What do I really need to be included into sequence diagram?

Question

I am kinda confused about what shall be included into the diagram and what shouldn't. For example, should I include a UI kind of stuff before the system? I notice that the system does not prompt user to select any kind of function or module he/she wants to use but it is very weird if I put the prompting message inside the account management module which is used to log in/signup/grant and revoke access kind of stuff.

Sequence diagram - Delete Book Record

The flow of the diagram is like this, the user will log in to their account if they failed to be authenticated, the system will display an error message and prompt he/her to log in again with correct login details. After that, the system will determine he/she is a librarian or not, if he/she is a librarian the system will display the librarian interface allowing the user to delete the book record, if there is no book record inside the system it will display a message of "No book record was found." and return to the homepage. If he/she is a member, the system will display the member interface, there won't be an option of deleting the book record for them, it has other functions/modules such as transaction and search engine but I do not draw it out cause that is not the main focus of the scenario, the user may choose to log out since they cannot delete the book. When log out failed the system will prompt the user to log out again.

My main focus for this sequence diagram is to show the process of a librarian deleting a book record inside the database, will the login part drawn away the main focus from the database? Do I draw the sequence diagram correctly? Does everything inside the use case need to be included in the sequence diagram or is it the other way around? Is the diagram too complex?

Answer 1

Simply spoken: if your diagram is confusing the reader it's of no help and not of worth. You must not include every bit in a SD (or any other UML diagram). A diagram is a certain view on some part of your model. And it has some context. Without the context it will be misleading, but with knowing it you explain certain details.

A simple way out of your dilemma is to split the diagram into multiple. One for the rough overview and detail diagams where needed. You can leave out obvious parts and simply add a note.