University:
California State University, Northridge
Course:
COMP 380/L | Introduction to Software Engineering and Lab
Academic year:

2011
Views:

196

Pages:

22
Author:

Joyce Guerra

Architecture Design & Sequence Diagram Part 2 Layered architecture • Used to model the interfacingg of sub‐systems. y • 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 j layer y is affected. adjacent • However, often artificial to structure systems in this way. The Layered architecture pattern 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. See Figure 6.6. A layered model of a system for sharing copyright documents held in different libraries, as shown in Figure 6.7. 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. 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. system 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. Example When used Advantages Disadvantages A generic layered architecture The architecture of the LIBSYS system Key points • A software architecture is a description of how a software ft system t is i organized. i d • 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 p p or viewssuch as a conceptual p view,, a logical g 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. Repository architecture • Sub‐systems y must exchange g data. This mayy be done in two ways: – Shared data is held in a central database or repository and may be accessed by all sub‐systems; 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. mechanism The Repository pattern 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. Figure 6.9 is an example of an IDE where the components use a repository of system design information. Each software tool generates information which is then available for use by other tools. 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 g time. You may y also use it in data-driven systems where the inclusion of data in the repository triggers an action or tool. 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. components All data can be managed consistently (e.g., backups done at the same time) as it is all in one place. 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. Example When used Advantages Disadvantages A repository architecture for an IDE Client‐server Client server architecture • Distributed system y model which shows how data and processing is distributed across a range of components. – Can C b be iimplemented l d on a single i l computer. • Set of stand‐alone servers which provide specific services such as printing, printing data management, management etc. etc • Set of clients which call on these services. • Network which allows clients to access servers. The Client Client–server server pattern 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. Figure 6.11 6 11 is an example of a film and video/DVD library organized as a client–server system. 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. 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. 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. Example When used Advantages Disadvantages A client–server architecture for a film lb library Pipe and filter architecture • Functional transformations p process their inputs p to produce outputs. • May be referred to as a pipe and filter model (as in UNIX shell). h ll) • Variants of this approach are very common. When transformations are sequential, sequential this is a batch sequential model which is extensively used in data processing systems. • Not really suitable for interactive systems. The pipe and filter pattern 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. Figure 6.13 6 13 is an example of a pipe and filter system used for processing invoices. Commonly used in data processing applications (both batch- and transaction-based) where inputs are processed in separate stages to generate related outputs. 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. 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. Example When used Advantages Disadvantages An example of the pipe and filter architecture h Application architectures • Application pp systems y are designed g to meet an organizational need. • As businesses have much in common, their application l systems also l tend d to h 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. Use of application architectures • As a startingg point p for architectural design. g • As a design checklist. • As a wayy of organising g g the work of the development p team. • As a means of assessing components for reuse. • As a vocabulary for talking about application types. Examples of application types • Data processing applications – Data driven applications that process data in batches without explicit user intervention during the processing. • Transaction p processingg applications pp – Data‐centred applications that process user requests and update information in a system database. • Event p processingg systems y – Applications where system actions depend on interpreting events from the system’s environment. • Language g g processing p g systems y – Applications where the users’ intentions are specified in a formal language that is processed and interpreted by the system. Application type examples • Focus here is on transaction processing and language processing systems. • Transaction processing systems – EE‐commerce commerce systems; – Reservation systems. • Language processing systems – Compilers; – Command interpreters. Transaction processing systems • Process user requests q 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. The structure of transaction processing applications l The software architecture of an ATM system

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