Business analysts use sequence or event diagrams to model the logic of management scenarios by showing the information passed between objects in the system through the execution of the scenario.
A sequence diagram shows how processes are connected during a scenario. The classes needed to implement the scenario and the messages they pass to one another; which are triggered by steps in the use case are displayed on the diagram.
The sequence diagram shows how the objects used in the scenario are connected, but not how they are related to one another.
Sequence diagrams are also used to show how software components interact. The diagram depicts information in a horizontal and vertical alignment.
The objects that send messages to each other are depicted as boxes that are aligned at the top of the page from the left to the right, with each object
occupying a column of space on the page bordered by a vertical line stretching down to the bottom of the page.
The messages that are sent from one object to the next are depicted with horizontal arrows. The order of the messages is represented in a top-down and left-to-right sequence starting with the first message at the top left of the page and successive messages appear to the right and below.
Sequence diagrams have some components, which include:
1. Lifeline: A lifeline displays the lifespan of an object during the scenario being modelled in a sequence diagram. A lifeline is drawn as a dashed line that vertically go down from each object box to the bottom of the page.
2 Activation Box: An activation box displays the period during which an operation is implemented. A call to activate is depicted by an arrow with a solid arrowhead leading to the activation object. The lifeline can be terminated with an X.
3. Message: A message is a connection between two objects. A message is shown as an arrow coming from the activation box of the object that sends the message to the activation box of the object that receives the message.
The name of the message is placed on top of the arrowed line. There are different types of messages, which include:
- Synchronous call: this is when control is transferred to the receiving object. The sender cannot act until a return message is received.
- Asynchronous call or signal: this allows the object to continue with its own processing after the signal is sent. The object may send many signals simultaneously, but may only accept one signal at a time.
Sequence diagrams have their strengths and limitations, which include the following:
Strengths
• It shows the relationships between the objects of a system in the chronological order that the interactions occur.
• It shows the relationship between the objects in a visual manner that allows the logic to be easily validated by stakeholders.
• Use cases can be broken down into one or more sequence diagrams to
provide more detail and understanding of a business process.
Limitations
• It takes a lot of time and effort to create a complete set of sequence diagrams for each use case of a system.
• They are usually used for modelling system flows and could be too technical for the stakeholders to understand.