Use case model drawing system sequence diagrams
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Use-case model: drawing system sequence diagrams. Objectives. Identify system events. Create system sequence diagrams for use cases. Moving on to Iteration 1. What have we done?

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Use-case model: drawing system sequence diagrams

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Use-case model: drawing system sequence diagrams


Objectives

  • Identify system events.

  • Create system sequence diagrams for use cases.


Moving on to Iteration 1

What have we done?

  • Some light requirements work was done in inception to help decide if the project was worth more serious investigation.

  • Planning for the first iteration has been completed

  • We have decided to tackle a simple cash-only success scenario of Process Sale(with no remote collaborations), with the goal of starting a "wide and shallow" design and implementation that touches on many major architectural elements of the new system.

  • In the first iteration, many tasks related to establishing the environment (tools, people, process, and setting) occur

    Our focuses from now on are:

  • Understand what problems are (Analysis), rather than how to solve problems (Design)

  • Analysis - use case and domain modeling analysis.


Introduction – System Sequence Diagram

  • A system sequence diagram is a fast and easily created artifact that illustrates input and output events related to the systems under discussion

  • Before proceeding to a logical design of how a software application will work, we should investigate and define the system behavior as a "black box“.


System Event and Response

  • Use cases describe how external actors interact with the software system.

  • During this interaction an actor generates events to a system, usually requesting some operation in response.

    For example, when a cashier enters an item's ID, the cashier is requesting the POS system to record that item's sale. That request event initiates an operation upon the system.


System sequence diagram

  • A system sequence diagram(SSD) is a picture that shows, for a particular scenario of a use case, the events that external actors generate, their order, and inter-system events.

    • The emphasis of the diagram is events that cross the system boundary from actors to systems.

    • An SSD should be done for the main success scenario of the use case, and frequent or complex alternative scenarios.

    • An SSD is generated from inspection of a use case

      Suggestion:

  • One SSD – one Use Case


Case Study: How to do SSD in the POS Project

  • Simple cash-only Process Salescenario:

    1. Customer arrives at a POS checkout with goods and/or services to purchase.

    2. Cashier starts a new sale.

    3. Cashier enters item identifier.

    4. System records sale line item and presents item description, price, and running total.

    Cashier repeats steps 3-4 until indicates done.

    5. System presents total with taxes calculated.

    6. Cashier tells Customer the total, and asks for payment.

    7. Customer pays and System handles payment.

    ...


Step 1: Define System Events and the System Boundary

the system boundary is usually chosen to be the software system itself; in this context, a system event is an external event that directly stimulates the software


Step 2: Naming System Events and Operations


Step 3: show Use Case

  • Simple cash-only Process Salescenario:

    1. Customer arrives at a POS checkout with goods and/or services to purchase.

    2. Cashier starts a new sale.

    3. Cashier enters item identifier.

    4. System records sale line item and presents item description, price, and running total.

    Cashier repeats steps 3-4 until indicates done.

    5. System presents total with taxes calculated.

    6. Cashier tells Customer the total, and asks for payment.

    7. Customer pays and System handles payment.

    ...


Step 4: Do SSD


SSD of UC - Process Sale


Relationships of SSDs to other artifacts


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