Comment by mrkeen
1 year ago
I'm not sold on double entry here.
If a new transaction enters the system now, I could follow the advice and record it as two sources of truth. Or I could just record it once.
If I could turn a single transaction into two ledger entries today, I could do the same later if I needed to.
the systems I’ve worked on that use your design end up in a state I call “the zoo” - because you never actually have simple transactions like you’re imagining. You end up with one object for credit card payments with stripe fees, another one for purchases with taxes, another one for ACH payouts, all meaning different things
double entry is actually the simplest projection of any possible billing system because anything that moves money can be described as a bundle of credits and debits that happen simultaneously and sum to zero. so you get accounts for taxes collected and fees charged automatically and don’t have to write custom logic for each thing
You are thinking it's duplicating work, but it isn't. You are simply recording the whole transaction in a double entry system. i.e. you know where the money came from and where it went. That's all a double entry system does, records both sides of the transaction.
This means when something goes wrong, you have a chance to figure out what in the world happened.
That's it. If you want to think of it as a state machine, you are storing all the state that happened with the resource you are tracking(usually money). This way when some part of the resource is missing, you can go figure out what happened.
Think about it another way: always record enough of the transaction, that if you were required to sit in a courtroom in front of a jury and explain what happened, you could do so. If you can't do that, you didn't store enough information.
There's no redundancy. Imagine you have 2 buckets, you reach into bucket A and grab some cash and put it in bucket B. $x dollars leaving bucket A is entry part 1, $x dollars entering bucket B is entry part 2. That's all it is. I honestly don't understand what single entry is by comparison - it sounds like losing an essential attribute of the transaction.
"Two sources of truth" is an anti pattern in computing and isn't redundancy. Use replication or backups for that.
You can't imagine simpler than what you described because it's single entry.
Double-entry is twice as complicated, makes sense only to accountants and not to computing people. Your example of 1 transaction would be doubly-kept as some nonsense like
no. single entry is when you track BucketA’s balance without regard to other buckets at all
Single would be recording the two accounts, one being the source account and the other the destination, and a quantity.
single entry is like a receipt at a store, is says what you owed and what you paid, but there’s no “other side” that is getting credited or debited the same amount
Keeping entries separate makes it easier to deal with data for specific accounts, the transaction will contain both of them.