Ch10 - Software Engineering 9th - Sommerville

Equipment

Hardware devices, some of which may be computers. Most devices will include an embedded system of some kind.

Influences on reliability

Hardware reliability -What is the probability of a hardware component failing and how long does it take to repair that component? Software reliability -How likely is it that a software component will produce an incorrect output. Software failure is usually distinct from hardware failure in that software does not wear out. Operator reliability -How likely is it that the operator of a system will make an error? Failures are not independent and they propagate from one level to another.

Organizations

Higher level strategic business activities that affect the operation of the system.

System evolution - Long Life and High Cost

Large systems have a long lifetime. They must evolve to meet changing requirements. 4 Reasons for the high cost of Evolution in STS: 1. Changes must be analysed from a technical and business perspective; 2. Sub-systems interact so unanticipated problems can arise; 3. There is rarely a rationale for original design decisions; 4. System structure is corrupted as changes are made to it. Existing systems which must be maintained are sometimes called legacy systems.

Society

Laws, regulation and culture that affect the operation of the system.

Requirements and design

Requirements engineering and system design are inextricably linked. Constraints posed by the system's environment and other systems limit design choices so the actual design to be used may be a requirement. Initial design may be necessary to structure the requirements. As you do design, you learn more about the requirements.

Application systems

Specific functionality to meet some organization requirements.

3 Organizational affects of STS - Change, Change, Change!

1. Process changes 2. Job changes 3. Organizational changes

3 Socio-technical system characteristics

1. Emergent properties 2. Non-deterministic 3. Moving organisational objectives against which to measure system Success

The 7 Layers of the STS stack

1. Equipment 2. Operating system 3. Communications and data management 4. Application systems 5. Business processes 6. Organizations 7. Society

Types of emergent property

1. Functional 2. Non-functional

The system design process

1. Partition requirements Organise requirements into related groups. 2. Identify sub-systems Identify a set of sub-systems which collectively can meet the system requirements. 3. Assign requirements to sub-systems Causes particular problems when COTS are integrated. 4. Specify sub-system functionality. 5. Define sub-system interfaces Critical activity for parallel sub-system development.

System categories

1. Technical computer-based systems 2. Socio-technical systems

4 STS Sub-system Development Characteristics

1. Typically parallel projects developing the hardware, software and communications. 2. May involve some COTS (Commercial Off-the-Shelf) systems procurement. 3. Lack of communication across implementation teams can cause problems. 4. There may be a bureaucratic and slow mechanism for proposing system changes, which means that the development schedule may be extended because of the need for rework.

Business processes

A set of processes involving people and computer systems that support the activities of the business.

Complex systems

A system is a purposeful collection of inter-related components working together to achieve some common objective. A system may include software, mechanical, electrical and electronic hardware and be operated by people. System components are dependent on other system components. The properties and behaviour of system components are inextricably inter-mingled. This leads to complexity.

System procurement

Acquiring a system (or systems) to meet some identified organizational need. Before procurement, decisions are made on: -Scope of the system -System budgets and timescales -High-level system requirements Based on this information, decisions are made on whether to procure a system, the type of system and the potential system suppliers.

6 System delivery and deployment Issues

After completion, the system has to be installed in the customer's environment 1. Environmental assumptions may be incorrect; 2. May be human resistance to the introduction of a new system; 3. System may have to coexist with alternative systems for some time; 4. May be physical installation problems (e.g. cabling problems); 5. Data cleanup may be required; 6. Operator training has to be identified.

How much development is required for Large Complex Systems?

At the minimum, specification and architectural design is usually necessary before procurement -You need a specification to let a contract for system development -The specification may allow you to buy a commercial off-the-shelf (COTS) system. Almost always cheaper than developing a system from scratch Large complex systems usually consist of a mix of off the shelf and specially designed components. The procurement processes for these different types of component are usually different.

Reliability as an emergent property

Because of component inter-dependencies, faults can be propagated through the system. System failures often occur because of unforeseen inter-relationships between components. It is practically impossible to anticipate all possible component relationships. Software reliability measures may give a false picture of the overall system reliability.

Evolution and dependability

Changes to a system are often a source of problems and vulnerabilities. Changes may be made without knowledge of previous design decisions made for security and dependability reasons. -Built-in safeguards may stop working. New faults may be introduced or latent faults exposed by changes. These may not be discovered because complete system retesting is too expensive.

Inter-disciplinary working

Communication difficulties -Different disciplines use the same terminology to mean different things. This can lead to misunderstandings about what will be implemented. Differing assumptions -Each discipline makes assumptions about what can and can't be done by other disciplines. Professional boundaries -Each discipline tries to protect their professional boundaries and expertise and this affects their judgments on the system.

Defenses in an ATC system

Conflict alert system -Raises an audible alarm when aircraft are on conflicting paths Recording of instructions -Allows instructions issues to be reviewed and checked. Sharing of information -The team of controllers cross-check each other's work.

Development and dependability

Decisions are made on dependability and security requirements and trade-offs made between costs, schedule, performance and dependability. Human errors may lead to the introduction of faults into the system. Testing and validation processes may be limited because of limited budgets. Problems in deployment mean there may be a mismatch between the system and its operational environment.

Security and dependability considerations

Design options limited by procurement decisions Purchased components may make some safeguards impossible to implement. Human errors made during development may introduce faults into the system. Inadequate testing may mean faults are not discovered before deployment. Configuration errors during deployment may introduce vulnerabilities. Assumptions made during procurement may be forgotten when system changes are made.

