Architecture Patterns - PDF

Hofstra University School of Engineering and Applied Sciences Computer Science 190 Software Engineering Instructor : Scott Jeffreys  Architectural design is concerned with understanding how a software system should be organized...

Hofstra University School of Engineering and Applied Sciences Computer Science 190 Software Engineering Instructor : Scott Jeffreys  Architectural design is concerned with understanding how a software system should be organized and designing the overall structure of that system.  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.  The output of the architectural design process is an architectural model that describes how the system is organized as a set of communicating components. 2  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.  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. 3  Stakeholder communication ◦ Architecture may be used as a focus of discussion by system stakeholders.  System analysis ◦ Means that analysis of whether the system can meet its non-functional requirements is possible.  Large-scale reuse ◦ The architecture may be reusable across a range of systems ◦ Product-line architectures may be developed. 4  Simple, informal block diagrams showing entities and relationships are the most frequently used method for documenting software architectures. ◦ But 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. 5  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. 6 7  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. 8  Performance ◦ Localize critical operations and minimize communications. Use large rather than fine-grain components.  Security ◦ Use a layered architecture with critical assets in the inner layers.  Safety ◦ Localize 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. 9  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. 10 11  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. 12 Name MVC (Model-View-Controller) 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. 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. Advantages 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. Disadvantages Can involve additional code and code complexity when the data model and interactions are simple. 13 14 15  Used to model the interfacing of sub-systems.  Organises 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. 16 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. 17 18  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 a this is an efficient data sharing mechanism. 19 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. 20 21  Distributed system model which shows how data and processing is distributed across a range of components. ◦ Can be implemented on a single computer.  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. Chapter 6 Architectural Design 07/01/2016 22 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. 23 24  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. 25 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. 26  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. 27  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. 28  Process user requests for information from a database or requests to update the database.  From a user perspective a transaction is: ◦ Any coherent sequence of operations that satisfies a goal; ◦ For example - find the times of flights from London to Paris.  Users make asynchronous requests for service which are then processed by a transaction manager. 29 30 31  A software architecture is a description of how a software system is organized.  Architectural design decisions include decisions on the type of application, the distribution of the system, the architectural styles to be used.  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 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. 32  Models of application systems architectures help us understand and compare applications, validate application system designs and assess large-scale components for reuse.  Transaction processing systems are interactive systems that allow information in a database to be remotely accessed and modified by a number of users. 33