UML SEQUENCE DIAGRAM EXAMPLE HOW TO

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. At the top of the screen, the system sends message 5 to the outside. If you want to show that the different interaction fragments communicate with each other, model a message (filled arrow) to the frame line. 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. When you create multiple sequence diagrams (or other interactions), the frame separates these representations. 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. The frame element is optional in UML diagrams as you can see in Figures 1 and 2, the diagram's label is placed in the top left corner in what I'll call the frame's 'namebox,' a sort of dog-eared rectangle, and the actual UML diagram is defined within the body of the larger enclosing rectangle.

UML SEQUENCE DIAGRAM EXAMPLE 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. P.S: this example shows 2 threads, but you can add more threads by adding more horizontal dashed lines.UML provides neither notation to model exception handling in sequence diagrams nor any reasoning why it is absent. Graphically (see link), the combined fragment is represented as a boxed region in your sequence diagram, parallel sequences being separated by horizontal dashed lines (each of the slices would then correspond to one thread of execution. So how to show that interactions may happen in parallel ?įor representing parallelism, you would use a combined fragment introduced by the operator par. In principle, the sequence of the interactions with your object is indicated by the vertical order of the messages. It could be thread instantiations directly, or it could be several objects that are created in the context of the two threads.īut this is not sufficient. The sequence diagram is a specific kind of interaction diagram. Interaction diagrams describe behavior in terms of messages exchanged between objects. This can be illustrated in UML with interaction diagrams, statechart diagrams, and activity diagrams. You would also represent with a lifeline each threaded object that interact with your shared object. Remember that the dynamic model describes the internal behavior of a system. So your object that is shared between the threads should appear once and only once in the diagram. In a sequence diagram, a lifeline represents an individual participant in the interaction.

Uml sequence diagram example