The sequence diagram is used primarily to show the interactions between objects in the sequential order that those interactions occur. Much like the class diagram, developers typically think sequence diagrams were meant exclusively for them. However, an organization's business staff can find sequence diagrams useful to communicate how the business currently works by showing how various business objects interact. Besides documenting an organization's current affairs, a business-level sequence diagram can be used as a requirements document to communicate requirements for a future system implementation. During the requirements phase of a project, analysts can take use cases to the next level by providing a more formal level of refinement. When that occurs, use cases are often refined into one or more sequence diagrams.
An organization's technical staff can find sequence diagrams useful in documenting how a future system should behave. During the design phase, architects and developers can use the diagram to force out the system's object interactions, thus fleshing out overall system design.
One of the primary uses of sequence diagrams is in the transition from requirements expressed as use cases to the next and more formal level of refinement. Use cases are often refined into one or more sequence diagrams. In addition to their use in designing new systems, sequence diagrams can be used to document how objects in an existing (call it "legacy") system currently interact. This documentation is very useful when transitioning a system to another person or organization.
Since this is the first article in my UML diagram series that is based on UML 2, we need to first discuss an addition to the notation in UML 2 diagrams, namely a notation element called a frame. The frame element is used as a basis for many other diagram elements in UML 2, but the first place most people will encounter a frame element is as the graphical boundary of a diagram. A frame element provides a consistent place for a diagram's label, while providing a graphical boundary for the diagram. 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.
Figure 1: An empty UML 2 frame element
In addition to providing a visual border, the frame element also has an important functional use in diagrams depicting interactions, such as the sequence diagram. On sequence diagrams incoming and outgoing messages (a.k.a. interactions) for a sequence can be modeled by connecting the messages to the border of the frame element (as seen in Figure 2). This will be covered in more detail in the "Beyond the basics" section below.
Figure 2: A sequence diagram that has incoming and outgoing messages
Notice that in Figure 2 the diagram's label begins with the letters "sd," for Sequence Diagram. When using a frame element to enclose a diagram, the diagram's label needs to follow the format of:
Diagram Type Diagram Name |
The UML specification provides specific text values for diagram types (e.g., sd = Sequence Diagram, activity = Activity Diagram, and use case = Use Case Diagram).
The main purpose of a sequence diagram is to define event sequences that result in some desired outcome. The focus is less on messages themselves and more on the order in which messages occur; nevertheless, most sequence diagrams will communicate what messages are sent between a system's objects as well as the order in which they occur. The diagram conveys this information along the horizontal and vertical dimensions: the vertical dimension shows, top down, the time sequence of messages/calls as they occur, and the horizontal dimension shows, left to right, the object instances that the messages are sent to.
When drawing a sequence diagram, lifeline notation elements are placed across the top of the diagram. Lifelines represent either roles or object instances that participate in the sequence being modeled. [Note: In fully modeled systems the objects (instances of classes) will also be modeled on a system's class diagram.] Lifelines are drawn as a box with a dashed line descending from the center of the bottom edge (Figure 3). The lifeline's name is placed inside the box.
Figure 3: An example of the Student class used in a lifeline whose instance name is freshman
The UML standard for naming a lifeline follows the format of:
Instance Name : Class Name |
In the example shown in Figure 3, the lifeline represents an instance of the class Student, whose instance name is freshman. Note that, here, the lifeline name is underlined. When an underline is used, it means that the lifeline represents a specific instance of a class in a sequence diagram, and not a particular kind of instance (i.e., a role). In a future article we'll look at structure modeling. For now, just observe that sequence diagrams may include roles (such as buyer and seller) without specifying who plays those roles (such as Bill and Fred). This allows diagram reuse in different contexts. Simply put, instance names in sequence diagrams are underlined; roles names are not.
Our example lifeline in Figure 3 is a named object, but not all lifelines represent named objects. Instead a lifeline can be used to represent an anonymous or unnamed instance. When modeling an unnamed instance on a sequence diagram, the lifeline's name follows the same pattern as a named instance; but instead of providing an instance name, that portion of the lifeline's name is left blank. Again referring to Figure 3, if the lifeline is representing an anonymous instance of the Student class, the lifeline would be: " Student." Also, because sequence diagrams are used during the design phase of projects, it is completely legitimate to have an object whose type is unspecified: for example, "freshman."
