But for "simple" exception handling, I think a break is a better way to represent it, then alt. ConclusionĪs always, the best method depends on the scenario. If the called method is (or should be) private, then it can safely be excluded from the sequence diagram as an implementation detail. 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. Representing a self-call on a UML sequence diagram (see step 7). 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. UML Sequence Diagram¶ A UML Sequence diagram shows how messages go back and forth between objects over time. 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. In the user authentication process we describe, the messages show how a user enters information into the browser to gain access to the webpage and how the web browser will then cross-check credential to the designated application.UML provides neither notation to model exception handling in sequence diagrams nor any reasoning why it is absent. What information is passed from one object to another? The other set of components you must identify are the messages the objects will send. The objects, sometimes called actors, are the principal players that perform tasks and send messages.įor example, in a login sequence, two of the objects would be the potential visitor or user and the login page a visitor uses to interact with the platform they’re attempting to log into. The first necessary set of components are the objects. To accomplish these goals, you must first identify the components of your system that should be diagrammed. Map out a real or hypothetical scenario that has many different components. See how objects and components interact with each other in sequence to complete a process. Model how a complex procedure or system operates. You can accomplish several critical tasks with a sequence diagram.
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