete.

[KirinDave]

Gonna repeat, it is DEFINITELY possible to spend the same money multiple times with one ATM card, using old ATM machines with other payment methods.

No, I will not further detail how here.

[spooneybarger]

I can verify this is possible.

[nine_k]

Interesting! A bit or research in this area may literally pay :)

[readams]

you can do this but the bank will know and the police will show up.

[erik_seaberg]

When a payment processing system doesn't promptly cancel auth holds, customers definitely complain about being prevented from spending their money. This stuff should be table stakes but some retail banks are just way behind.

[KirinDave]

That is the reverse problem. Flippin' gas stations STILL have a problem with this that you feel acutely because the large holds. A similar problem exists with hotels, where they'll put a lock on a ton of money in your account.

[marknadal]

Great video. I'm impressed you guys built the checker tool, that isn't an easy task - we built something similar called PANIC (https://github.com/gundb/panic-server) that lets us simulate failure cases and then run the test across a distributed system to see if it passes.

I had a question, in the video at a certain point you say that you must modify the code to disallow concurrent debits. This makes sense in theory, but wouldn't it fail in practice? If two machines in different regions are running this code, they would not know that there is a concurrent debit. How si that addressed?