Choose a model, controller, observer, or component

Choose the extension point from the caller's task. A model exposes site-aware data and operations. A controller handles an HTTP endpoint. An observer responds to a Zotonic notification. A filter transforms a template value; a scomp renders a template component.

Keep shared domain logic in support functions and call it from these boundaries. Avoid making a controller the only way another Erlang module can use a feature.

Look at an existing implementation with the same return contract before adding a callback. Similar names do not guarantee identical callback signatures.