The engine that bills, rewritten while it ran.
A SaaS already billing on a calculation core with no tests and no typing, where every change was checked by hand. We rewrote it without stopping the product, demanding identical output against the previous engine, and left it on Azure with separate environments and rollback deployments.
- Context
- Financial SaaS - live product
- Window
- Phased rewrite, cutover at the end
- Delivery
- Calculation core rewritten with tests, infrastructure as code on Azure, deployment and rollback pipelines, and separate environments.
Where each figure comes from is detailed below.
Where it came from
The product was already on the market and already billing on subscription; what failed was not the idea but the engineering around the calculation. The core had no tests and no typing, every change was verified by hand, and deploying meant logging into the machine. In a financial product that translates directly: touching the engine is scary, and what is scary rarely gets touched.
The approach
The rewrite did not start with the new code but with armoring the old one: we collected real calculation cases and turned them into a suite that demands identical output — the contract for the whole migration. With that net in place we rewrote the engine in a typed language, separating the calculation domain from the delivery layer, with code review on every change, and both systems ran side by side until they matched across every case in the suite. In parallel, the platform: infrastructure as code on Azure, separate environments, and deployment and rollback pipelines.
What reached production
The same product returning the same numbers: that was the requirement, and it is what the suite lets you verify. What changed is what can be done now: touch the engine with a net that catches problems before a user does, and reach production through a pipeline, with rollback, without logging into any machine.
Delivery components
- Comparison suite against the previous engine
- Infrastructure as code, separate environments
- Deployment and rollback pipelines
Anonymized case. We work under NDA: we do not publish the client name, their logo or any detail that would identify them. We discuss the technical detail on a call.
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