How to separate the responsibility in application?

For example, I want to separate the online shop into three parts. User: the user related information, for example, they login. logout. Communication: sending email, or newsletter modules. ShoppingCart: displaying order.

Ok, these three module is the main function of of my little online store. Clearly, the user medule, deal with its own stuff, for example, user change their profile pic(ok, I think it is non-sense to have a profile pic for user, just think it is an example.)

user ---call---> user

But I got a problem here, is when the user doing some functions, which require cross-module call.... Let me give a example, if the user lost the password, the user needs to use the communication method to send a new password to him/her...The situation will become somethings like that:

user ----call---> communication 

A worse situation is use all the m开发者_运维知识库odules, here is the situation: The user using a shopping chart to deal with his/her shopping, after that, he /she make the order, and a invoice use communication modules to send to the user.

user ----call---> shoppingCart ---call---> Communication 

Therefore, each module is not separate, all modules knows each others.... But I don't want to do that, for example, this time I am doing a new application, for example, I doing a video sharing web site which only use "user" and "communication", I don't really need the "shoppingChart. ", and having a new video module.....

It is ok for me "upgrade" my user and communication method to deal with the video module, but the question is, if I got something bugs fix, for example, the getFullName method is doing something wrong, when I need to "upgrade" back the online shop application, I need to take the "video" module too.....

What I wanna to ask is, how to separate their responsibility, and how to make the code more reusable? Thank you.

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It is good practice to minimize the coupling in your application, but removing it entirely is not always possible.

My recommendation would be to build base classes User, Communication, and ShoppingCart that provide only basic interfaces, such as getFullName()

Then, for each application, write separate wrappers that are able to interact with your base classes. You may have an OnlineShopping class and a VideoSharing class, that contain the functions you need that are specific for each application.

There are a number of structural patterns that may help you out with your design. Also, take advantage of inheritance for functionality that is similar across all applications.