Furthermore, existing procedures would have to change, and this may upset the peo- peo-ple in the organisation and cause difficulties with the bank's auditors

Figure 2.11 illustrates the logical parts of a legacy system and their relationships:

Figure 2.11 Legacy system components

[ . s..

Application software

2 4 II Legacy systems 39

Runs·on Runs-on Uses Uses Constrain

System

hardware [

Application data

Business processes

1. System hardware

In many cases" legacy systems have been written for main-frame hardware that is no longe:r available, that is expensive to maintain and that may not be compatible with current organisational IT purchasing policies.

2. Support software

The legacy system may rely on a range of support software from the operating system and utilities provided by the hardware manufacturer through to the compilers used for system development. Again, these may be obsolete and no longer supported by their original providers.

3. Application software

The applicEltion system that provides the business services is usually composed of a numbc:r of separate programs that have been devel-oped at different times. Sometimes the term

means this applica-tion software system rather than the entire system.

4. Application data

These are the data that are processed by the application sys-tem. In many legacy systems, an immense volume of data has accumulated over the lifetime of the system. This data may be inconsistent and may be dupli-cated in several files.

5. Business processes

These are processes that are used in the business to achieve some business objective. An exarnple of a business process in an insurance com-pany would be issuing an insurance policy; in a manufacturing comcom-pany, a busi-ness process would be accepting alll order for products and setting up the associated manufacturing process. Business processes may be designed around a legacy system and constrained by the functionality that it provides.

6. Business policies and rules

These are definitions of how the business should be carried out and constraints on the business. Use of the legacy application system may be embedded in these policies and rules.

An alternative way of looking at the:se components of a legacy system is as a series

of layers, as shown in Figure 2.12. Each layer depends on the layer immediately below

it and interfaces with that layer.

interfaces are maintained, then you should be able

to make changes within a layer without affecting either of the adjacent layers.

40 Chapter 2 I. Socio-technical systems

Figure2.1 2Layered model of a legacy system

Socia-technical system Business processes

Application software Support software

In practice, this simple encapsulation rarely works, and changes to one layer of the system may require consequent changes to layers that are both above and below the changed level. The reasons for this are:

1.

Changing one layer in the system may introduce new facilities, and higher lay-ers in the system may then be changed to take advantage of these facilities. For example, a new database introduced at the support software layer may include KEY POINTS II

Socio-technical systems include computer hardware, software and people, and are situated within an organisation. They are designed to help the organisation meet some broad goal.

The emergent properties of a system are characteristic of the system as a whole rather than of its component parts. They include properties such as performance, reliability, usability, safety and security. The success or failure of a system is often dependent on these emergent properties.

The systems engineering process includes specification, design, development, integration and testing. System integration, where sub-systems from more than one supplier must be made to work together, is particularly critical.

Human and organisational factors such as organisational structure and politics have a significant effect on the operation of socio-technical systems.

Within an organisation, there are complex interactions between the processes of system procurement, development and operation.

A legacy system is an old system that still provides essential business services.

Legacy systems are not just application software systems. They are socio-technical systems so include business processes, application software, support software and system hardware.

Chapter 2 ,. Exercises 41

facilitiestoaccess the data through a web browser, and business processes may be modified to take advantage of this facility.

2. Changing the software may slow lhe system down so that new hardware is needed to improve the system performance. The increase in performance from the new hardware may then mean that further software changes which were previously impractical become possible.

3. It is often impossible to maintain hardware interfaces, especially if a radical change to a new type of hardware is proposed. For example, if a company moves from mainframe hardware to cl:ent-server systems (discussed in Chapter II) these usually have different operating systems. Major changes to the applica-tion software may therefore be required.

FURl-HER READING

J··I _

'Software system engineen ng: A tutorial'. A goodgenl~ral overview of systems engineering, although Thayer focuses exclusively oncomputer·basl~d systems and does not discuss socio-technical issues. (R. H. Thayer, IEEE Computer, April 2002.)

'Legacy information systems: Issues and directions'. A.n overview of the problems of legacy systems with a particular focus on the problems of legacy data.

O.

SeptemberjOctober1999.)

Systems Engineering: Coping with Complexity.At the time of this writing, this is still the best available systems engineering book. It focuses on systems engineering processes with good chapters on reqUirements, architecture and project management. (R. Stevens, et aI.,1998, Prentice Hall.)

'Airport95:Automated bag;gage system'. An excellent, readable case study of what can go wrong with a systems engineering project and how software tends to get the blame for wider systems failures. (ACM Software Enqineering Notes, 21, March1996.)

EXERCISES

1

-2.1 Explain why other sy:5tems within a system's environment can have unanticipated effects on the functioning of a~,ystem.

2.2 Explain why specifying a system to be used by E'mergency services for disaster management is an inherentlywickl~d problem.

2.3 Suggest how the software systems used in a car can help with the decommissioning (scrapping) of theov,~rall system.

42 Chapter 2 • Socio-technical systems

2.4 Explain why it is important to produce an overall description of a system architecture at an early stage in the system specification process.

2.5 Consider a security system that is an extended version of the system shown in Figure2.6, which is intended to protect against intrusion and to detect fire. It incorporates smoke sensors, movement sensors, door sensors, video cameras under computer control, located at various places in the building, an operator console where the system status is reported, and external communication facilities to call the appropriate services such as the police and fire departments. Draw a block diagram of a possible design for such a system.

2.6 A flood warning system is to be procured which will give early warning of possible flood dangers to sites that are threatened by floods. The system will include a set of sensors to monitor the rate of change of river levels, links to a meteorological system giving weather forecasts, links to the communication systems of emergency services (police, coastguard, etc.), video monitors installed at selected locations, and a control room equipped with operator consoles and video monitors.

Controllers can access database information and switch video displays. The system database includes information about the sensors, the location of sites at risk and the threat conditions for these sites (e.g., high tille, southwesterly winds), tide tables for coastal sites, the inventory and location of flood control equipment, contact details for emergency services, local radio stations, and so on.

Draw a block diagram of a possible architecture for such a system. You should identify the principal sub-systems and the links between them.

2.7 A multimedia virtual museum system offering virtual experiences of ancient Greece is to be developed for a consortium of European museums. The system should provide users with the facility to view 3-D models of ancient Greece through a standard web browser and should also support an immersive virtual reality experience. What political and organisational difficulties might arise when the system is installed in the museums that make up the consortium?

2.8 Explain why legacy systems may be critical to the operation of a business.

2.9 Explain why legacy systems can cause difficulties for companies that wish to reorganise their business processes.

2.10 What are the arguments for and against considering system engineering as a profession in its own right such as electrical engineering or software engineering?

2.11 You are an engineer involved in the development of a financial system. During installation, you discover that this system will make a significant number of people redundant. The people in the environment deny you access to essential information to complete the system installation. To what extent should you, as a systems engineer, become involved in this? Is it your professional responsibility to complete the installation as contracted? Should you simply abandon the work until the procuring organisation has sorted out the problem?

3J