How to read a system you didn’t build
The method I use in the first week of every engagement, when the documentation is stale and the people who wrote it have left.
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Follow the money path first
In commerce that means one order, end to end, through every service it touches. It gives you the real dependency graph rather than the one on the architecture slide.
Read the incidents, not the wiki
A year of incident tickets tells you where the system is actually fragile, which teams own what in practice, and which fixes were never finished.
Draw it badly, then ask
A deliberately rough diagram in front of the team gets corrected fast. A polished one gets nodded at. I want the corrections.
The system is whatever survived contact with production — not what was designed.