I want to add extra fields when a node is displayed, the data of this fields are fetched from an external db.
I'm uncertain if I've to use the
hook_node_view_alter function or
I don't understand if the extra fields to add are in the node form or into the page(display of node). If it's the first thing I think you can use hook_form_alter to add the extra fields and in the second case you can use hook_preprocess_node to add the variables to 'send' to the front-end of site.