What Is A Physical Network Diagram?
- What is a UML sequence diagram? 4. Sequence diagram: an “interaction diagram” that models a single scenario executing in a system. 2nd most used UML diagram (behind class diagram). Shows what messages are sent and when. Relating UML diagrams to other design artifacts:. CRC cards → class diagrams. Use cases → sequence.
- UML Sequence Diagram is a type of Interaction Diagrams that describes the interactions between objects and classes that are involved in the scenario and the sequence of messages exchanged between them. UML Sequence Diagram represents the processes and objects that occur at the same time as lifelines.
Sequence diagrams are commonly used by developers to depict interactions between objects in a use case (actions that show interactions between an actor and a system to achieve a goal). Such diagrams typically illustrate how different parts of a system interact with each other, and in what order, to achieve an outcome. They are most commonly used by software developers but have been increasingly adopted within business settings as well, due to its ability to show how a system operates clearly.
This interactive behavior is represented in UML by two diagrams known as Sequence diagram and Collaboration diagram. The basic purpose of both the diagrams are similar. Sequence diagram emphasizes on time sequence of messages and collaboration diagram emphasizes on the structural organization of the objects that send and receive messages.
How Can a Sequence Diagram Helps You?
So when should sequence diagrams be used, and what can they be used for?
- To illustrate interactions between actors (system users) and the system, or various parts of a system.
- To show the time order in which events occur.
- To be used as a handy form of documentation on how a system functions. This documentation can be passed on to other persons or organizations.
Sequence Diagram Symbols
Now that we have discussed what it is and its purpose, let’s take a look at some commonly used symbols and components in a sequence diagram.
Objects or parts of a system that interact during the sequence.
Two or more message sequences exist, and a choice must be made between the two of them.
Represents a repetitive sequence.
A sequence can only occur under certain circumstances. Otherwise, it will fail.
Represents an active object during an interaction between two objects. The length of the bar represents the duration of an object’s activeness.
Ends the lifeline of objects or messages.
Can be a human user or other external subjects.
Represents system data.
Represents software element in a system.
Organizes entity and boundary elements.
Used to contain additional useful information for the modeller/developer.
Sender of the message must wait for the receiver to process the message before continuing to send another message.
Receiver of the message passes information back to the sender.
Message sender continues sending messages without waiting for the receiver to process its messages.
When an object sends a message to itself.
How to Draw a Sequence Diagram in EdrawMax?
EdrawMax’s intuitive software allows you to create your sequence diagrams. Follow this outline to create one that fits your needs with minimum effort.
- Step 1: In your browser, visit https://www.edrawmax.com/online/ to access the online version, or download the desktop version at https://www.edrawsoft.com/download-edrawmax.html
- Step 2: In the white sidebar, click on Software Development. Then, click on the UML Modelling box that appears near the top of the screen.
- Step 3: Choose a template, or click on the UML Modelling box with the “ + ” to open a new document. Alternatively, you may type in “sequence diagram” in the search bar above and use a ready-made template.
- Step 4: The symbol library can be found to the left of the screen. Scroll down further to find more sequence diagram-related symbols.
- Step 5: Simply drag the components you need from the symbol library onto the blank document to start creating. Each element can be moved, resized, or customized to fit your needs.
Sequence Diagram Examples
Here are examples of sequence diagrams created on EdrawMax.
Diagram 1: a customer logging into their online shopping account. This diagram shows what occurs during the verification process.
Diagram 2: ATM withdrawal. This diagram shows what interactions occur between the customer, the ATM machine and the bank during a cash withdrawal.
In any seemingly simple operation, there are always multiple parts interacting to create a certain outcome. Sequence diagrams are a fantastic way of visualizing and illustrating complex processes, allowing you to break down all of the interactions and see how a system operates on a macro scale. Use Edraw Max’s sequence diagram tools to create your sequence diagrams now!
A sequence diagram is a form of interaction diagram which shows objects as lifelines running down the page, with their interactions over time represented as messages drawn as arrows from the source lifeline to the target lifeline. Sequence diagrams are good at showing which objects communicate with which other objects; and what messages trigger those communications. Sequence diagrams are not intended for showing complex procedural logic.
A lifeline represents an individual participant in a sequence diagram. A lifeline will usually have a rectangle containing its object name. If its name is 'self', that indicates that the lifeline represents the classifier which owns the sequence diagram.
