Business code · never changes
// talks only to the interface
repo.findAll()
repo.save(user)
repo.delete(id)
📚
UserRepository
the only thing
that knows
the backend
→ In-Memory
Storage backend · swappable
Same API, swappable storage
Click a backend chip on the right, then run ＋ Add user. Watch the caller code stay identical while the storage changes.