Introduction to Sequence Diagrams

A sequence diagram is a form of interaction diagram that shows the development of objects along a lifeline, and the interactions between objects over time are represented as messages from a source lifeline to a target lifeline.

Sequence diagrams can be used to:

Composition of a sequence diagram

Example of a sequence diagram

The following is an example of a sequential diagram that identifies the types of elements and relationships.

Elements of a sequence diagram

Icon	Element	Description
	Role	Actors are users of the system; A user can refer to a human user in the model, a machine, or even another system or subsystem.
	Lifeline	A lifeline represents a unique connectable element that is an individual participant in an interaction.
	Boundary classes	Boundary is a stereotype of a class that models some system boundary, usually a user interface. Boundaries are used in analytics to describe the interaction between users and systems.
	Control class	Control is a stereotype of a class that models a control entity or manager. Control classes are used to control and dispatch other classes, and are typically used in analysis (including robustness), sequences, and communication graphs.
	Entity classes	Entity is a stereotype of a class that models information or stored data in a system.
	Combine clips	Fragment elements can represent iterative or conditionally executed interactive processes in a sequence diagram.
	Endpoint	Endpoints are used in interaction graphs to reflect lost or found messages in order.
	The gateway of the figure	A Diagram Gate is a simple graphical way to indicate the point at which a message can pass in and out of an interactive fragment.
	Status / Continuation	State Constant/Continuation, the element provides two distinct purposes for the sequence diagram, the state invariant and the continuation part.
	In turn	You can use interaction elements to insert an interaction diagram as a child of a class element.

Legend: Fragment legend

A fragment is a combination of an interactive fragment in a sequence diagram. A complex sequence diagram can be divided into several parts, each part is called an interaction fragment, each interaction segment is surrounded by a large box, and its name is displayed in the interval in the upper left corner of the box, representing the information of the sequence diagram.

A composite fragment can contain several fragments, each with a Boolean constraint. Combine Fragments appears as a transparent window, where each fragment is separated by a horizontal line and can have its own trigger.

Here's an example of a combined fragment:

This sequence diagram illustrates how to model a streamlined procurement process using combined fragments.

There are 2 fragments in the sequence diagram above

• One of them is a loop fragment that indicates that the cashier counts each purchased item until the price of all items is completed, and then the cashier asks the customer to pay.

•  The other is a conditional judgment combination fragment, which represents a judgment on a payment method that is then split to show the processing fragment of the two conditions, cash and credit card. After the payment fragment execution is complete, the cashier provides the receipt to the customer.

The order of the interaction fragment conditions can be changed directly on the diagram:

1. Select an interaction fragment that defines multiple criteria; Up and down arrows appear to the right of each condition.
2. Click the corresponding arrow to change the order.

How-to guide: Select and move combo clips

In order to select a grouped region, you must click near the inner edge or drag a selection rectangle around the region.

Once included in a fragment or fragment condition, the message will continue to be included as it moves up and down in the chart. To move a message out of a fragment, or to a different location in a sequence within a fragment, hold down the Alt key while dragging the message into place. As the message in it moves up or down, the fragment on the sequence diagram will resize to continue to contain the message.

To move a composite fragment independently of its contents, make sure the "move freely" element iconis visible; If it doesn't appear, click the "Move contents" iconand drag the element border.

Interaction Clips within a Combine Clip condition cannot be moved outside of the condition unless the clip is in "move freely" mode. Moving a conditional row moves any objects and messages below the conditional row down or up based on how much it moves.

When a contained clip is resized, a clip containing other clips is resized (unless the clip is in "move freely" mode).

How-to guide: Fill opacity

While interaction fragments often contain many other elements, there may be reasons why these elements are hidden and when they are fully displayed, or it may simply indicate that they are there, depending on the immediate purpose of the diagram. You can apply these nuances to the display of elements behind interactive fragments and overlaid or overlapping elements by changing the opacity of the elements.

Before setting the opacity, check if the element has a fill color.

