[tylerhou]

> "This feature would take 1 day if we had database layer, but because of technical debt we own, it will take 4 days instead". In this case "3 days" is the "interest payment"

Nit: Not the entire 3 days is interest in this analogy because you are also paying down principal.

For example: "Implementing the database layer properly would have taken 5 days; because we took shortcuts (for business reasons) it only took us 3 days. Now we have to implement feature X, and because of our previous shortcut feature X will take 4 days, so 3 + 4 days in total. But if we had implemented the database layer properly in the first place the new feature would have only taken 1 day, so 5 + 1 days in total."

Then in the three-day payment, two are towards principal (that's how much you "borrowed" previously), and one is towards interest.

[codesections]

> Nit: Not the entire 3 days is interest in this analogy because you are also paying down principal.

I didn't read the GP as saying that they were paying down principle. They don't say what the hypothetical feature is, but my assumption was that it's a feature that they could implement without a database layer but that would be faster if they had a database layer. That is, I assumed that even after the 4 days of development for the new feature, they still won't have a db layer – which is why the extra three days is pure "interest".