程序代写 Software Engineering – cscodehelp代写
Software Engineering
Dr Kingsley Sage
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Development process
• Waterfall • Agile
Waterfall process model
• Akathe“plan based” approach
• This basic concept has been around for a long time …
• It works well in the right circumstances
– Where we can actually generate a full and proper set of requirements
– So the customer knows what they want and has been able to communicate that to us
– So creativity is not at much of a premium
– But strength of planning, delivery and reliability are important
– And we have the time to have a full and thorough technical development
– Basically the time to do the job properly and on a long lasting basis
Isn’t that what we should always do?
• Sometimes yes
• Rapid development and delivery is now often the most important requirement for software systems
– Businesses operate in a fast –changing requirement and it is practically impossible to produce a set of stable software requirements
– Software has to evolve quickly to reflect changing business needs
• Rapid software development
– Specification, design and implementation are inter-leaved
– System is developed as a series of versions with stakeholders involved in version evaluation
• Dissatisfaction with the overheads involved in software design methods of the 1980s and 1990s led to the creation of Agile methods. These methods:
– Focus on the code rather than the design
– Are based on an iterative approach to software development
– Are intended to deliver working software quickly and evolve this quickly to meet changing requirements
• The aim of agile methods is to reduce overheads in the software process (e.g. by limiting documentation) and to be able to respond quickly to changing requirements without excessive rework
An Agile manifesto
Great, forgot about documentation then ..
• Agile is NOT an excuse for poor design, rubbish coding and non-existent documentation
• However, these things can be the reality of a poorly managed Agile development process
• https://effectivesoftwaredesign.com/2014/03/17/the- end-of-agile-death-by-over-simplification/
Principles of Agile
Applicability of Agile
• Product development where a software company is developing a small or medium-sized product for sale.
• Custom system development within an organization, where there is a clear commitment from the customer to become involved in the development process and where there are not a lot of external rules and regulations that affect the software.
• Because of their focus on small, tightly-integrated teams, there are problems in scaling agile methods to large systems.
Problems with Agile methods
• It can be difficult to keep the interest of customers who are involved in the process.
• Team members may be unsuited to the intense involvement that characterises agile methods.
• Prioritising changes can be difficult where there are multiple stakeholders.
• Contracts may be a problem as with other approaches to iterative development (how do you you measure how well your team is doing?)
Agile and software maintenance
• Most organizations spend more on maintaining existing software than they do on new software development. So, if agile methods are to be successful, they have to support maintenance as well as original development.
– Are systems that are developed using an agile approach maintainable, given the emphasis in the development process of minimizing formal documentation?
– Can agile methods be used effectively for evolving a system in response to customer change requests?
• Problems may arise if original development team cannot be maintained.
Plan based vs waterfall
“Doing” XP: Extreme programming
• Perhaps the best-known and most widely used agile method.
• Extreme Programming (XP) takes an ‘extreme’ approach to iterative development.
– New versions may be built several times per day;
– Increments are delivered to customers every 2 weeks;
– All tests must be run for every build and the build is only accepted if tests run successfully.
• Nothing at all to do with Windows XP!
XP and Agile
• Incremental development is supported through small, frequent system releases.
• Customer involvement means full-time customer engagement with the team.
• People not process through pair programming, collective ownership and a process that avoids long working hours.
• Maintaining simplicity through constant refactoring of code.
The XP release cycle
Requirements scenarios
• In XP, a customer or user is part of the XP team and is responsible for making decisions on requirements.
• User requirements are expressed as scenarios or user stories.
• These are written on cards and the development team break them down into implementation tasks. These tasks are the basis of schedule and cost estimates.
• The customer chooses the stories for inclusion in the next release based on their priorities and the schedule estimates.
A prescribing medication story
Example task cards
XP and change
• Conventional wisdom in software engineering is to design for change. It is worth spending time and effort anticipating changes as this reduces costs later in the life cycle.
• XP, however, maintains that this is not worthwhile as changes cannot be reliably anticipated.
• Rather, it proposes constant code improvement (refactoring) to make changes easier when they have to be implemented.
Refactoring
• Programming team look for possible software improvements and make these improvements even where there is no immediate need for them.
• This improves the understandability of the software and so reduces the need for documentation.
• Changes are easier to make because the code is well structured and clear.
• However, some changes requires architecture refactoring and this is much more expensive.
Examples of refactoring
• Re-organization of a class hierarchy to remove duplicate code.
• Tidying up and renaming attributes and methods to make them easier to understand.
• The replacement of inline code with calls to methods that have been included in a program library.
• Two very different approaches to the development lifecycle
– Waterfall – Agile
• But that does not mean exclusively “one of the other”
• Hybrid approached can try to take the best of both
• And your coursework will likely have hybrid elements
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