Opacity can be set using the icons in the toolbar of the following two pop-up elements:

• Click on the interaction fragment element andicon:
  

  

• Right-click on the Interaction Fragment element and look above the context menu：
  

  

Click theicon and select:

Legend: Endpoints

The message endpoint element defines the termination of a state or numeric lifeline in the time graph. It indicates the message:

•  An undefined point that originates outside the lifeline, which can be referred to as message finding.

•  Sent to an undefined point outside the object's lifeline, which can be referred to as a message loss.

Endpoints are used in interaction graphs (sequences, timing, communication, or interaction overviews) to reflect lost or found messages in order.

Using the sequence diagram, drag the message from the appropriate lifeline to the endpoint. For sequence diagrams, messages that connect lifelines to endpoints require some sequential specification to draw the connection. The following example depicts missing messages in a sequence diagram.

Legend: Diagram gate

A diagram gate is a simple graphical way to indicate where a message can be transferred to or outside of an interaction fragment. A fragment may be required to receive or deliver a message; Internally, ordered messages reflect this need, indicating the entrance at the boundary of the fragment frame. Any external messages that are synchronized with this internal message must correspond appropriately. Portals can appear in interaction diagrams (sequence, time, communication, or interaction overview), interaction events, and combined fragments (to specify expressions).

Legend: Status / Persistence

State/Persistence - Elements serve two different purposes for an interactive (sequence) diagram, namely state invariant and persistent. When you create an element, you'll be prompted to determine its purpose.

Legend: Interactions

You can use interaction elements to insert an interaction diagram as a child of a class element. Interactive elements can contain any of the following types of diagrams:

Interactive elements in an EA are considered the behavior of the classifier encapsulated in it. It can have parameters and return types that are modeled using the Behavior tab of the Properties dialog of the interactive element.

Relationships of sequence diagrams

Message

Sequence diagrams use messages passed from element to element to describe a workflow or activity that changes over time.

Self message

Self-Message reflects a new process or method that is invoked in an action that invokes a lifeline. It is the specification of the message, usually in a sequence diagram.

Self-Message Calls represent nested calls; A new activation level is added with each call.

Self message returns

You can describe the return of your own message call.

Recursion

Internal communication manifests itself as a recursive invocation of an operation, or a method invoking other methods that belong to the same object. Displays nested control focus for execution events on the lifeline.

Call

A call is a message connector that extends the activation level of the previous message. All self-information creates a new activation level, but this control usually ends with the next message (unless the activation level is manually adjusted). Self-message calls, as shown in the image of the first call, represent nested calls; A new activation level is added with each call. Unlike regular messages between elements, calls between elements continue existing activations in the source element, meaning that the call was initiated within the activation range of the previous message.

Sequence Diagram Modeling Guide

Denote the Lifecycle of an Element

Capture the element lifecycle with a message represented as a New or Delete message type

This example shows two elements with specific creation and deletion times.

Layout of Sequence Diagrams

Icon	Element name	Description
	Message	Messages indicate the flow of information or control transitions between elements.
	Self message	Self messages reflect a new process or method that is invoked in an action that invokes a lifeline
	Recursion	Recursion is a type of message used in sequence graphs to indicate a recursive function.
	Call	A call is a message connector that extends the activation level of the previous message.

This example shows the layout of a sequence diagram in use.

Sequence Element Activation

A sequence element in a sequence diagram has a rectangle drawn along its lifeline. These rectangles describe the amount of time an element is active throughout the processing period. This visual representation can be suppressed by right-clicking on the sequence diagram and selecting "Suppress Activations".

Usually, the EA calculates the period of activation. There are several context menu options on Sequence Message that you can use to accomplish this task. To access the context menu, right-click on the message and select Activations.

A branch with a previous message

After you set up your lifeline and sequence messages with the appropriate message grouping and activation levels, you may want to indicate that the two messages emitted from the lifeline in different message groups and at different activation levels are branched, or executed at the same time. Consider this example:

Description: Message 1.1 is passed from object 5 to object 6, then message 1.2 is passed to object 7, and message 2.1 is passed back to object 5. The messages appear to be in the order of 1.1, 1.2, and 2.1. However, it would like to point out that message 2.1, although separate, is concurrent with message 1.2.

