Draw boundaries you can defend

Good architecture is mostly about where you draw the lines. A boundary in the right place lets teams move independently. A boundary in the wrong place turns every change into a negotiation.

Put boundaries where the rate of change differs. Code that changes together should live together, and code that changes for different reasons should be separated. This keeps a single change from rippling across the system.

Make each boundary own its data. When two modules share a table, they are not really separate, and the coupling will surface at the worst time. A clear owner for each piece of state is worth the extra interface.

Write down the decision and the reason. A short note on why a boundary exists saves the next engineer from undoing it by accident, and gives you something to defend the choice with later.