In my previous article, you learned about UML diagrams. In this article, we take a close look at the sequence diagram.

The examples provided below are based on the UML 2 specification. The UML 2.0 specification came out in 2005, but the most recent version is now UML 2.5.1 (which came out in 2017).

Explore the other articles in my UML Basics series, which covers the class diagram, the component diagram, and the sequence diagram.

As I reviewed this article in 2023, I realized that this article is so old that it references checks and physical cash. In 2023, I maybe write four checks a year and use physical cash every couple of months. However, I do still use money in digital form several days a week and I also use sequence diagrams several times a week. (I hope you enjoy this article and its cultural anachronisms.)

The diagram's purpose

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.

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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.

The notation

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 (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 basics

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.

Lifelines

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."

Messages

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

Guards

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. 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]

Combined fragments (alternatives, options, and loops)

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

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.) 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.

Option

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.

Loops

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

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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

Beyond the basics

I've covered the basics of the sequence diagram, which should allow you to model most of the interactions that will take place in a common system. The following section will cover more advanced notation elements that can be used in a sequence diagram.

Referencing another sequence diagram

When doing sequence diagrams, developers love to reuse existing sequence diagrams in their diagram's sequences. (Note: It is possible to reuse a sequence diagram of any type (e.g., programming or business). I just find that developers like to functionally break down their diagrams more.) Starting in UML 2, the "Interaction Occurrence" element was introduced. The addition of interaction occurrences is arguably the most important innovation in UML 2 interactions modeling. Interaction occurrences add the ability to compose primitive sequence diagrams into complex sequence diagrams. With these you can combine (reuse) the simpler sequences to produce more complex sequences. This means that you can abstract out a complete, and possibly complex, sequence as a single conceptual unit.

An interaction occurrence element is drawn using a frame. The text "ref" is placed inside the frame's namebox, and the name of the sequence diagram being referenced is placed inside the frame's content area along with any parameters to the sequence diagram. The notation of the referenced sequence diagram's name follows the pattern of:

sequence diagram name[(arguments)] [: return value]

Two examples:

Example 1: Retrieve Borrower Credit Report(ssn) : borrowerCreditReport

or

Example 2: Process Credit Card(name, number, expirationDate, amount : 100)

In example 1, the syntax calls the sequence diagram called Retrieve Borrower Credit Report and passes it the parameter ssn. The Retreive Borrower Credit Report sequence returns the variable borrowerCreditReport.

In example 2, the syntax calls the sequence diagram called Process Credit Card and passes it the parameters of name, number, expiration date, and amount. However, in example 2 the amount parameter will be a value of 100. And since example 2 does not have a return value labeled, the sequence does not return a value (presumably, the sequence being modeled does not need the return value).

Figure 11. A sequence diagram that references two different sequence diagrams

Figure 11 shows a sequence diagram that references the sequence diagrams "Balance Lookup" and "Debit Account." The sequence starts at the top left, with the customer sending a message to the teller object. The teller object sends a message to the theirBank object. At that point, the Balance Lookup sequence diagram is called, with the accountNumber passed as a parameter. The Balance Lookup sequence diagram returns the balance variable. Then the option combination fragment's guard condition is checked to verify the balance is greater then the amount variable. In cases where the balance is greater than the amount, the Debit Account sequence diagram is called, passing it the accountNumber and the amount as parameters. After that sequence is complete, the withdrawCash message returns cash to the customer.

It is important to notice in Figure 11 that the lifeline of theirBank is hidden by the interaction occurrence Balance Lookup. Because the interaction occurrence hides the lifeline, that means that the theirBank lifeline is referenced in the "Balance Lookup" sequence diagram. In addition to hiding the lifeline in the interaction occurrence, UML 2 also specifies that the lifeline must have the same theirBank in its own "Balance Lookup" sequence.

There will be times when you model sequence diagrams that an interaction occurrence will overlap lifelines that are not referenced in the interaction occurrence. In such cases the lifeline is shown as a normal lifeline and is not hidden by the overlapping interaction occurrence.

In Figure 11, the sequence references the "Balance Lookup" sequence diagram. The "Balance Lookup" sequence diagram is shown in Figure 12. Because the example sequence has parameters and a return value, its label —located in the diagram's namebox—follows a specific pattern:

Diagram Type Diagram Name [(Parameter Type : Parameter Name)] :

[: Return Value Type]

Two examples:

Example 1: SD Balance Lookup(Integer : accountNumber) : Real

or

Example 2: SD Available Reports(Financial Analyst : analyst) : Reports

Figure 12 illustrates example 1, in which the Balance Lookup sequence uses parameter accountNumber as a variable in the sequence, and the sequence diagram shows a Real object being returned. In cases such as this, where the sequence returns an object, the object being returned is given the instance name of the sequence diagram.

