Software Engineering Chapter 6 Architectural Design

Pipe and Filter Pattern

*Not really suitable for interactive systems* When used: Commonly used in data processing applications (both batch- and transaction-based) where inputs are processed in separate stages to generate related outputs. Pros: Easy to understand and supports transformation reuse. Workflow style matches the structure of many business processes. Evolution by adding transformations is straightforward. Can be implemented as either a sequential or concurrent system Cons: The format for data transfer has to be agreed upon between communicating transformations. Each transformation must parse its input and unparse its output to the agreed form. This increases system overhead and may mean that it is impossible to reuse functional transformations that use incompatible data structures

Architecture and system characteristics

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

Compiler Components

*Pipe and Filter Compiler Architecture* 1) lexical analyzer 2) Symbol Table 3) Syntax Analyzer 4) Syntax Tree 5)Semantic Analyzer 6) Code Generator

Layered Architecture Pattern

*When used*: Used when building new facilities on top of existing systems; when the development is spread across several teams with each team responsibility for a layer of functionality; when there is a requirement for multi-level security. *Pros*: Allows replacement of entire layers so long as the interface is maintained. Redundant facilities (e.g., authentication) can be provided in each layer to increase the dependability of the system. *Cons*: In practice, providing a clean separation between layers is often difficult and a high-level layer may have to interact directly with lower-level layers rather than through the layer immediately below it. Performance can be a problem because of multiple levels of interpretation of a service request as it is processed at each layer.

The Client-Server Pattern

*When used*: Used when data in a shared database has to be accessed from a range of locations. Because servers can be replicated, may also be used when the load on a system is variable. *Pros*: servers can be distributed across a network. General functionality (e.g., a printing service) can be available to all clients and does not need to be implemented by all services. *Cons*: Each service is a single point of failure so susceptible to denial of service attacks or server failure. Performance may be unpredictable because it depends on the network as well as the system. May be management problems if servers are owned by different organizations

Model-View-Controller

*When used*: Used when there are *multiple ways to view and interact with data*. Also used when the *future requirements* for interaction and presentation of data are *unknown*. *Pros*: Allows the data to change independently of its representation and vice versa. Supports presentation of the same data in different ways with changes made in one representation shown in all of them. *Cons*: Can involve additional code and code complexity when the data model and interactions are simple.

Repository Pattern

*When used*: You should use this pattern when you have a system in which large volumes of information are generated that has to be stored for a long time. You may also use it in datadriven systems where the inclusion of data in the repository triggers an action or tool. *Pros*: Components can be independent-they do not need to know of the existence of other components. Changes made by one component can be propagated to all components. All data can be managed consistently (e.g., backups done at the same time) as it is all in one place. *Cons*: The repository is a single point of failure so problems in the repository affect the whole system. May be inefficiencies in organizing all communication through the repository. Distributing the repository across several computers may be difficult.

Box and line diagrams

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

Advantages of Explicit Architecture

1) *Stakeholder communication* - Architecture may be used as a focus of discussion by system stakeholders 2) *System Analysis* - It should be possible to analyze if the system can meet its non-functional requirements 3) *Large-Scale reuse* - The architecture may be reusable across a range of systems - Product-line architectures may be developed.

Use of architectural models

1) As a way of facilitating discussion about the system design 2) As a way of documenting

Key Points of Chapter 6

1. A software architecture is a description of how a software system is organized. 2. Architectural design decisions include decisions on the type of application, the distribution of the system, the architectural styles to be used. 3. 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. 4. 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. 5. Models of application systems architectures help us understand and compare applications, validate application system designs and assess large-scale components for reuse. 6. Transaction processing systems are interactive systems that allow information in a database to be remotely accessed and modified by a number of users. 7. 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.

What is application architecture

1. Application systems are designed to meet an organisational need. 2. As businesses have much in common, their application systems also tend to have a common architecture that reflects the application requirements. 3. 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 Deisgn

1. Architectural Design is concerned with understanding how a software system should be organized and designing the overall structure of that system 2.Architectural design is the critical link between design and requirements engineering, as it identifies the main structural components in a system and the relationships between them. 3. The output of the architectural design process is an architectural model that describes how the system is organized as a set of communicating components.

Architectural Abstraction

1. 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. 2. 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.

Information Systems Architecture

1. Layered architecture 2. Transaction-based systems 3. layers include: - The user interface - User communications - Information retrieval - System database

Architectural Patterns

1. Model View Controller 2. Layered Architecture 3. Repository Architecture 4. Client-Server Architecture 5. Pipe and Filter Architecture

Explain why design conflicts might arise when designing an architecture for which both availability and security requirements are the most important non-functional requirements.

Design conflicts might arise when designing an architecture for which both availability and security requirements are the most important non-functional requirements. This is because availability requirements need to include redundant components and mechanism for fault tolerance, yet security requirements need to use a layered architecture with critical assets in the inner layers. The design conflict is that security requirements need the critical assets in the inner layers only so they can be protected but availability requirements need a redundant copy of the critical assets at each layer for an easier access.

Structure of Transaction Processing Applications

I/O processing <-> Application logic <-> Transaction manager <-> Database

Architectural view

Logical view Physical view development view process view

Using the generic model of a language processing system presented here, design the architecture of a system that accepts natural language commands and translates these into database queries in a language such as SQL.

Please see group chat's hw6

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.

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.

Application Types

• Data processing applications • Transaction processing applications • Event processing systems • Language processing systems