Activity Diagram is an important behavioral diagram in the UML diagram to describe the dynamic aspects of the system. Activity Diagrams are often used in business process modeling for instance, to illustrate how ATM withdrawal actually works. Activity Diagrams can also describe the steps in a use case diagram as they are modeled sequential and concurrent. The following Activity Diagram shows how a Customer starts a session at the ATM counter for the first time. In order to withdraw cash, the customer has to request the PIN, which will then be authorized by the registered Bank. Once the bank authorizes that the PIN matches the respective bank account of the customer, it allows the customer to withdraw funds from the account. If the requested amount matches the sum-balance, then the bank authorizes it internally and processes the withdrawal request. The ATM Withdrawal Activity Diagram represents the Customer, ATM, and Bank, and is pretty helpful to understand how the withdrawal request at any ATM counter works.
