340 2 study pt2

The design process for identifying the subsystems making up a system and the framework for subsystem control and communication is

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

Architecture in the large is concerned with the architecture of

complex enterprise systems that include other systems, programs, and program components. These enterprise systems are distributed over different computers, which may be owned and managed by different companies.

Architectures may be documented from several different perspectives or views such as a

conceptual view, a logical view, a process view, and a development view.

Architectural design decisions include

decisions on the type of application, the distribution of the system, the architectural styles to be used

Client-server architecture Distributed system model which shows how data and processing is distributed across a range of components

§Can be implemented on a single computer.

²Availability

§Include redundant components and mechanisms for fault tolerance.

Each architectural model only shows one view or perspective of the system

§It might show how a system is decomposed into modules, how the run-time processes interact or the different ways in which system components are distributed across a network. For both design and documentation, you usually need to present multiple views of the software architecture.

²Performance

§Localise critical operations and minimise communications. Use large rather than fine-grain components.

Safety

§Localise safety-critical features in a small number of sub-systems.

Repository architecture ²Sub-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

Information systems architecture ²These are transaction-based systems as interaction with these systems generally involves database transactions. Layers include

§The user interface §User communications §Information retrieval §System database

Security

§Use a layered architecture with critical assets in the inner layers.

²Maintainability

§Use fine-grain, replaceable components.

Architecture and system characteristics

²Performance §Localise critical operations and minimise communications. Use large rather than fine-grain components. ²Security §Use a layered architecture with critical assets in the inner layers. ²Safety §Localise safety-critical features in a small number of sub-systems. ²Availability §Include redundant components and mechanisms for fault tolerance. ²Maintainability §Use fine-grain, replaceable components.

Architectural representations

²Simple, informal block diagrams showing entities and relationships are the most frequently used method for documenting software architectures. ²But these have been criticized because they lack semantics, do not show the types of relationships between entities nor the visible properties of entities in the architecture. ²Depends on the use of architectural models. The requirements for model semantics depends on how the models are used.

Advantages of explicit architecture

Stakeholder communication; System analysis; Large-scale reuse

A software architecture is

a description of how a software system is organized.

Transaction processing systems are interactive systems that allow

information in a database to be remotely accessed and modified by a number of users.

When large amounts of data are to be shared, the repository model of sharing is

most commonly used a this is an efficient data sharing mechanism

Architecture in the small is concerned with the architecture of individual programs. At this level, we are concerned with

the way that an individual program is decomposed into components.

Language processing systems are used

to translate texts from one language into another and to carry out the instructions specified in the input language. They include a translator and an abstract machine that executes the generated language.

Architectural design decisions

²Architectural design is a creative process so the process differs depending on the type of system being developed. ²However, a number of common decisions span all design processes and these decisions affect the non-functional characteristics of the system. ²Is there a generic application architecture that can be used? ²How will the system be distributed? ²What architectural styles are appropriate? ²What approach will be used to structure the system? ²How will the system be decomposed into modules? ²What control strategy should be used? ²How will the architectural design be evaluated? How should the architecture be documented?

Use of application architectures

²As a starting point for architectural design. ²As a design checklist. ²As a way of organising the work of the development team. ²As a means of assessing components for reuse. ²As a vocabulary for talking about application types.

Use of architectural models

²As a way of facilitating discussion about the system design §A high-level architectural view of a system is useful for communication with system stakeholders and project planning because it is not cluttered with detail. Stakeholders can relate to it and understand an abstract view of the system. They can then discuss the system as a whole without being confused by detail. ²As a way of documenting an architecture that has been designed §The aim here is to produce a complete system model that shows the different components in a system, their interfaces and their connections.

Examples of application types

²Data processing applications ²Transaction processing applications ²Event processing systems ²Language processing systems

Pipe and filter architecture

²Functional transformations process their inputs to produce outputs. ²May be referred to as a pipe and filter model (as in UNIX shell). ²Variants 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.

§Compilers; §Command interpreters.

²Language processing systems

²Set of clients which call on these services.

²Network which allows clients to access servers.

Application architectures 1. Application systems are designed to meet 2. As businesses have much in common, their application systems also 3. A generic application architecture is an architecture for a type of software system that

1. Organizational need 2. tend to have a common architecture that reflects the application requirements. 3. may be configured and adapted to create a system that meets specific requirements.

Chapter 6 - Architectural Design

Architectural design decisions Architectural views Architectural patterns Application architectures

Architectural patterns are a means of reusing knowledge about generic system architectures.

They describe the architecture, explain when it may be used and describe its advantages and disadvantages.

Information systems have a generic architecture that

can be organised as a layered architecture

Set of stand-alone servers which provide specific services such as

printing, data management, etc.

Architecture reuse

²Systems in the same domain often have similar architectures that reflect domain concepts. ²Application product lines are built around a core architecture with variants that satisfy particular customer requirements. ²The architecture of a system may be designed around one of more architectural patterns or 'styles'. §These capture the essence of an architecture and can be instantiated in different ways. §Discussed later in this lecture.

§E-commerce systems; §Reservation systems.

²Transaction processing systems

Box and Line Diagrams

²Very abstract - they do not show the nature of component relationships nor the externally visible properties of the sub-systems. ²However, useful for communication with stakeholders and for project planning.

Models of application systems architectures help us

²understand and compare applications, validate application system designs and assess large-scale components for reuse.