Component Isolation For Failure Tolerance

Particularly when components are expected to change frequently in the future, or when they are overly complex and likely (relative to the rest of the app) to fail, isolate them.

Isolation is deeper than simple separation of modules or files, it definitely should be separated that way too, but the rest of your app should be ensured that failure to perform in that separated module does not cause failures elsewhere.

This is to engineer against future problems in your system when you know certain pieces are more prone to failure than others.