Story · Test tooling
Mapping 40,000 dependencies inside a mainframe
How do you safely change a decades-old mainframe when no one can see how its 40,000 pieces connect? You build something that can.
The task was to secure a large IBM mainframe while it took on a new data delivery from a Swedish authority — a project full of mainframe changes plus a new external application, where a small change in one place could ripple somewhere unexpected.
The established way to understand those ripples was manual source-code inspection. With more than 40,000 internal dependencies, that does not scale. On my own initiative I built a tool that maps every dependency in the system and outputs it as a searchable, visual graph you can actually explore.
Alongside it, I mapped out every possible combination of system input and generated all of it together with the expected output, plus tooling to compare expected against actual results — a debug aid the whole team could use, not just me.
We got most of the way to perhaps the first-ever fully automated test pipeline that included the mainframe. Testing finished on time, business acceptance passed almost without a hitch, and the system went to production as scheduled.
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