The Airbus flight control system; The Ariane 5 launch accident. Ian Sommerville and Pearson Education. They may not be edited, sold or distributed. This new 10 th edition of Sommerville introduces students to software engineering by focusing on the proven processes and techniques fundamental to the creation of reliable software systems. This all leads to a shared feeling of product ownership and a very cohesive team.
Plan-based development Team cohesion is the responsibility of the project manager and he or she has to take explicit actions to encourage this. The general approach relies on formal meetings that are relatively infrequent and this does not lead to the development of a cohesive team.
Managing changes in project team membership Scrum This is a topic that is rarely discussed in Scrum but is a fundamental problem because so much information is informal and reliant on people remembering what has been agreed.
When someone leaves, it can be very difficult to bring a replacement team member up to speed, especially if very little project documentation is available. Therefore, if a team member leaves, then a new team member with comparable expertise can read what has been done and, after understanding this, should be able to serve as a replacement. Project planning is often essential when developing software with larger teams to a ensure that the right people are available when they are needed to be involved in the development process and b ensure that the delivery schedules of different parts of the system developed by different teams are aligned.
Requirements analysis and documentation is important to decide how to distribute the work across teams and to ensure that each team has some understanding of what other teams are doing. Design documentation especially interface specifications are important so that teams can develop independently without having access to software that is under development.
Risk management may be required to ensure that all of the teams understand the risks faced and can organize their work to minimize these risks. Risk management may also be useful to cope with different delivery schedules used by different teams.
That is, they adopt the outlook of the development team and lose sight of the needs of their user colleagues. Suggest three ways how you might avoid this problem and discuss the advantages and disadvantages of each approach.
Involve multiple users in the development team. Advantages are you get multiple perspectives on the problem, better coverage of user tasks and hence requirements and less likelihood of having an atypical user. Disadvantages are cost, difficulties of getting user engagement and possible user conflicts. Change the user who is involved with the team. Advantages are, again, multiple perspectives. Disadvantages are each user takes time to be productive and possible conflicting requirements from different users.
Validate user suggestions with other user representatives. Advantages are independent check on suggestions; disadvantage is that this slows down the development process as it takes time to do the checks. Users select their destination and input a credit card and a personal identification number. The rail ticket is issued and their credit card account charged.
When the user presses the start button, a menu display of potential destinations is activated, along with a message to the user to select a destination and the type of ticket required. Once a destination has been selected, the ticket price is displayed and customers are asked to input their credit card. Its validity is checked and the user is then asked to input their personal identifier PIN. When the credit transaction has been validated, the ticket is issued.
Ambiguities and omissions include: 1. Can a customer buy several tickets for the same destination together or must they be bought one at a time? Can customers cancel a request if a mistake has been made? How should the system respond if an invalid card is input? What happens if customers try to put their card in before selecting a destination as they would in ATM machines?
Must the user press the start button again if they wish to buy another ticket to a different destination? Should the system only sell tickets between the station where the machine is situated and direct connections or should it include all possible destinations?
Possible non-functional requirements for the ticket issuing system include: 1. Between and in any one day, the total system down time should not exceed 5 minutes. Between and in any one day, the recovery time after a system failure should not exceed 2 minutes. Between and in any one day, the total system down time should not exceed 20 minutes.
All these are availability requirements — note that these vary according to the time of day. Failures when most people are traveling are less acceptable than failures when there are few customers. After the customer presses a button on the machine, the display should be updated within 0. The ticket issuing time after credit card validation has been received should not exceed 10 seconds. When validating credit cards, the display should provide a status message for customers indicating that activity is taking place.
This tells the customer that the potentially time consuming activity of validation is still in progress and that the system has not simply failed.
The maximum acceptable failure rate for ticket issue requests is 1: I have not specified the acceptable number of incorrect tickets as this depends on whether or not the system includes trace facilities that allow customer requests to be logged.
If so, a relatively high failure rate is acceptable as customers can complain and get refunds. If not, only a very low failure rate is acceptable. Obviously, these requirements are arbitrary and there are many other possible answers. You simply have to examine their credibility. Keeping track of the relationships between functional and non-functional requirements is difficult because non-functional requirements are sometimes system level requirements rather than requirements which are specific to a single function or group of functions.
One approach that can be used is to explicitly identify system-level non- functional requirements that are associated with a functional requirement and list them separately. All system requirements that are relevant for each functional requirement should be listed.
They can be related by including them in a table as shown below. Related non-functional Non-functional Functional requirement system requirements requirements The system shall provide Safety requirement: No Timing requirement: The an operation which release of steam shall be valve must open allows operators to open permitted if maintenance completely within 2 the release valve to vent work is being carried out seconds of the operator steam into the on any steam generation initiating the action.
Notice that in this example, the system non-functional requirement would normally take precedence over the timing requirement, which applied to the specific operation. Obviously, any sensible answer that provides a way of linking functional and non- functional requirements is acceptable here. There are a variety of different types of ATM so, obviously, there is not a definitive set of use cases that could be produced.
However, I would expect to see use cases covering the principal functions such as withdraw cash, display balance, print statement, change PIN and deposit cash. The use case description should describe the actors involved, the inputs and outputs, normal operation and exceptions.
The Withdraw cash option is selected. The customer is promoted with a request for the amount of cash required and inputs the amount. Before the cash is dispensed, the card is returned to the customer who is prompted by the machine to take their card.
Exception: Invalid card. Card is retained by machine; Customer advised to seek advice. I have designed by textbook to be a hybrid print-web book and the supplements here provide additional information over…. Software Engineering, Tenth Edition A comprehensive textbook on software engineering The tenth edition of my Software Engineering textbook was published in April Twitter: iansommerville.
Email: iansommerville at geemail. Introduction The latest tenth edition of my Software Engineering textbook was published in Case Studies All of the case studies that are shown here are used in the book.
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