Moving to Azure, without downtime

Map what you have

Before you move anything, you need to know what is there: applications, servers, databases and, above all, what depends on what. Which application calls which database, which jobs run overnight, which partner connects from which IP address.

Talk to the people who use the systems, not just the people who run them. They know when things are quiet, and which old tool one team still relies on.

Build the landing zone first

The landing zone is the foundation every application will sit on: network, identity, security policies, naming, tagging and budgets. Building it first means each application you migrate lands in a clean, well-governed environment, not in a temporary setup that ends up being permanent.

Replicate and test

Rather than moving everything at once, you build the new environment alongside the old one and replicate the data continuously. You then test the applications on the Azure side with real scenarios, while the old system carries on running.

This is where the surprises turn up: a hard-coded IP address, a certificate, a timeout nobody had noticed. Far better to find them now than on the night of the switchover.

Switch over in one go

The switchover itself should be short and rehearsed: final data sync, DNS change, checks, done. Write the steps down, do a dry run, and agree in advance on what "working" means and who makes the call.

Let users and partners know. An hour of planned maintenance is rarely a problem. An outage nobody saw coming always is.

Keep a way back

Do not switch off the old environment the next morning. Keep it ready until the new one has been through a full cycle: a month-end close, a busy day, the overnight jobs. If something goes wrong, going back should be a simple decision, not a new project.