YASSIR Senior Software Engineer interview questions

There's a highly loaded web site for selling books. It has ability to search for a book by name and/or description. This app is a standalone service with PostgreSQL database in use. You need to extend functionality of the application and add ability to save books into user's wishlist and later that user should receive email notification when books from a wishlist available for purchasing. User should receive the email no more than once a day and all new books should be presented in a single email message. New books can be published in existed part of application, you don't need to implement it. Please, describe and visualise high level architecture of how you extend the system. Take into account that new changes shouldn't affect existed application performance.