Comment by kukkeliskuu
3 hours ago
I have been working on payment systems and it seems that in almost all discussions about transactions, people talk about toy versions of bank transactions that have very little to do with what actually happens.
You don't even need to talk about credit cards to have multiple kinds of accounts (internal bank accounts for payment settlement etc.), multiple involved systems, batch processes, reconciliation etc. Having a single atomic database transaction is not realistic at all.
On the other hand, the toy transaction example might be useful for people to understand basic concepts of transactions.
I don't have a lot of payment experience, but AFAIK actual payment systems work in an append-only fashion, which makes concurrency management easier since you're just adding a new row with (timestamp, from, to, value, currency, status) or something similar. However, how can you efficiently check for overdrafts in this model? You'd have to periodically sum up transactions to find the sender's balance and compare it to a known threshold.
Is this how things are usually done in your business domain?
And then they take that toy transaction model and think that they're on ACID when they're not.
Are you stepping out of SQL to write application logic? You probably broke ACID. Begin a transaction, read a value (n), do a calculation (n+1), write it back and commit: The DB cannot see that you did (+1). All it knows is that you're trying to write a 6. If someone else wrote a 6 or a 7 in the meantime, then your transaction may have 'meant' (+0) or (-1).
Same problem when running on reduced isolation level (which you probably are). If you do two reads in your 'transaction', the first read can be at state 1, and the second read can be at state 2.
I think more conversations about the single "fully consistent" db approach should start with it not being fit-for-purpose - even without considering that it can't address soft-modification (which you should recognise as a need immediately whenever someone brings up soft-delete) or two-generals (i.e. consistency with a partner - you and VISA don't live in the same MySql instance, do you? Or to put it in moron words - partitions between your DB and VISA's DB "don't happen often" (they happen always!))
RE: "All it knows is that you're trying to write a 6. If someone else wrote a 6 or a 7 in the meantime, then your transaction may have 'meant' (+0) or (-1)."
This is not how it works at all. This is called dirty writes and is by default prevented by ACID compliant databases, no matter the isolation level. The second transaction commit will be rejected by the transaction manager.
Even if you start a transaction from your application, it does not change this still.
The point is to give people who don't realise that they have been dealing with eventual consistency all along, that it's right there, in their lives, and they already understand it.
You're right I go into too much detail (maybe I got carried away with the HN audience :-) and you are right that multiple accounts is something else that people generally already understand and demonstrate further eventual consistency principles.
I wasn't criticizing you, just making the point that when people talk about toy example bank transactions, they usually want to just introduce the basic understanding. And I think it ok, but I would prefer that they would also mention that REALLY the operations are complex.
I modified my comment above that by multiple types of accounts I meant that banks have various accounts for settlements with the other banks etc. even in the common payment case.