Sandbox

Let's say a component's frontend and admin need to access the same data. You create an abstraction function for it and you want to include it in both the frontend and admin. How should I do this? Define the function in frontend and include it in admin or define it in a totally separate file available to both?

EDIT: another question. Is there a way to include something in the admin menu without making an install file?

Another problem, I made an install file after all but after a successful install I still can't add my component to the menu.