UML sequence diagrams model the flow of logic within your system in a visual manner, enabling you both to
document and validate your logic, and are commonly used for both analysis and design purposes. Sequence diagrams
are the most popular UML artifact for dynamic modeling, which focuses on identifying the behavior within your
system. Other dynamic modeling techniques include
timing diagramming, and
interaction overview diagramming. Sequence diagrams, along with
class diagrams and
physical data models are in my opinion the most important design-level models for modern business
Sequence diagrams are typically used to model:
Usage scenarios. A usage scenario is a description of a potential way your system is used. The
logic of a usage scenario may be part of a use case, perhaps an alternate course. It may also be one
entire pass through a use case, such as the logic described by the basic course of action or a portion
of the basic course of action, plus one or more alternate scenarios. The logic of a usage scenario may
also be a pass through the logic contained in several use cases. For example, a student enrolls in the
university, and then immediately enrolls in three seminars.
The logic of methods. Sequence diagrams can be used to explore the logic of a complex
operation, function, or procedure. One way to think of sequence diagrams, particularly highly detailed
diagrams, is as
visual object code.
The logic of services. A service is effectively a high-level method, often one that can be
invoked by a wide variety of clients. This includes web-services as well as business transactions
implemented by a variety of technologies such as CICS/COBOL or CORBA-compliant object request brokers
Let's start with three simple examples. Figure 1 depicts a UML sequence diagram for
Enroll in University use case, taking a system-level approach where the interactions between the actors and
the system are shown.
Figure 2 depicts a sequence diagram for the detailed logic of a service to
determine if an applicant is already a student at the university.
Figure 3 shows the logic for how to enroll in a seminar. I will often develop a system-level sequence
diagram with my stakeholders to help to both visualize and validate the logic of a usage scenario. It also helps
me to identify significant methods/services, such as checking to see if the applicant already exists as a
student, which my system must support.
Figure 1. System-level sequence diagram.
The reason why they're called sequence diagrams should be obvious: the sequential nature of the logic is
shown via the ordering of the messages (the horizontal arrows). The first message starts in the top left corner,
the next message appears just below that one, and so on.
Figure 2. Service-level sequence diagram.
The boxes across the top of the diagram represent classifiers or their instances, typically use cases,
objects, classes, or actors. Because you can send messages to both objects and classes, objects respond to
messages through the invocation of an operation and classes do so through the invocation of static operations,
it makes sense to include both on sequence diagrams. Because actors initiate and take an active part in usage
scenarios, they can also be included in sequence diagrams. Objects have labels in the standard UML format
, where "name"¯ is optional (objects that haven't been given a name on the diagram are called anonymous
objects). Classes have labels in the format ClassName, and actors have names in the format Actor Name
. Notice how object labels are underlined, classes and actors are not. For example, in Figure 3, you see the Student object has the name
aStudent, this is called a named object, whereas the instance of Seminar is an anonymous object. The
instance of Student was given a name because it is used in several places as a parameter in messages,
whereas the instance of the Seminar didn't need to be referenced anywhere else in the diagram and thus
could be anonymous. In
Figure 2 the Student class sends messages to the PersistenceFramework class (which could have
been given the stereotype
<> but wasn't to keep the diagram simple). Any message sent to a class is implemented as a
static method, more on this later.
Figure 3. Enrolling in a seminar (method).
The dashed lines hanging from the boxes are called object lifelines, representing the life span of the object
during the scenario being modeled. The long, thin boxes on the lifelines are activation boxes, also called
method-invocation boxes, which indicate processing is being performed by the target object/class to fulfill a
message. I will only draw activation boxes when I'm using a tool that natively supports them, such as a
sophisticated CASE tool, and when I want to explore performance issues. Activation boxes are too awkward to draw
on whiteboards or with simple drawing tools such that don't easily support them.
The X at the bottom of an activation box, an example of which is presented in
Figure 4, is a UML convention to indicate an object has been removed from
memory. In languages such as C++ where you need to manage memory yourself you need to invoke an object's
destructor, typically modeled a message with the stereotype of
<>. In languages such as Java or C# where memory is managed for you and objects that are no longer
needed are automatically removed from memory, something often referred to as garbage collection, you do not need
to model the message. I generally don't bother with modeling object destruction at all and will instead trust
that the programmers, often myself, will implement low-level details such as this appropriately.
Figure 4 presents a complex UML sequence diagram for the basic course of action for the Enroll in Seminar
use case. This is an alternative way for modeling the logic of a usage scenario, instead of doing it at the
system-level such as Figure 1 you simply dive straight into modeling the detailed logic
at the object-level. I'll take this approach when I'm working with developers who are experienced sequence
diagrammers and I have a large working space (either a huge whiteboard or a CASE tool installed on a workstation
with a very large screen and good graphic card). Most of the time I'll draw system-level diagrams first and then
create small diagrams along the lines of what is shown in Figures
2 and 3.
