Define a component boundary
Give one invented runtime component one clear responsibility. Short card.
A component boundary is a one-responsibility contract: what this invented unit owns, what it refuses, and what another unit must own instead. Keep the card short - then deepen ownership with object pooling when you need reuse.
Key points
Name one job
Write the single decision or lifecycle this component is allowed to own.
List refusals
Name two things that must live elsewhere so this unit stays small.
Check the handoff
If another system needs data, define the explicit input instead of reaching inward.