[perakojotgenije]

integer/year works almost flawlessly as invoice ID. It is unique and readable.

If the company has more than one unit that creates invoices than just add unit ID, so it becomes invoice_id/unit_id/year. Still unique and very much readable.

[yencabulator]

To avoid the German Tank Problem and leaking your business vitals to competition, I'd recommend making invoices be scoped to the client, e.g. <client_id>-<number>. And then client_id is either random or for small business e.g. unique slug derived from client name, for example you could get PERAK-0001.

https://en.wikipedia.org/wiki/German_tank_problem#Historical...