Using Alternative Combined Fragments in Sequence Diagram

An alternative combined fragment is used to specify an area of a group of lifelines/ actors to show conditional flow in a sequence diagram. It also models the if-then-else logic in the sequence diagram. In this article, we will demonstrate how to draw a simple sequence diagram by using an alternative combined fragment.

  1. Create a sequence diagram as follows:

    Sequence diagram

    Sequence diagram

  2. Select Alt. Combined Fragment in the diagram toolbar. Press on the blank area of the diagram and drag to cover the Person and ATM Machine lifeline.

    Create an alterative combined fragment

    Create an alternative combined fragment

  3. Draw a message from ATM Machine lifeline to Person lifeline in the upper compartment of the alternative combined fragment. Name it as Ask for password. Draw another message from ATM Machine lifeline to Person lifeline in the lower compartment of the alternative combined fragment afterwards. Name it as Display invalid card.

    Draw two messages

    Draw two messages in an alternative combined fragment

  4. Right-click on the combined fragment and select Open Specification… from the pop-up menu to open the Combined Fragment Specification dialog box.

    Open Specification

    Open Specification

  5. When the Combined Fragments Specification window appears, open the Interaction Operands tab, select Operand, and then click Open Specification… button. In the Interaction Operand Specification dialog box, open the Guard tab and enter Valid card in Constraint text field. Click OK button to confirm the changes.

    Enter Valid card for Operand

    Enter Valid card for Operand

  6. Similarly, enter Invalid card in Constraint text field for Operand2by following the previous step.

    Enter Invalid card for Operand2

    Enter Invalid card for Operand2

  7. Guard conditions are displayed in the alternative combined fragment as shown below:

    Guard conditions are displayed

    Guard conditions are displayed

How to try the sample

Download ATM machine and then unzip it to get the .vpp file.

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