Chapter 6

A module

is a system component that provides services to other components but would not normally be considered as a separate system.

A sub-system

is a system in its own right whose operation is independent of the services provided by other sub-systems.

System organization

A shared data repository style; A shared services and servers style; An abstract machine or layered style.

Client-server characteristics

Advantages Distribution of data is straightforward; Makes effective use of networked systems. May require cheaper hardware; Easy to add new servers or upgrade existing servers. Disadvantages No shared data model so sub-systems use different data organisation. Data interchange may be inefficient; Redundant management in each server; No central register of names and services - it may be hard to find out what servers and services are available.

Repository model characteristics

Advantages Efficient way to share large amounts of data; Sub-systems need not be concerned with how data is produced Centralised management e.g. backup, security, etc. Sharing model is published as the repository schema. Disadvantages Sub-systems must agree on a repository data model. Inevitably a compromise; Data evolution is difficult and expensive; No scope for specific management policies; Difficult to distribute efficiently.

Modular decomposition

Another structural level where sub-systems are decomposed into modules. Two modular decomposition models covered An object model where the system is decomposed into interacting object; A pipeline or data-flow model where the system is decomposed into functional modules which transform inputs to outputs.

Reference architectures

Architectural models may be specific to some application domain. Two types of domain-specific model Generic models which are abstractions from a number of real systems and which encapsulate the principal characteristics of these systems. Covered in Chapter 13. Reference models which are more abstract, idealised model. Provide a means of information about that class of system and of comparing different architectures. Generic models are usually bottom-up models; Reference models are top-down models.

Control styles

Are concerned with the control flow between sub-systems. Distinct from the system decomposition model.

Case reference model

Data repository services: Storage and management of data items. Data integration services: Managing groups of entities. Task management services: Definition and enaction of process models. Messaging services: Tool-tool and tool-environment communication. User interface services: User interface development.

Client-server model

Distributed system model which shows how data and processing is distributed across a range of components. Set of stand-alone servers which provide specific services such as printing, data management, etc. Set of clients which call on these services. Network which allows clients to access servers.

Event-based control

Each sub-system can respond to externally generated events from other sub-systems or the system's environment. Two principal event-driven models Broadcast models. An event is broadcast to all sub-systems. Any sub-system which can handle the event may do so; Interrupt-driven models. Used in real-time systems where interrupts are detected by an interrupt handler and passed to some other component for processing.

Function-oriented pipelining

Functional transformations process their inputs to produce outputs. May be referred to as a pipe and filter model (as in UNIX shell). Variants of this approach are very common. When transformations are sequential, this is a batch sequential model which is extensively used in data processing systems. Not really suitable for interactive systems.

Object model advantages

Objects are loosely coupled so their implementation can be modified without affecting other objects. The objects may reflect real-world entities. OO implementation languages are widely used. However, object interface changes may cause problems and complex entities may be hard to represent as objects.

Centralised control

One sub-system has overall responsibility for control and starts and stops other sub-systems. Call-return model Top-down subroutine model where control starts at the top of a subroutine hierarchy and moves downwards. Applicable to sequential systems. Manager model Applicable to concurrent systems. One system component controls the stopping, starting and coordination of other system processes. Can be implemented in sequential systems as a case statement.

Architecture and system characteristics

Performance Security Safety Availability Maintainability

Advantages of explicit architecture

Stakeholder communication System analysis Large-scale reuse

The repository model

Sub-systems must exchange data. This may be done in two ways: Shared data is held in a central database or repository and may be accessed by all sub-systems; Each sub-system maintains its own database and passes data explicitly to other sub-systems. When large amounts of data are to be shared, the repository model of sharing is most commonly used.

Pipeline model advantages

Supports transformation reuse. Intuitive organisation for stakeholder communication. Easy to add new transformations. Relatively simple to implement as either a concurrent or sequential system. However, requires a common format for data transfer along the pipeline and difficult to support event-based interaction.

Architectural design

The design process for identifying the sub-systems making up a system and the framework for sub-system control and communication

Software architecture

The output of this design process is a description of the software architecture.

Architectural conflicts

Using large-grain components improves performance but reduces maintainability. Introducing redundant data improves availability but makes security more difficult. Localising safety-related features usually means more communication so degraded performance.