A controlled input path
One defined way for work to arrive: a form submission, an inbox rule, a file drop, a CRM event, an export, a ticket, or a button somebody presses on purpose. Version one should have exactly one. Every extra entrance doubles the error handling and halves the chance the sprint finishes. The input path is also where the data boundary is enforced, which means it is the part that decides what the system is even capable of seeing.