LARMAN CD MODULES

Module 5

Creating System Sequence Diagrams

Introduction

At this point in the CD Modules we have created our Use Cases in text and perhaps the odd Use Case Diagram. In Module 5, Larman introduces us to the System Sequence Diagram (SSD).

What is a System Sequence Diagram?

The SSD is a visual summary of the events that will be hitting our system or, in other words, what the system has to react to. These are commonly called "system events" and are external input events that are generated by some actor.

How do we create a System Sequence Diagram?

General Idea:

The idea is to take a Use Case (text document) and create a UML diagram showing the input and output events for that particular Use Case. In other words, we are forming a relationship between the text document and the SSD. It is important to note that we are treating our system as a "black box" for the purposes of the SSD. As a black box, we do not show what is happening inside the system, we are just showing that the system is receiving events and responding to them.

Show collaboration/interaction with other systems:

The SSD can also be used to show collaboration between different systems. For example, in our beloved NextGen POS system we can show the POS collaborating with the Credit Authority Service. After all, these are all identified as actors in our Use Case Model. If an external system has to reply, we show that the reply is returned.

Naming our Events:

When naming an event in the SSD, it is proper at this stage to come up with a semantic description of the intention of the event (as it is unlikely at this stage that we will have any idea what our methods will ultimately be named). Also, by using a semantic description we are abstracting the event from any underlying technology.

The Diagram:

Speaking of diagrams, the following diagram is a SSD for our very familiar use case: Process Sale.

We can see that the initial input event came from the Cashier, when the cashier started a new sale. From the Cashier, there are further input events as new items are added to the sale. As a new item is added, the system responds with a description and total. Once the sale is complete an "end sale" event comes from the cashier, and the system responds with the total for the sale, including any tax calculated. Finally a "payment" event comes from the cashier, and the system responds with the change due and the receipt.

Note, this diagram illustrates the "black box" system concept. At no stage are we depicting how the System goes about doing its job.

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