Non-functional emergent properties

Examples are reliability, performance, safety, and security. These relate to the behaviour of the system in its operational environment. They are often critical for computer-based systems as failure to achieve some minimal defined level in these properties may make the system unusable.

Human error - Is it Human Fault or the Designer/Developers' Fault?

Human errors occur in operational processes that influence the overall dependability of the system. Viewing human errors: -The person approach makes errors the responsibility of the individual and places the blame for error on the operator concerned. Actions to reduce error include threats of punishment, better training, more stringent procedures, etc. -The systems approach assumes that people are fallible and will make mistakes. The system is designed to detect these mistakes before they lead to system failure. When a failure occurs, the aim is not to blame an individual but to understand why the system defenses did not trap the error.

Communications and data management

Middleware that provides access to remote systems and databases.

System operation

Operational processes are the processes involved in using the system for its defined purpose. For new systems, these processes may have to be designed and tested and operators trained in the use of the system. Operational processes should be flexible to allow operators to cope with problems and periods of fluctuating workload.

Emergent properties

Properties of the system of a whole that depend on the system components and their relationships. Properties of the system as a whole rather than properties that can be derived from the properties of components of a system Emergent properties are a consequence of the relationships between system components They can therefore only be assessed and measured once the components have been integrated into a system Volume, Reliability, Security, Repairability, Usability

Moving organisational objectives against which to measure system Success

Org. obj. change. The extent to which the system supports organisational objectives does not just depend on the system itself. A successful system is deemed unsuccessful with new objectives. Complex systems are developed to address 'wicked problems' - problems where there cannot be a complete specification. Different stakeholders see the problem in different ways and each has a partial understanding of the issues affecting the system. Consequently, different stakeholders have their own views about whether or not a system is 'successful' Success is not judged against the original reasons for system's procurement. Hence, cannot be objectively measured.

System development (of STS)

Plan-driven approach -Why? --The need for parallel development of different parts of the system --System Size --Cost Inevitably involves engineers from different disciplines who must work together -Much scope for misunderstanding here. -As explained, different disciplines use a different vocabulary and much negotiation is required. Engineers may have personal agendas to fulfill.

Systems engineering

Procuring, specifying, designing, implementing, validating, deploying and maintaining socio-technical systems. Concerned with the services provided by the system, constraints on its construction and operation and the ways in which it is used to fulfil its purpose or purposes.

Operating system

Provides a set of common facilities for higher levels in the system.

Procurement issues

Requirements may have to be modified to match the capabilities of off-the-shelf components. The requirements specification may be part of the contract for the development of the system. There is usually a contract negotiation period to agree changes after the contractor to build a system has been selected.

Systems

Software engineering is not an isolated activity but is part of a broader systems engineering process. Software systems are therefore not isolated systems but are essential components of broader systems that have a human, social or organizational purpose. Example Wilderness weather system is part of broader weather recording and forecasting systems These include hardware and software, forecasting processes, system users, the organizations that depend on weather forecasts, etc.

Organizational changes

Systems may change the political power structure in an organization. If an organization depends on a system then those that control the system have more power.

Job changes

Systems may de-skill users or cause changes to the way they work. The status of individuals in an organization may be affected by the introduction of a new system.

Process changes

Systems may require changes to business processes so training may be required. Significant changes may be resisted by users.

Technical computer-based systems

Systems that include hardware and software but where the operators and operational processes are not normally considered to be part of the system. The system is not self-aware. Example: A word processor used to write a book.

Socio-technical systems

Systems that include technical systems but also operational processes and people who use and interact with the technical system. Socio-technical systems are governed by organisational policies and rules. Example: A publishing system to produce a book.

System integration - Ideally Incremental

The process of putting hardware, software and people together to make a system. Should ideally be tackled incrementally so that sub-systems are integrated one at a time. -Allows Regression Testing -The system is tested as it is integrated. Interface problems between sub-systems are usually found at this stage. May be problems with uncoordinated deliveries of system components.

Contractors and sub-contractors

The procurement of large hardware/software systems is usually based around some principal contractor. Sub-contracts are issued to other suppliers to supply parts of the system. Customer liases with the principal contractor and does not deal directly with sub-contractors.

Decision drivers

The state of other organizational systems The need to comply with external regulations External competition Business re-organization Available budget

Holistic system design

There are interactions and dependencies between the layers in a system and changes at one level ripple through the other levels -Example: Change in regulations (society) leads to changes in business processes and application software. For dependability, a systems perspective is essential -Contain software failures within the enclosing layers of the STS stack. -Understand how faults and failures in adjacent layers may affect the software in a system.

Functional emergent properties

These appear when all the parts of a system work together to achieve some objective. For example, a bicycle has the functional property of being a transportation device once it has been assembled from its components.

Non-deterministic

They do not always produce the same output when presented with the same input because the systems's behaviour is partially dependent on human operators. Software systems are deterministic; systems that include humans are non-deterministic A socio-technical system will not always produce the same sequence of outputs from the same input sequence Human elements -People do not always behave in the same way System changes -System behaviour is unpredictable because of frequent changes to hardware, software and data.

System requirements definition

Three types of requirement defined at this stage -Abstract functional requirements. System functions are defined in an abstract way; -System properties. Non-functional requirements for the system in general are defined; -Undesirable characteristics. Unacceptable system behaviour is specified. Should also define overall organisational objectives for the system.

System defenses

To improve security and dependability, designers should think about the checks for human error that should be included in a system. As I discuss in later lectures, there should be multiple (redundant) barriers which should be different (diverse) No single barrier can be perfect. -There will be latent conditions in the system that may lead to failure. However, with multiple barriers, all have to fail for a system failure to occur.