#System sequence diagram if condition how to

Using a break interaction operator, it would look something like this: Let's take a look when the situation when something happens, what causes the ATM to decline de withdrawal. When the guard condition is true, the current interaction run is abandoned and the clause in the break interaction operand runs. The break interaction operator is similar to the break mechanism in other programming languages. This looks nice, for a small diagram, but once the diagram starts to grow and you get a lot of nested alt interaction operations, you might start to loose the overview. Using an alt interaction operator, it would look something like this: The else clause of the alternative combined fragment runs when no other option is selected. If there is no guard, the operand always runs when it is selected.

However, as for any operand, the selected operand in the alternative structure runs only if the guard condition tests true. Only one of the offered alternatives runs on any pass through the interaction. The server confirms the withdrawal is approved and, the service passes this on to the ATM's UI and the ATM dispenses the money.Īn alternative interaction operator represents the logic equivalent of an if-then-else statement. The customer enters the data, the ATM calls a service, which calls the bank's server. I'll describe both methods using a classical example: withdraw money from an ATM when the balance is too low. They both use interaction operators, alt and break.

#System sequence diagram if condition how to

1Īfter searching for quite a while on how to model exception handling in a UML Sequence Diagram I found two ways to represent exception handling in a UML Sequence Diagram. There are several proposed notations for exception handling. Some clumsy approaches to model try-catch blocks are by utilizing combined fragments - alt (alternatives) and breaks, while adding stereotypes for reply messages representing thrown exceptions. UML provides neither notation to model exception handling in sequence diagrams nor any reasoning why it is absent.