Figure 4. Basic course of action for the Enroll in Seminar use case.
Messages are indicated on UML sequence diagrams as labeled arrows, when the source and target of a message is
an object or class the label is the signature of the method invoked in response to the message. However, if
either the source or target is a human actor, then the message is labeled with brief text describing the
information being communicated. For example, in Figure 4
the EnrollInSeminar object sends the message isEligibleToEnroll(theStudent) to the instance
of Seminar. Notice how I include both the method's name and the name of the parameters, if any, passed
into it. The Student actor provides information to the SecurityLogon object via the
messages labeled name and student number (these really aren't messages, they are actually user
Return values are optionally indicated using a dashed arrow with a label indicating the return value. For
example, the return value
theStudent is indicated coming back from the
Student class as the result of invoking a message, whereas no return value is indicated as the result of
sending the message isEligibleToEnroll(theStudent)
to Seminar. My style is not to indicate the return values when it's obvious what is being returned, so I
don't clutter my sequence diagrams (as you can see, sequence diagrams get complicated fairly quickly).
Figure 5shows an alternate way to indicate return values using the format
message: returnValue for messages, as you can see with
Notice the use of stereotypes throughout the diagram. For the boxes, I applied the stereotypes
<> indicating they represent an actor, a controller class, or a user interface (UI) class, respectively.
I've also used visual stereotypes on some diagrams - a stick figure for actors; the robustness diagram visual
stereotypes for controller, interface, and entity objects; and a drum for the database. Stereotypes are also
used on messages. Common practice on UML diagrams is to indicate creation and destruction messages with the
<>, respectively. For example, you see the SecurityLogon
object is created in this manner (actually, this message would likely be sent to the class that would then
result in a return value of the created object, so I cheated a bit). This object later destroys itself in a
similar manner, presumably when the window is closed.
I used a UML note in
Figure 4; notes are basically free-form text that can be placed on any UML
diagram, to provide a header for the diagram ,indicating its title and identifier (as you may have noticed, I
give unique identifiers to all artifacts that I intend to keep). Notes are depicted as a piece of paper with the
top-right corner folded over. I also used a note to indicate future work that needs to be done, either during
analysis or design, in this diagram-the
qualifications() message likely represents a series of messages sent to the student object. Common UML
practice is to anchor a note to another model element with a dashed line when appropriate, in this case the note
is attached to the message.
Figure 4 models the logic of the basic course of action for the Enroll in
Seminar use case how would you go about modeling alternate courses? The easiest way to do so is to create a
single sequence diagram for each alternate course, as you see depicted in
Figure 5. This diagram models only the logic of the alternate course, as you can tell by the numbering of
the steps on the left-hand side of the diagram, and the header note for the diagram indicates it is an alternate
course of action. Also notice how the ID of this diagram includes that this is alternate course C, yet
another modeling rule of thumb I have found useful over the years.
Figure 5. An alternate course of action for the Enroll in Seminar use
Let's consider other sequence diagramming notation.
Figure 5 includes an initial message, Student chooses seminar, which is indicated by the filled in
circle. This could easily have been indicated via a method invocation, perhaps enrollIn(seminar).
shows another way to indicate object creation - sending the new message to a class. We've actually seen
three ways to achieve this, the other two being to send a message with the
<> stereotype and/or to send a message into the side of the classifier symbol (for example in Figure 4
the message going into the side of EnrollInSeminar
or in Figure 6
the message going into the side of StudentInfoPage. My advice is to choose one style and stick to it.
7 each depict a way to indicate looping logic. One way is to show a
frame with the label loop and a constraint indicating what is being looped through, such as for each seminar
Figure 6. Another approach is to simply precede a message that will be invoked several times with an
asterisk, as you see in
Figure 7 with the inclusion of the Enroll in Seminar use case.
Figure 6. Outputting transcripts.
includes an asynchronous message, the message to the system printer which has the partial arrowhead. An
asynchronous message is one where the sender doesn't wait for the result of the message, instead it processes
the result when and if it ever comes back. Up until this point all other messages have been synchronous,
messages where the sender waits for the result before continuing on. It is common to send asynchronous messages
to hardware devices or autonomous software services such as message buses.
The method of modeling the inclusion of use cases using in Figure 7
is something that I first proposed in
The Elements of UML Style although I have no doubt that others use this approach as well. I basically
show the use case as a bubble across the top of the diagram, just like any other classifier, and show a message
sent to it with the
<> stereotype. This is consistent with both use case diagramming and sequence diagramming practices.
Figure 7. Enrolling in the University.
