Nobody sets out to build a hundred applications. You get there one reasonable decision at a time. "Yes, we can do that!" "We'll use our CRM for that." A few years later you are sitting on a large on-premise estate, with a CRM doing jobs it was never meant to do and integrations running in every direction that nobody fully understands.

That is where a large Norwegian organization was when we started. This talk is the story of how we turned it into one platform on Azure, while the business kept running as usual. We put abstractions in front of the old core systems so they could be replaced without the rest of the organization noticing, built the new services on .NET and Azure one piece at a time, and shut the old applications down as we went. That last part sounds obvious. It is also the easiest one to skip.

It was not all smooth. A renewal like this is expensive and hard to estimate, and someone has to pay for it. We had to slow down before we could speed up, and we spent quite a bit of time deciding which legacy bugs to fix and which ones to live with until the system was gone. I'll be honest about what worked and what didn't.

You will also see the architecture we ended up with and the stack behind it, from API Management and Service Bus to .NET minimal APIs, a library of Bicep modules and integration tests with Testcontainers.

For architects, tech leads and .NET developers sitting on a legacy estate of their own, or trying not to build one.