Fintech Engineering Handbook

It seems like a clever idea (fast integer math, no rounding problems for addition and subtraction), but it'll bite you incredibly hard if you ever stumble upon an edge case such as working with a partner that has a different implied number of digits for a given currency. This is especially relevant for stablecoins, which often have a different number of implied decimal digits than the "fiat" currency they represent.

Also, consider representing amounts as a string type in JSON-based APIs. JSON does not specify decimal precision, so you (and all your users/vendors) will always have to make sure your parser/serializer doesn't internally lose precision by going via floating point. This can get ugly fast, and while a string seems conceptually less neat, it completely bypasses that problem. (Some will call this an anti-pattern [1], but I'd rather not fight this particular battle for ideological purity on the shoulders of my users or shareholders.)

[1] https://blog.json-everything.net/posts/numbers-are-numbers-n...

E.g. If I ever see a monetary value stored in something else than integers I'm going to run away screaming (thank you Rust decimals represented as JSON floats). It's always integers unless you have a VERY good reason to do otherwise (though exported view can be in anything, even in weird bitcoded formats).

FX exchange. Resolution of FX isn't a point-in-time thing, things like buyer rate-in-time, seller rate-in-time, agreement, agreement tolerance, agreed upon resolution timestamp come in the effect.

Immutability - that's why you want to have event sourcing everywhere that touches money:

    # Resolved stream
    A -> B -> E

    # Actual stream
    A0 -> Edit(A0, A) -> B -> C -> D -> Rollback(B) -> E

For example the parts talking of retries, idempotency, event ordering, etc. This applies to all systems that require any degree of accuracy, even if no money is directly involved. I've seen so many systems built on the assumption that "we can always retry", but you can only retry if you fail cleanly in the first place, and if the downstream system offers the same level of idempotency that you think it does. Quite often these are not put to the test.

I would recommend anyone starting in fintech to take some time to understand accounting principles and the ledger in a bit more depth than just debits vs credits - this is likely what is most unfamiliar to programmers.

Also financial software is very data-heavy and I learned more about databases in my time working in fintech than the 15 years before that. I think going into a bit more detail about even the basics (indexes) will save a lot of headaches.

I see webhooks documented all the time, but I have yet to use them in practice, nor have my customers requested them. Is the above not true, or are they widely used in some sectors and not others?