Sometimes a sequence diagram will have a lifeline with an actor element symbol at its head. This will usually be the case if the sequence diagram is owned by a use case. Boundary, control and entity elements from robustness diagrams can also own lifelines.
Messages are displayed as arrows. Messages can be complete, lost or found; synchronous or asynchronous; call or signal. In the following diagram, the first message is a synchronous message (denoted by the solid arrowhead) complete with an implicit return message; the second message is asynchronous (denoted by line arrowhead), and the third is the asynchronous return message (denoted by the dashed line).
A thin rectangle running down the lifeline denotes the execution occurrence, or activation of a focus of control. In the previous diagram, there are three execution occurrences. The first is the source object sending two messages and receiving two replies; the second is the target object receiving a synchronous message and returning a reply; and the third is the target object receiving an asynchronous message and returning a reply.
A self message can represent a recursive call of an operation, or one method calling another method belonging to the same object. It is shown as creating a nested focus of control in the lifeline’s execution occurrence.
Lost and Found Messages
Lost messages are those that are either sent but do not arrive at the intended recipient, or which go to a recipient not shown on the current diagram. Found messages are those that arrive from an unknown sender, or from a sender not shown on the current diagram. They are denoted going to or coming from an endpoint element.
Lifeline Start and End
A lifeline may be created or destroyed during the timescale represented by a sequence diagram. In the latter case, the lifeline is terminated by a stop symbol, represented as a cross. In the former case, the symbol at the head of the lifeline is shown at a lower level down the page than the symbol of the object that caused the creation. The following diagram shows an object being created and destroyed.
Duration and Time Constraints
By default, a message is shown as a horizontal line. Since the lifeline represents the passage of time down the screen, when modelling a real-time system, or even a time-bound business process, it can be important to consider the length of time it takes to perform actions. By setting a duration constraint for a message, the message will be shown as a sloping line.
Combined FragmentsIt was stated earlier that sequence diagrams are not intended for showing complex procedural logic. While this is the case, there are a number of mechanisms that do allow for adding a degree of procedural logic to diagrams and which come under the heading of combined fragments. A combined fragment is one or more processing sequence enclosed in a frame and executed under specific named circumstances. The fragments available are:
- Alternative fragment (denoted “alt”) models if…then…else constructs.
- Option fragment (denoted “opt”) models switch constructs.
- Break fragment models an alternative sequence of events that is processed instead of the whole of the rest of the diagram.
- Parallel fragment (denoted “par”) models concurrent processing.
- Weak sequencing fragment (denoted “seq”) encloses a number of sequences for which all the messages must be processed in a preceding segment before the following segment can start, but which does not impose any sequencing within a segment on messages that don’t share a lifeline.
- Strict sequencing fragment (denoted “strict”) encloses a series of messages which must be processed in the given order.
- Negative fragment (denoted “neg”) encloses an invalid series of messages.
- Critical fragment encloses a critical section.
- Ignore fragment declares a message or message to be of no interest if it appears in the current context.
- Consider fragment is in effect the opposite of the ignore fragment: any message not included in the consider fragment should be ignored.
- Assertion fragment (denoted “assert”) designates that any sequence not shown as an operand of the assertion is invalid.
- Loop fragment encloses a series of messages which are repeated.
There is also an interaction occurrence, which is similar to a combined fragment. An interaction occurrence is a reference to another diagram which has the word 'ref' in the top left corner of the frame, and has the name of the referenced diagram shown in the middle of the frame.
A gate is a connection point for connecting a message inside a fragment with a message outside a fragment. EA shows a gate as a small square on a fragment frame. Diagram gates act as off-page connectors for sequence diagrams, representing the source of incoming messages or the target of outgoing messages. The following two diagrams show how they might be used in practice. Note that the gate on the top level diagram is the point at which the message arrowhead touches the reference fragment - there is no need to render it as a box shape.
An object can have more than one lifeline coming from it. This allows for inter- and intra-object messages to be displayed on the same diagram.
State Invariant / Continuations
A state invariant is a constraint placed on a lifeline that must be true at run-time. It is shown as a rectangle with semi-circular ends.
Uml System Sequence Diagram Software
A continuation has the same notation as a state invariant, but is used in combined fragments and can stretch across more than one lifeline.