𝗦𝗮𝗺𝗲 𝗳𝗲𝗮𝘁𝘂𝗿𝗲. 𝗧𝘄𝗼 𝗱𝗶𝗳𝗳𝗲𝗿𝗲𝗻𝘁 𝘀𝘆𝘀𝘁𝗲𝗺𝘀.

The case: "Show a user's transaction history."

Team 1 builds a banking app.

Team 2 builds a social media app.

Same screens. Same API. Same feature.

Completely different architecture.

That's because of two things every system needs.

𝗙𝘂𝗻𝗰𝘁𝗶𝗼𝗻𝗮𝗹 𝗿𝗲𝗾𝘂𝗶𝗿𝗲𝗺𝗲𝗻𝘁𝘀 tell you what the system should do.

Show the history. Display the numbers. Load the screen.

𝗡𝗼𝗻 𝗳𝘂𝗻𝗰𝘁𝗶𝗼𝗻𝗮𝗹 𝗿𝗲𝗾𝘂𝗶𝗿𝗲𝗺𝗲𝗻𝘁𝘀 tell you how it should behave.

How fast. How accurate. How available.

For the bank, a stale balance is unacceptable. It needs strong consistency.

For social media, a like count off by one is fine. It needs speed, not perfect accuracy.

𝗦𝗮𝗺𝗲 𝗳𝘂𝗻𝗰𝘁𝗶𝗼𝗻. 𝗗𝗶𝗳𝗳𝗲𝗿𝗲𝗻𝘁 𝗯𝗲𝗵𝗮𝘃𝗶𝗼𝗿. 𝗗𝗶𝗳𝗳𝗲𝗿𝗲𝗻𝘁 𝗱𝗲𝘀𝗶𝗴𝗻.

Most engineers focus on functional requirements first.

But it's the non functional ones that shape the real architecture.

𝘉𝘶𝘪𝘭𝘥 𝘧𝘰𝘳 𝘸𝘩𝘢𝘵 𝘪𝘵 𝘥𝘰𝘦𝘴.

𝘋𝘦𝘴𝘪𝘨𝘯 𝘧𝘰𝘳 𝘩𝘰𝘸 𝘪𝘵 𝘮𝘶𝘴𝘵 𝘣𝘦𝘩𝘢𝘷𝘦.