The first message of a sequence diagram always starts at the top and is typically located on the left side of the diagram for readability. Subsequent messages are then added to the diagram slightly lower then the previous message.
To show an object (i.e., lifeline) sending a message to another object, you draw a line to the receiving object with a solid arrowhead (if a synchronous call operation) or with a stick arrowhead (if an asynchronous signal). The message/method name is placed above the arrowed line. The message that is being sent to the receiving object represents an operation/method that the receiving object's class implements. In the example in Figure 4, the analyst object makes a call to the system object which is an instance of the ReportingSystem class. The analyst object is calling the system object's getAvailableReports method. The system object then calls the getSecurityClearance method with the argument of userId on the secSystem object, which is of the class type SecuritySystem. [Note: When reading this sequence diagram, assume that the analyst has already logged into the system.]
Figure 4: An example of messages being sent between objects
Besides just showing message calls on the sequence diagram, the Figure 4 diagram includes return messages. These return messages are optional; a return message is drawn as a dotted line with an open arrowhead back to the originating lifeline, and above this dotted line you place the return value from the operation. In Figure 4 the secSystem object returns userClearance to the system object when the getSecurityClearance method is called. The system object returns availableReports when the getAvailableReports method is called.
Again, the return messages are an optional part of a sequence diagram. The use of return messages depends on the level of detail/abstraction that is being modeled. Return messages are useful if finer detail is required; otherwise, the invocation message is sufficient. I personally like to include return messages whenever a value will be returned, because I find the extra details make a sequence diagram easier to read.
When modeling a sequence diagram, there will be times that an object will need to send a message to itself. When does an object call itself? A purist would argue that an object should never send a message to itself. However, modeling an object sending a message to itself can be useful in some cases. For example, Figure 5 is an improved version of Figure 4. The Figure 5 version shows the system object calling its determineAvailableReports method. By showing the system sending itself the message "determineAvailableReports," the model draws attention to the fact that this processing takes place in the system object.
To draw an object calling itself, you draw a message as you would normally, but instead of connecting it to another object, you connect the message back to the object itself.
Figure 5: The system object calling its determineAvailableReports method
The example messages in Figure 5 show synchronous messages; however, in sequence diagrams you can model asynchronous messages, too. An asynchronous message is drawn similar to a synchronous one, but the message's line is drawn with a stick arrowhead, as shown in Figure 6.
Figure 6: A sequence diagram fragment showing an asynchronous message being sent to instance2
When modeling object interactions, there will be times when a condition must be met for a message to be sent to the object. Guards are used throughout UML diagrams to control flow. Here, I will discuss guards in both UML 1.x as well as UML 2.0. In UML 1.x, a guard could only be assigned to a single message. To draw a guard on a sequence diagram in UML 1.x, you placed the guard element above the message line being guarded and in front of the message name. Figure 7 shows a fragment of a sequence diagram with a guard on the message addStudent method.
Figure 7: A segment of a UML 1.x sequence diagram in which the addStudent message has a guard
In Figure 7, the guard is the text "[pastDueBalance = 0]." By having the guard on this message, the addStudent message will only be sent if the accounts receivable system returns a past due balance of zero. The notation of a guard is very simple; the format is:
[Boolean Test] |
For example,
[pastDueBalance = 0] |
In most sequence diagrams, however, the UML 1.x "in-line" guard is not sufficient to handle the logic required for a sequence being modeled. This lack of functionality was a problem in UML 1.x. UML 2 has addressed this problem by removing the "in-line" guard and adding a notation element called a Combined Fragment. A combined fragment is used to group sets of messages together to show conditional flow in a sequence diagram. The UML 2 specification identifies 11 interaction types for combined fragments. Three of the eleven will be covered here in "The Basics" section, two more types will be covered in the "Beyond The Basics" section, and the remaining six I will leave to be covered in another article. (Hey, this is an article, not a book. I want you to finish this piece in one day!)
Alternatives are used to designate a mutually exclusive choice between two or more message sequences. [Note: It is indeed possible for two or more guard conditions attached to different alternative operands to be true at the same time, but at most only one operand will actually occur at run time (which alternative "wins" in such cases is not defined by the UML standard).] Alternatives allow the modeling of the classic "if then else" logic (e.g., if I buy three items, then I get 20% off my purchase; else I get 10% off my purchase).
