Objects as well as classes can be targets on a sequence diagram, which means that messages can be sent to them. To relate the comment to whatever diagram elements it is about, connect them with dashed lines. Note that this highlight is not part of the diagram itself.Īs with all UML diagrams, comments are shown in a rectangle with a folded-over corner : To clarify how execution switches from one object to another, a blue highlight was added to represent the flow of control. The diagram above shows how objects interact in the "rent item" collaboration when the item is not available during the requested period. The UML sequence diagram gallery contains many examples, but here's a typical sequence diagram based on a system use case scenario : For each key collaboration, diagrams are created that show how objects interact in various representative scenarios for that collaboration. Lower equals Later).Ī popular use for them is to document the dynamics in an object-oriented system. An important characteristic of a sequence diagram is that time passes from top to bottom : the interaction starts near the top of the diagram and ends at the bottom (i.e. But for "simple" exception handling, I think a break is a better way to represent it, then alt.UML sequence diagrams are used to show how objects interact in a given situation. ConclusionĪs always, the best method depends on the scenario. This is ofcourse a very simple example, and you can have a lot more interaction in the break interaction operator, like logging or transaction handling. I think it even makes things better, because you can see where a sequence stops when an exception occurs. What I like about this way of displaying exception handling is that it makes the diagram much less cluttered. In this example the exception isn't caught until it enters the UI, in this case the ATM. 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. 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.
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