Prototyping Notes
Software prototyping
- Animating and demonstrating system requirements.
Objectives
- To describe the use of prototypes in requirements validation;
- To discuss evolutionary and throw-away prototyping;
- To introduce rapid prototyping techniques;
- To explain the need for user interface prototyping.
Topics covered
- Prototyping in the software process;
- Prototyping techniques;
- User interface prototyping.
Uses of system prototypes
- The principle is to help customers and developers understand the requirements for the system;
- The prototype may be used for user training before a final system is delivered;
- The prototype may be used for back-to-back testing.
Prototyping benefits
- Misunderstanding between software users and developers are exposed;
- Missing services may be detected;
- Confusing services may be identified;
- A working system is available early in the system;
- The prototype may serve as a basis for deriving a system specification.
Prototyping objectives
- The objective of evolutionary prototyping is to deliver a working system to end-users. The development starts with those requirements which are best understood;
- The objective of throw-away prototyping is to validate or derive the systems requirements. The prototyping process starts with those requirements which are poorly understood.
Evolutionary prototyping
- Must be used for system where the specification cannot be developed in advance;
- Based on techniques which allow rapid system iterations;
- Verification is impossible as there is no specification. Validation means demonstrating the adequacy of the system.
Evol. Prototyping problems
- Existing management processes assume a waterfall method of development;
- Continual change tends to corrupt system structure so long-term maintenance is expensive;
- Specialist skills are required which may not be available in all development teams;
- Organisations must accept that the lifetime of systems developed this way will inevitable be short.
Throw-away prototyping
- Used to reduce requirements risk;
- The prototype is developed from an initial specification, delivered for experiment then discarded;
- The throw-away prototype should NOT be considered a final system
o Some system characteristics may have been left out;
o There is no specification for long-term maintenance;
o The system will be poorly structured and difficult to maintain.
Prototypes as specifications
- Some parts of the requirements may be impossible to prototype and so don’t appear in the specification;
- An implementation has no legal standing as a contract;
- Non-functional requirements cannot be adequately tested in a system prototype.
Incremental development
- System is developed and developed in increments after establishing an overall architecture;
- Users may experiment with delivered increments while others are being developed. Therefore, these serve as a form of prototype system;
- Intended to combine some of the advantages of prototyping but with a more manageable process and better system structure.
Prototyping techniques
- Executable specification languages;
- Very high-level languages;
- Application generators and 4GL’s;
- Composition of reusable components.
Executable specification languages
- The system is specified in a formal language;
- This specification is processed and an executable system is automatically generated;
- At the end of the process, the specification may serve as a basis for re-implementation of the system.
Problems with this approach
- Graphical user interfaces cannot be prototyped;
- Formal specification development is not a rapid process;
- The executable system is usually slow and inefficient;
- Executable specifications only allow functional requirements to be prototyped.
Very high-level languages
- Languages which include powerful data management facilities;
- Need a large run-time support system. Not normally used for large system development;
- Some languages offer excellent UI development facilities;
- Some languages have an integrated support environment whose facilities may be used in the prototype.
Smalltalk
- Very powerful system for prototyping interactive systems;
- Object-oriented language so systems are resilient to change;
- The smalltalk environment includes support software such as graphical user interface generation tools.
Fourth-generation languages
- Domain specific languages for business systems based around a database management system;
- Normally include a database query language, a screen generator, a report generator, and a spreadsheet;
- May be integrated with a CASE toolset;
- Cost-effective for small to medium sized business systems.
Prototyping with reuse
- The system is prototyped by gluing together existing components;
- Likely to become more widely used as libraries of objects become available;
- Needs a composition language such as Unix shell language;
- Visual basic is largely based on this approach.
User interface prototyping
- It is impossible to pre-specify the look and feel of a user interface in an effective way. Prototyping is essential;
- UI development consumes an increasing part of overall system development costs;
- Prototyping may use very high level languages such as Smalltalk or Lisp;
- User interface generators may be used to draw the interface and simulate its functionality.
Key points
- A prototype can be used to give end-users a concrete impression of the system’s capabilities;
- Prototyping may be evolutionary prototyping or throw-away prototyping;
- Rapid development is essential for prototype systems;
- Prototype structures become corrupted by constant change. Hence long-term evolution is difficult.
Key points
- In a throw-away prototype start with the least well-understood parts; in an evolutionary prototype, start with the best understood parts;
- Prototyping methods include the use of executable specification languages, very high-level languages, fourth-generation languages and prototype construction from reusable components;
- Prototyping is essential for parts of the system such as the user interface which cannot be effectively pre-specified.
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