In this case:

The source anchor for message 2.1 then changes to the same source as message 1.2, which is the previous message. They are separate but concurrent messages from the same lifeline.

If you don't need to show that the message is a branch later, right-click on the message behind it (2.1) and deselect the Branch with previous message option.

Lifeline Activation Levels

By adding an activation layer to a single lifeline, a complex processing system can be easily reflected in a sequence diagram.

Example 1 :

The class calls method Sample A, which in turn calls Sample A1.

Here's how it works:

1. Click in the Diagram Toolbox to display the Find Toolbox Item dialog box and specify Interaction.
2. Click the Interaction Relationships icon in the Self-message panel.
3. Click on Lifeline.

Example 2 :

The lifeline now visually depicts method Sample A1 that is called during processing Sample A.

Example 3 :

In this case, more self messages have been added.

The message Sample A2a is called from Sample A2, which in turn is called from Sample A (instead of Sample A1).

Sample A1 is called from Sample A.

Message Label Visibility

Hide and show the labels used in the sequence message as follows:

1. Right-click on the message in the sequence diagram and select Set Label Visibility.

2. The Label Visibility dialog box is displayed.

3. Select or clear the check boxes for each message label you want to show or hide, individually.

4. Click the OK button to save the settings.

Change the Top Margin (create usable space at the top of the diagram)

In order to change the top edge of the sequence diagram from the default of 50 units, right-click on the diagram and select the "Set Top Margin" option. You can set the top blank to any value between 30 and 250 units. You can then use this space, for example, to add a Note or Text element to provide a document on a diagram.

Change the Timing Details

In this image, on the open order message:

On the Get Cart message:

You can adjust the message angle by typing a value in the Duration constraint field.

You can also create a Duration Constraint Between Messages row by dragging the General Ordering arrow up to the point where the previous message joins the source lifeline of the current message. A dialog box appears where you can enter constraint values. Once you have created a line, you can move it to any point in the middle of the current message and the previous message to avoid overlapping with other message timing details. The value can be edited or deleted through the Timing Details dialog box or by right-clicking on the row itself and selecting the appropriate context menu option.

Business Modeling/Interaction

Business Modeling Diagrams and Business Interaction Diagrams can model the structure and behavior of business systems. A business modeling diagram is based on a class (UML structure) diagram, while a business interaction diagram is based on a sequence (UML behavior) diagram. Both diagram types have the same default Toolbox, which consists of pages of Business Modeling elements. The available elements include prototype objects, prototype actors (business actors), use cases (business use cases), and collaboration (business use case implementations).

The diagram shows what an element looks like from the Business Modeling page of the Diagram Toolbox that is dragged and dropped onto the Business Modeling Diagram.

This diagram shows what an element looks like when dragged and dropped onto a business interaction diagram from the Business Modeling page of the Diagram toolbox.

Sequence Diagrams and Version Control

You can create sequence diagrams that use elements from other packages as lifelines in the diagram. In this case, the diagram can be corrupted when the element package is checked in and out under version control. This is because during the checkout process, the elements are first removed from the model and then re-imported, and although they are restored in the diagram, there is no message connecting them.

Therefore, if the diagram and its elements are in different packages, the round-trip of the element package through version control can corrupt the sequence diagram.

The solution is to drag and drop each class as an object onto the sequence diagram - when dragging and dropping the class onto the sequence diagram, select the Instance as Element (Object) option in the Paste Element dialog box. This creates a new object in the chart's parent package based on the selected Class element. Then, create messages between the objects.

So, to make sure that the sequence diagram isn't corrupted by other packages through versioned round-trips, keep in mind that:

This diagram shows a browser window with two packages: P1 which contains the elements and P2 which contains a sequence diagram that uses these elements. The chart itself is also shown.

This diagram does not break when going round trip through version control because all lifelines are objects, and those objects are in the same package as the sequence diagram.

Sequence Diagram Options

When modeling with sequence diagrams, there are a number of rendering features specific to that diagram type, such as controlling the focus of the indicator. In addition to the more general chart settings defined on other chart pages of the Preferences dialog box, you can define default settings for these features.

If you would like to learn more:

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