From Excel to the web and back again.

Starting point

The best mortgage comparison tool on the Czech market lived in Excel. Dozens of sheets, over 2,500 formulas, over 5,200 constants, and a VBA interface. One expert, not a programmer, updated it week after week because banks keep changing their products. The client wanted a web application but didn't want to lose the ability to keep editing the logic themselves.

What we considered

• Rewrite the formulas in Python. Clean, but the expert would lose Excel.
• Drive Excel remotely via Google Sheets or the Office 365 API. A simple calculation took 0.6 to 1.8 s. Too slow.
• Drive Excel locally from Python. Fragile and hard to scale.

What we did

Excel stayed the source of truth. The open-source library Koala parses formulas into a dependency tree and recalculates only what changed. We added the missing Excel functions, and a recalculation on the server takes 20 ms. 20,000 scenarios generated directly in Excel verified the match with the original.

The prototype took five nights, the MVP a few months. Then reports, SSO, multi-company operation, CRM integration, and checks against bank calculators. A pool of precomputed instances on Docker Swarm handles hundreds of requests per minute from srovnejto.cz.

Result

FINfest 2021 Application of the Year. Six years of development, roughly 7,800 commits. The expert still edits the logic in Excel and the web picks it up.

What we learned

• Prototype and measure the key architectural decision, don't guess it.
• Excel is excellent for parameters and their relationships. No need to replace it at any cost.
• Tests against reference data caught every deviation before a user did.

“The company owner always made sure that the project was running smoothly.”

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