Figure 7 is also interesting because it shows how to model conditional
logic. In this case a frame with the label
alt is used along with a guard, in this case
applicant on eligibility list. The frame is separated into regions separated by dashed lines. In this case
there are two regions, one for each alternative, although you can have as many regions as you require (to
support the visual equivalent of a case statement). Each region requires a guard.
Earlier I stated that sequence diagrams are effectively a form of visual coding, or perhaps another way to
think of it is that sequence diagrams can be used for very detailed design. When I developed the sequence
Figure 4 I made several decisions that could potentially affect my other
models. For example, as I modeled Step 10, I made the design decision that the fee display screen also handled
the verification by the student that the fees were acceptable.
Also, as I was modeling Steps 2 and 3, I came to the realization that students should probably have passwords
to get into the system. When you're following the AM practices of
Active Stakeholder Participation and
Model With Others it's easy to find out if ideas such as this make sense because all you need to do is
ask. In this case I discovered I was wrong: the combination of name and student number is unique enough for our
purposes and the university didn't want the added complexity of password management. This is an interesting
decision that would potentially be recorded as a business rule because it is an operating policy of the
university, indicating the need to follow the practice
Iterate To Another Artifact and jot down the rule if we're interested in keeping a permanent record of
How to Draw Sequence Diagrams
I've been trying to explain to people how to draw sequence diagrams for years, and what I've discovered is
that the people who get it are either very good at thinking in a logical manner and/or they are good at writing
software code. Sequence diagramming really is visual coding, even when you are modeling a usage scenario via a
system-level sequence diagram.
When I'm creating a sequence diagram I'll start by identifying the scope of what I'm trying to model, and
because I prefer to follow the AM practice Model in Small Increments I'll typically tackle small usage
scenarios at the system level or a single method/service at the detailed object level. A diagram such as Figure 4
is too complex to be useful in my experience.
I'll then work through the logic with at least one more person, laying out classifiers across the top as I
need them. I automatically add the object lifelines but as I indicated earlier will typically not invest time
adding activation boxes. The heart of the diagram is in the messages, which I add to the diagram one at a time
as I work through the logic. I rarely indicate return values, instead I'll give messages intelligent names which
often make it clear what is being returned.
It is interesting to note that as you sequence diagram you will identify new responsibilities for classes and
objects, and, sometimes, even new classes. The implication is that you may want to update your class model
appropriately, agile modelers will follow the practice Create Several Models in Parallel, something that
CASE tools will do automatically. Remember, each message sent to a class invokes a static method/operation on
that class each message sent to an object invokes an operation on that object.
Regarding style issues for sequence diagramming, I prefer to draw messages going from left-to-right and
return values from right-to-left, although that doesn't always work with complex objects/classes. I justify the
label on messages and return values, so they are closest to the arrowhead. I also prefer to layer the sequence
diagrams: from left-to-right. I indicate the actors, then the controller class(es), and then the user interface
class(es), and, finally, the business class(es). During design, you probably need to add system and persistence
classes, which I usually put on the right-most side of sequence diagrams. Laying your sequence diagrams in this
manner often makes them easier to read and also makes it easier to find layering logic problems, such as user
interface classes directly accessing persistence classes.
Keeping it Agile
The most important things that you can do is to keep your diagrams simple, both content wise and tool wise. I
will sketch sequence diagrams on whiteboards to think something through, either to verify the logic in a use
case or to design a method or service. I rarely keep sequence diagrams as I find their true value is in their
A common mistake is to try to create a complete set of sequence diagrams for your system. I've seen teams
waste months creating several sequence diagrams for each of their use cases, one for the basic course of action
and one for each alternate course. My advice is to only create a sequence diagram when you have complex logic
that you want to think through - if the logic is straightforward the sequence diagram won't add any value, you
had might as well go straight to code.
This artifact description is excerpted from Chapter 11 of
The Object Primer 3rd Edition: Agile Model Driven
Development with UML 2.
The notation used in these diagrams, particularly the hand drawn ones, may not conform perfectly to the
current version of the UML for one or more of reasons:
- The notation may have evolved from when I originally developed the diagrams. The UML evolves over time,
and I may not have kept the diagrams up to date.
- I may have gotten it wrong in the first place. Although these diagrams were thoroughly reviewed for the
book, and have been reviewed by thousands of people online since then, an error may have gotten past of
us. We're only human.
- I may have chosen to apply the notation in "non-standard" ways. An agile modeler is more interested in
created models which communicate effectively than in conforming to notation rules set by a committee.
- It likely doesn't matter anyway, because the
modeling tool(s) that you're using likely won't fully support the current version of the UML
notation perfectly anyway. Bottom line is that you're going to be constrained by your tools anyway.
If you're really concerned about the nuances of "official" UML notation then read the current version of the