CS 410/510 - Software Engineering

Architectural Design

Reference: Sommerville, Software Engineering, 10 ed., Chapter 6

The big picture

Architectural design is a process for identifying the sub-systems making up a system and the framework for sub-system control and communication. The output of this design process is a description of the software architecture. Architectural design is an early stage of the system design process. It represents the link between specification and design processes and is often carried out in parallel with some specification activities. It involves identifying major system components and their communications.

Software architectures can be designed at two levels of abstraction:

Three advantages of explicitly designing and documenting software architecture:

Software architecture is most often represented using simple, informal block diagrams showing entities and relationships. Pros: simple, useful for communication with stakeholders, great for project planning. Cons: lack of semantics, types of relationships between entities, visible properties of entities in the architecture.

Uses of architectural models:

As 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.
As 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.

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 and these decisions affect the non-functional characteristics of the system:

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/styles, which capture the essence of an architecture and can be instantiated in different ways.

The particular architectural style should depend on the non-functional system requirements:

Architectural views

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.

4+1 view model of software architecture:

Architectural patterns

Patterns are a means of representing, sharing and reusing knowledge. An architectural pattern is a stylized description of a 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.


Pattern name Model-View-Controller (MVC)
Description Separates presentation and interaction from the system data. The system is structured into three logical components that interact with each other. The Model component manages the system data and associated operations on that data. The View component defines and manages how the data is presented to the user. The Controller component manages user interaction (e.g., key presses, mouse clicks, etc.) and passes these interactions to the View and the Model.
Problem description The display presented to the user frequently changes over time in response to input or computation. Different users have different needs for how they want to view the program.s information. The system needs to reflect data changes to all users in the way that they want to view them, while making it easy to make changes to the user interface.
Solution description This involves separating the data being manipulated from the manipulation logic and the details of display using three components: Model (a problem-domain component with data and operations, independent of the user interface), View (a data display component), and Controller (a component that receives and acts on user input).
Consequences Advantages: views and controllers can be easily be added, removed, or changed; views can be added or changed during execution; user interface components can be changed, even at runtime. Disadvantages: views and controller are often hard to separate; frequent updates may slow data display and degrade user interface performance; the MVC style makes user interface components (views, controllers) highly dependent on model components.

Layered architecture

Name Layered architecture
Description Organizes the system into layers with related functionality associated with each layer. A layer provides services to the layer above it so the lowest-level layers represent core services that are likely to be used throughout the system.
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.
Advantages 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.
Disadvantages 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.

Repository architecture

Name Repository
Description All data in a system is managed in a central repository that is accessible to all system components. Components do not interact directly, only through the repository.
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 data-driven systems where the inclusion of data in the repository triggers an action or tool.
Advantages 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.
Disadvantages 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.

Client-server architecture

Name Client-server
Description In a client-server architecture, the functionality of the system is organized into services, with each service delivered from a separate server. Clients are users of these services and access servers to make use of them.
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.
Advantages The principal advantage of this model is that 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.
Disadvantages 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.

Pipe and filter architecture

Name Pipe and filter
Description The processing of the data in a system is organized so that each processing component (filter) is discrete and carries out one type of data transformation. The data flows (as in a pipe) from one component to another for processing.
When used Commonly used in data processing applications (both batch- and transaction-based) where inputs are processed in separate stages to generate related outputs.
Advantages 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.
Disadvantages 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.

Application architectures

Application systems are designed to meet an organizational 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. application architectures can be used as a:

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.