I am creating a system that allows an admin to login, manage items, customers, reservations and many more. But as I have asked my professor, he told me that before the admin can access the managed items, customers etc., the admin should go through the login process in the data flow diagram first. But how can I illustrate that?

Below is an image of what I have tried. I only included the add item function here. I am not entirely sure if it is correct. I am confused by how the item details should change, because as I have read, the process changes the data that is flowing through it. Anyway this is for another question.

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2 Answers 2


You need seperate "admin logged out" and "admin logged in" states. The "admin logged in state" should connect to "add item" and to each of the administrator actions. Logging out should return to the "admin logged out" state.

  • What would be the data that is flowing through the connection from admin logged in state to the add item process?
    – RalpawTato
    Commented Nov 24, 2016 at 15:43
  • This is an interesting idea. However it is against the principle of least surprise in the DFD context where entities are considered different sources or destination and not different states of the same source/destination (e.g. DeMarco/Yourdon defines it as being a person or an organization lying outside the context of a system)
    – Christophe
    Commented Mar 18 at 20:30

A data-flow diagram is a way of representing the flow of data through a process or a system. This diagram only shows how your data is transmitted to the system and it doesn't show any activity so that it requires login for enjoying all the functionality, and your diagram exactly does that. If you want to enhance your diagram, you can separate the authentication into sign-in and sign-out.

See What is Data Flow Diagram? for further reference.

  • For OP: yeah, it's a trap trying to coerce any diagram tool to represent "everything". Use DFD for data-source, function, and data-destination. That's the most essential, then additional complexity can be added to the diagram in turn. But I recommend you keep it as simple as possible, and make several for key non-obvious elements in the system, not for every element. Commented Jan 1, 2021 at 15:32
  • Indeed, this is the correct answer. I have found not a single evidence of login in none of the authoritative DFD authors. If the teacher insists, the right way would be to let the login and the logout processes update a data store of logged in users, and have every other process interacting with the entity check the data store. This would make the diagram very complex without clear any value added.
    – Christophe
    Commented Mar 18 at 20:38

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