Behavioral modeling 1: sequence and communication diagrams
Objects exchanging messages in time order to realize a use case: lifelines, activations, message types, alt/opt/loop fragments, and the same information as a communication diagram. On the exam you are asked to draw a sequence diagram for a scenario.
The functional model (use case) said what the system does, and the structural model (class diagram) said with which objects it does it. The behavioral model shows how these objects work together over time. There are three kinds of diagrams: interaction diagrams (sequence and communication), behavioral state machines (state machine), and the activity diagram that often accompanies them. This page covers the interaction diagrams; the second page covers the state machine and activity diagram.
The exam question is mostly: “Draw a sequence diagram for the normal flow of use case X.” You read the use case description step by step and turn each step into one or more messages.
Golden rules:
- One diagram = one scenario of one use case.
- Lifelines come from the classes in the class diagram. If you invented a new class, add it to the class diagram too.
- A message is an operation of the receiver. The class that receives the
checkEligibility()message must have this operation. - Watch the arrowheads: filled = synchronous, open = asynchronous, dashed = return.
Elements of the sequence diagram
| Element | Notation | Meaning |
|---|---|---|
| Actor | Stick figure, with a lifeline under it | The role that starts the interaction |
| Object / lifeline | An anOrder : Order box + dashed vertical line | An object taking part in the interaction |
| Activation | Narrow rectangle on the lifeline | The period when the object is active |
| Synchronous message | Solid line, filled arrow | A call, a reply is expected |
| Asynchronous message | Solid line, open arrow | Send and continue |
| Return message | Dashed line, open arrow | Return value |
| Create message | <<create>> or create(), the target box is at that level | A new object is created |
| Destroy | X at the end of the lifeline | The object is destroyed |
| Guard | [condition] | The message is sent only if the condition is true |
| Combined fragment | Frame, with alt, opt, loop at the top left | Control structure |
Writing the object name: aStudent : Student (a specific object), : Copy (an anonymous object of that class). In analysis, anonymous objects are usually used.
Combined fragments
| Fragment | Code equivalent | Example guard |
|---|---|---|
alt | if / else | [copy exists] / [else] |
opt | if (no else) | [total > 300] |
loop | for / while | [for each cart item] |
par | parallel | two compartments at the same time |
How to draw a sequence diagram
- Choose the use case and the scenario (usually the normal flow).
- Identify the participating actor and objects; arrange them from left to right, with the first one to speak on the left.
- Draw the lifelines. Put objects created later at the level where they are created.
- Turn every step of the description into a message, in order from top to bottom.
- Add the activation boxes.
- Show conditions and repetitions with fragments.
- Check: does the receiver of each message have that operation? Is it consistent with the class diagram?
Worked example: Reserve Book
The normal flow of the fully dressed description in week 7. In analysis, putting a control object (ReservationController) in between organizes the flow: the actor talks to this object, and it manages the other objects.
S = aStudent C = :ReservationController
B = :Book K = :Copy
R = :Reservation (born with message 4)
S C B K
| | | |
|--1---->| | |
| |--2---->| |
| |<- 2r- -| |
| |--3--+ | |
| |<----+ | |
| |--4 create------------>[ R ]
| |--5------------->| |
| |--6 (async)------------->|
|<--7----| | | |
| | | | |The numbers are the messages in the table below; 2r is the return of message 2 (the copy list). Message 3 is a message C sends to itself (self-call). The R box does not start at the top but at the level of message 4. On the exam this table and the drawing complement each other.
| No | Sender | Receiver | Message | Type |
|---|---|---|---|---|
| 1 | aStudent | :ReservationController | searchBook(query) | Synchronous |
| 2 | :ReservationController | :Book | getAvailableCopies() | Synchronous, return: copy list |
| 3 | :ReservationController | :ReservationController | checkEligibility(student) | Message to itself (self-call) |
| 4 | :ReservationController | :Reservation | create(student, copy) | Create, Reservation is born here |
| 5 | :ReservationController | :Copy | setStatus(“On Hold”) | Synchronous |
| 6 | :ReservationController | :Reservation | sendConfirmation() | Asynchronous (the e-mail is not waited for) |
| 7 | :ReservationController | aStudent | showConfirmation(code) | Synchronous, displaying the reply |
Count: 5 lifelines (aStudent, :ReservationController, :Book, :Copy, :Reservation), 7 messages (excluding returns).
Adding an alternate flow: the alt fragment
If 3a (“no available copy”) is added, an alt frame comes after message 2:
+-alt-----------------------------------+
| [copy list not empty] |
| 3 checkEligibility ... 7 showConf. |
|- - - - - - - - - - - - - - - - - - - -|
| [else] |
| 3' offerWaitingList() |
+---------------------------------------+PITFALL · forcing a return onto every message. A return message is drawn only when a meaningful value is returned (the copy list, the reservation code). Putting a dashed arrow on every synchronous call clutters the diagram but is not wrong; the real mistake is mixing up synchronous and asynchronous.
Communication diagram
The same interaction, shown on the links between objects. There is no time axis; the order is read from the numbers. It shows at a glance who talks to whom (the structure).
:Book
| 2: getAvailableCopies()
|
aStudent ------ C ------ :Copy
1: searchBook(q)| 5: setStatus("On Hold")
7: showConf() |
| 4: create()
| 6: sendConfirmation()
:Reservation
C = :ReservationController
A link from C to itself: 3: checkEligibility()Numbering rules:
| Number | Meaning |
|---|---|
| 1, 2, 3 | Top-level messages sent in order |
| 2.1, 2.2 | Sent while handling message 2 (nested) |
| 2.1.1 | Sent inside 2.1 |
*[i=1..n] | Repetition (iteration) |
[condition] | Guard |
Example: if :Book asked each copy for its status while handling getAvailableCopies(), these messages would be written as 2.1 *[each copy]: getStatus().