Figure 12. A sequence diagram that takes the parameter of accountNumber and returns a Real object

Figure 13 illustrates example 2, in which a sequence takes a parameter and returns an object. However, in Figure 13 the parameter is used in the sequence's interaction.

Figure 13. A sequence diagram that uses its parameter in its interaction and returns a Reports object

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Gates

The previous section showed how to reference another sequence diagram by passing information through parameters and return values. However, there is another way to pass information between sequence diagrams.

Gates can be an easy way to model the passing of information between a sequence diagram and its context. A gate is merely a message that is illustrated with one end connected to the sequence diagram's frame's edge and the other end connected to a lifeline. A reworking of Figures 11 and 12 using gates can be seen in Figures 14 and 15. The example diagram in Figure 15 has an entry gate called getBalance that takes the parameter of accountNumber. The getBalance message is an entry gate, because it is the arrowed line that is connected to the diagram's frame with the arrowhead connected to a lifeline. The sequence diagram also has an exit gate that returns the balance variable. The exit gate is known, because it's a return message that is connected from a lifeline to the diagram's frame with the arrowhead connected to the frame.

Figure 14. A reworking of Figure 11, using gates this time

Figure 15. A reworking of Figure 12, using gates this time

Combined fragments (break and parallel)

In the "basics" section presented earlier, I covered the combined fragments known as "alternative," "option," and "loop." These three combined fragments are the ones most people will use the most. However, there are two other combined fragments that a large share of people will find useful: break and parallel.

Break

The break combined fragment is almost identical in every way to the option combined fragment, with two exceptions. First, a break's frame has a namebox with the text "break" instead of "option." Second, when a break combined fragment's message is to be executed, the enclosing interaction's remainder messages will not be executed because the sequence breaks out of the enclosing interaction. In this way the break combined fragment is much like the break keyword in a programming language like C++ or Java.

Figure 16. A reworking of the sequence diagram fragment from Figure 8, with the fragment using a break instead of an alternative

Breaks are most commonly used to model exception handling. Figure 16 is a reworking of Figure 8, but this time Figure 16 uses a break combination fragment because it treats the balance < amount condition as an exception instead of as an alternative flow. To read Figure 16, you start at the top left corner of the sequence and read down. When the sequence gets to the return value "balance," it checks to see if the balance is less than the amount. If the balance is not less than the amount, the next message sent is the addDebitTransaction message, and the sequence continues as normal. However, in cases where the balance is less than the amount, then the sequence enters the break combination fragment and its messages are sent. Once all the messages in the break combination have been sent, the sequence exits without sending any of the remaining messages (e.g., addDebitTransaction).

An important thing to note about breaks is that they only cause the exiting of an enclosing interaction's sequence and not necessarily the complete sequence depicted in the diagram. In cases where a break combination is part of an alternative or a loop, then only the alternative or loop is exited.

Parallel

Today's modern computer systems are advancing in complexity and at times perform concurrent tasks. When the processing time required to complete portions of a complex task is longer than desired, some systems handle parts of the processing in parallel. The parallel combination fragment element needs to be used when creating a sequence diagram that shows parallel processing activities.

The parallel combination fragment is drawn using a frame, and you place the text "par" in the frame's namebox. You then break up the frame's content section into horizontal operands separated by a dashed line. Each operand in the frame represents a thread of execution done in parallel.

Figure 17. A microwave is an example of an object that does two tasks in parallel

While Figure 17 may not illustrate the best computer system example of an object doing activities in parallel, it offers an easy-to-understand example of a sequence with parallel activities. The sequence goes like this: A hungryPerson sends the cookFood message to the oven object. When the oven object receives that message, it sends two messages to itself at the same time (nukeFood and rotateFood). After both of these messages are done, the hungryPerson object is returned yummyFood from the oven object.

Summary and next steps

The sequence diagram is a good diagram to use to document a system's requirements and to flush out a system's design. The reason the sequence diagram is so useful is because it shows the interaction logic between the objects in the system in the time order that the interactions take place.

To learn more, check out my next article about UML class diagrams.

Explore the UML sequence diagram

Explore the UML sequence diagram