Architecture

transaction processing systems are...

interactive systems that allow information in a database to be remotely accessed and modified by a number of users

MVC

model: data view: how data is presented to user controller: user interaction, passes to view and model

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.

generic layered architecture

user interface UI management, authentication, authorization business logic/ app functionality system support

Architectural models

• Used to document an architectural design. • Static structural model that shows the major system components. • Dynamic process model that shows the process structure of the system. • Interface model that defines sub-system interfaces. • Relationships model such as a data-flow model that shows sub-system relationships. • Distribution model that shows how sub-systems are distributed across computers.

Layered architecture

• Used to model the interfacing of sub-systems. • Organizes the system into a set of layers (or abstract machines) each of which provide a set of services. • Supports the incremental development of sub-systems in different layers. When a layer interface changes, only the adjacent layer is affected. • However, often artificial to structure systems in this way.

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.

Box and line diagrams

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

Architectural views

• What views or perspectives are useful when designing and documenting a system's architecture? • What notations should be used for describing architectural models? • 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.

System architecture affects the...

• performance, • robustness, • distributability, and • maintainability

Architecture and system characteristics

Performance Security

software architecture

output of this design process

Availability

- Include redundant components and mechanisms for fault tolerance.

Performance

- 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.

Security

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

Maintainability

- Use fine-grain, replaceable components.

architectural design

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

Compiler components

• A lexical analyzer, which takes input language tokens and converts them to an internal form. • A symbol table, which holds information about the names of entities (variables, class names, object names, etc.) used in the text that is being translated. • A syntax analyzer, which checks the syntax of the language being translated. • A syntax tree, which is an internal structure representing the program being compiled. • A semantic analyzer that uses information from the syntax tree and the symbol table to check the semantic correctness of the input language text. • A code generator that 'walks' the syntax tree and generates abstract machine code.

Language processing systems

• Accept a natural or artificial language as input and generate some other representation of that language. • May include an interpreter to act on the instructions in the language that is being processed. • Used in situations where the easiest way to solve a problem is to describe an algorithm or describe the system data

Architectural design

• An early stage of the system design process. • Represents the link between specification and design processes. • Often carried out in parallel with some specification activities. • It involves identifying major system components and their communications.

Application architectures

• Application systems are designed to meet an organisational need. • As businesses have much in common, their application systems also tend to have a common architecture that reflects the application requirements. • A generic application architecture is an architecture for a type of software system that may be configured and adapted to create a system that meets specific requirements.

Architectural design decisions

• Architectural design is a creative process so the process differs depending on the type of system being developed. • However, a number of common decisions span all design processes.

Use of application architectures

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

Use of architectural models

• As a way of facilitating discussion about the system design • As a way of documenting an architecture that has been designed

System structuring

• Concerned with decomposing the system into interacting sub-systems. • The architectural design is normally expressed as a block diagram presenting an overview of the system structure. • More specific models showing how sub- systems share data, are distributed and interface with each other may also be developed.

Examples of application types

• Data processing applications: Data driven applications that process data in batches without explicit user intervention during the processing. • Transaction processing applications: Data-centred applications that process user requests and update information in a system database. • Event processing systems: Applications where system actions depend on interpreting events from the system's environment. • Language processing systems: Applications where the users' intentions are specified in a formal language that is processed and interpreted by the system.

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.

Pipe and filter architecture

• 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.

Web-based information systems

• Information and resource management systems are now usually web-based systems where the user interfaces are implemented using a web browser. • For example, e-commerce systems are Internet-based resource management systems that accept electronic orders for goods or services and then arrange delivery of these goods or services to the customer.

Information systems architecture

• Information systems have a generic architecture that can be organized as a layered architecture. • These are transaction-based systems as interaction with these systems generally involves database transactions. • Layers include: - The user interface - User communications - Information retrieval - System database

Architectural patterns

• Patterns are a means of representing, sharing and reusing knowledge. • An architectural pattern is a stylized description of good design practice, which has been tried and tested in different environments. • Patterns should include information about when they are and when the are not useful. • Patterns may be represented using tabular and graphical descriptions.

Transaction processing systems

• Process user requests for information from a database or requests to update the database. • Users make asynchronous requests for service which are then processed by a transaction manager.

System organization

• Reflects the basic strategy that is used to structure a system. • Three organizational styles are widely used: - A shared data repository style; - A shared services and servers style; - An abstract machine or layered style.

Architectural representations

• Simple, informal block diagrams showing entities and relationships are the most frequently used method for documenting software architectures. • But these have been criticised because they lack semantics, do not show the types of relationships between entities nor the visible properties of entities in the architecture. • Depends 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

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.

Architecture reuse

• Systems in the same domain often have similar architectures that reflect domain concepts. • Application product lines are built around a core architecture with variants that satisfy particular customer requirements. • The 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.

Architectural styles

• The architectural model of a system may conform to a generic architectural model or style. • An awareness of these styles can simplify the problem of defining system architectures. • However, most large systems are heterogeneous and do not follow a single architectural style.

Server implementation

• These systems are often implemented as multi-tier client server/architectures - The web server is responsible for all user communications, with the user interface implemented using a web browser; - The application server is responsible for implementing application-specific logic as well as information storage and retrieval requests; - The database server moves information to and from the database and handles transaction management.

Application type examples, transaction processing and language processing systems.

• Transaction processing systems - E-commerce systems; - Reservation systems. • Language processing systems - Compilers; - Command interpreters.