Sequence or communication?
| Criterion | Sequence | Communication |
|---|---|---|
| Emphasis | Time order | Object connections |
| Reading | Top to bottom | From the numbers |
| Fragments (alt/opt/loop) | Yes | Limited (with guard and *) |
| Interaction with many objects | Widens, overflows horizontally | More compact |
The two diagrams carry the same information; some tools generate one from the other automatically.
The CRUDE matrix
After the sequence diagrams, a table can be built that summarizes how the classes interact with each other.
| Student | Book | Copy | Reservation | |
|---|---|---|---|---|
| ReservationController | R | R | RU | CE |
| Reservation | R | R |
Reading a cell: ReservationController Creates the Reservation object and Executes one of its operations (sendConfirmation). It reads and updates Copy (R, U). If a class never sends a message to another but there is a relationship between them in the class diagram, question whether the relationship is needed (week 11).
Common mistakes
| Wrong idea | The truth |
|---|---|
| Putting the whole system in one sequence diagram | One use case, one scenario |
| Putting a class that is not in the class diagram on a lifeline and leaving it | Add the new class to the class diagram too |
| Synchronous and asynchronous arrows are the same | Filled arrow synchronous, open arrow asynchronous |
| A return is a solid line | A return is a dashed line |
| opt for if/else | alt for if/else; opt is a single condition, no else |
| A communication diagram has no order | The order is shown by the numbers |
| 2.1 comes after 3 | 2.1 is inside 2; it ends before 3 |
| Drawing the created object at the top | It starts at the level of the create message |
Test yourself
Exam-style questions. Solve them on paper first, then reveal the answer.
What does a sequence diagram emphasize compared to a communication diagram?
- The time order of messages
- The structure of links between objects
- The states of one object
- The attributes of classes
Show answer
A sequence diagram shows time on the vertical axis: the order of messages from top to bottom. A communication diagram shows the same messages with numbers on the links between objects; it emphasizes the structure, and the order is read from the numbers.
In a sequence diagram, what is the thin rectangle drawn on a lifeline?
Show answer
A lifeline is the dashed vertical line showing the existence of the object. The narrow rectangle on it is the activation: during that time the object is doing some work or waiting for the call it made to return.
Which arrow represents a synchronous message?
- Solid line with a filled arrowhead
- Solid line with an open arrowhead
- Dashed line with an open arrowhead
- Line with a hollow triangle
Show answer
Synchronous message: solid line, filled arrowhead; the sender waits for the reply. Asynchronous: solid line, open (line) arrowhead; the sender does not wait. Return: dashed line, open arrowhead. The hollow triangle is generalization and is not used in a sequence diagram.
Which combined fragment shows that a group of messages happens only if a condition is true, with no else branch?
- alt
- opt
- loop
- par
Show answer
opt: a single compartment, runs if the condition is true. alt: if/else, several compartments each with its guard. loop: repetition. par: parallel compartments.
How many messages (excluding return messages) are in the Reserve Book sequence diagram on this page?
Show answer
1 searchBook, 2 getAvailableCopies, 3 checkEligibility, 4 create (Reservation), 5 setStatus, 6 sendConfirmation, 7 showConfirmation. The returns (dashed arrows) were not counted separately. Total 7 messages.
How many lifelines are in the same diagram?
Show answer
aStudent (the actor), :ReservationController, :Book, :Copy, :Reservation. Because Reservation is created in the middle of the diagram by a create message, its box starts lower down, but it is still a lifeline.
In a communication diagram, a message is numbered 2.1. What does that mean?
Show answer
Nested numbering shows the call structure: the object that receives message 2 sends 2.1 and 2.2 while doing its work. 3 comes after 2 has completely finished.
Food-delivery: 'The customer submits an order. The OrderController creates an Order. For each item in the cart, the Order creates an OrderLine. If the total is over 300 TL, a free-delivery flag is set.' Which fragments do you need?
Show answer
A repeated message goes into a loop frame (guard: [for each item]). A conditional message with no else goes into an opt frame (guard: [total > 300]). If there were an else, alt would be used.
What does an X at the bottom of a lifeline mean?
Show answer
The large X at the end of a lifeline shows that the object is destroyed. Example: a temporary Reservation object whose time has expired.
In the CRUDE matrix, what does each letter stand for?
Show answer
The CRUDE matrix is a table with the classes in the rows and the columns. A cell shows how the class in the row interacts with the class in the column: Create, Read, Update, Delete, Execute. It is filled in from the messages in the sequence diagrams and is used to verify relationships between classes.
A sequence diagram is usually drawn for what scope?
- The whole system at once
- One scenario of one use case
- One class only
- The physical network
Show answer
A sequence diagram is usually drawn for one scenario of a use case (mostly the normal flow). For important alternate flows, a separate diagram or an alt fragment is added. Putting the whole system in one diagram makes it unreadable.
Verification: Conceptual and drawing/counting questions were checked against the UML 2 interaction diagram rules (synchronous message with a filled arrowhead, asynchronous with an open arrowhead, return as a dashed line; alt/opt/loop fragments; nested numbering 1, 1.1, 1.2 in the communication diagram). The numbers of messages and lifelines in the diagrams on the page were counted by hand and compared twice.