As you will notice in Figure 8, an alternative
combination fragment element is drawn using a frame. The word "alt" is placed
inside the frame's namebox. The larger rectangle is then divided into what UML 2
calls operands. [Note: Although operands look a lot like lanes on a highway, I
specifically did not call them lanes. Swim lanes are a UML notation used on
activity diagrams. Please refer to The Rational Edge's earlier article about Activity Diagrams.]
Operands are separated by a dashed line. Each operand is given a guard to test
against, and this guard is placed towards the top left section of the operand on
top of a lifeline. [Note: Usually, the lifeline to which the guard is attached
is the lifeline that owns the variable that is included in the guard
expression.] If an operand's guard equates to "true," then that operand is the
operand to follow.
Figure 8: A sequence diagram fragment that contains an alternative combination fragment
As an example to show how an alternative combination fragment is read, Figure 8 shows the sequence starting at the top, with the bank object getting the check's amount and the account's balance. At this point in the sequence the alternative combination fragment takes over. Because of the guard "[balance >= amount]," if the account's balance is greater than or equal to the amount, then the sequence continues with the bank object sending the addDebitTransaction and storePhotoOfCheck messages to the account object. However, if the balance is not greater than or equal to the amount, then the sequence proceeds with the bank object sending the addInsuffientFundFee and noteReturnedCheck message to the account object and the returnCheck message to itself. The second sequence is called when the balance is not greater than or equal to the amount because of the "[else]" guard. In alternative combination fragments, the "[else]" guard is not required; and if an operand does not have an explicit guard on it, then the "[else]" guard is to be assumed.
Alternative combination fragments are not limited to simple "if then else" tests. There can be as many alternative paths as are needed. If more alternatives are needed, all you must do is add an operand to the rectangle with that sequence's guard and messages.
The option combination fragment is used to model a sequence that, given a certain condition, will occur; otherwise, the sequence does not occur. An option is used to model a simple "if then" statement (i.e., if there are fewer than five donuts on the shelf, then make two dozen more donuts).
The option combination fragment notation is similar to the alternation combination fragment, except that it only has one operand and there never can be an "else" guard (it just does not make sense here). To draw an option combination you draw a frame. The text "opt" is placed inside the frame's namebox, and in the frame's content area the option's guard is placed towards the top left corner on top of a lifeline. Then the option's sequence of messages is placed in the remainder of the frame's content area. These elements are illustrated in Figure 9.
Figure 9: A sequence diagram fragment that includes an option combination fragment
Reading an option combination fragment is easy. Figure 9 is a reworking of the sequence diagram fragment in Figure 7, but this time it uses an option combination fragment because more messages need to be sent if the student's past due balance is equal to zero. According to the sequence diagram in Figure 9, if a student's past due balance equals zero, then the addStudent, getCostOfClass, and chargeForClass messages are sent. If the student's past due balance does not equal zero, then the sequence skips sending any of the messages in the option combination fragment.
The example Figure 9 sequence diagram fragment includes a guard for the option; however, the guard is not a required element. In high-level, abstract sequence diagrams you might not want to specify the condition of the option. You may simply want to indicate that the fragment is optional.
Occasionally you will need to model a repetitive sequence. In UML 2, modeling a repeating sequence has been improved with the addition of the loop combination fragment.
The loop combination fragment is very similar in appearance to the option combination fragment. You draw a frame, and in the frame's namebox the text "loop" is placed. Inside the frame's content area the loop's guard is placed towards the top left corner, on top of a lifeline. [Note: As with the option combination fragment, the loop combination fragment does not require that a guard condition be placed on it.] Then the loop's sequence of messages is placed in the remainder of the frame's content area. In a loop, a guard can have two special conditions tested against in addition to the standard Boolean test. The special guard conditions are minimum iterations written as "minint = [the number]" (e.g., "minint = 1") and maximum iterations written as "maxint = [the number]" (e.g., "maxint = 5"). With a minimum iterations guard, the loop must execute at least the number of times indicated, whereas with a maximum iterations guard the number of loop executions cannot exceed the number.
Figure 10: An example sequence diagram with a loop combination fragment
The loop shown in Figure 10 executes until the reportsEnu object's hasAnotherReport message returns false. The loop in this sequence diagram uses a Boolean test to verify if the loop sequence should be run. To read this diagram, you start at the top, as normal. When you get to the loop combination fragment a test is done to see if the value hasAnotherReport equals true. If the hasAnotherReport value equals true, then the sequence goes into the loop fragment. You can then follow the messages in the loop as you would normally in a sequence diagram
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