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Creating my own Point of Sale system, Part 0: Start

Obrázek
I had several topics I wanted to write about (both from my dabblings in machine learning and travel stories), but a more urgent thing came up. With a change in Czech legislature, all shopkeepers are now required to authorize all transactions at the Ministry of Finances' servers. They call it EET - Elektronicka Evidence Trzeb. (electronic records of sales) Sadly, POS just reminds me of part-of-speech tags. To deal with the ambiguity, I'll just refer to it as EET system, even though it contains more.

So what exactly is EET?

From march on, every time we make a sale, we have to contact their servers and send them the amount with our hashed ID we got from them by registering. Afterwards, they send an unique receipt ID back, which we have to print and give to the customer. The customer can then at any point check the receipt on their website. The specifications are all published and the state doesn't require any license to use it.
It's rather annoying and requires certain in…