Question

1. (30 points) For each of the architectural styles described in lecture 7 (4 architecture styles), give an advantage and disadvantage of using each style and give an example of a real world application whose software design might incorporate that style. 2. (40 points) For each of the systems described below, which software architecture would be most appropriate? Any reasons? sketch the appropriate software architecture. i. a system of automated banking machines, acting as distributed kiosks that bank customers can use to deposit and withdraw cash from their accounts. ii. image-processing software that allows users to apply various operations to modify their pictures. a weather forecasting application that analyzes tens of thousands of data elements collected from various sensors; the sensors periodically transmit new data values. 3. (10 points) List 5 important design principles to consider when designing your software architecture or class. 4. (20 points) Can a system ever be completely "decoupled"? That is can the degree of coupling be reduced so much that there is no coupling between components or class? Why or why not?

Doesn't need to be exactly from slide you can use outside info!

Answer 1

1. Architectural styles in software engineering

There are four types of Architectural styles in software engineering are as follows

A) Layered architecture

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.

· 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.

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.

Application

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.          

B) Repository architecture

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.

· Sub-systems must exchange data. This may be done in two ways:

o Shared data is held in a central database or repository and may be accessed by all sub-systems;

o 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.

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.

Applications

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.

C) Client-server architecture

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.

· Distributed system model which shows how data and processing is distributed across a range of components, but can also 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.

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.

Applications

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.

D) Pipe and filter architecture

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.

· 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.

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.

3. Software Design Principles

Software design principles are concerned with providing means to handle the complexity of the design process effectively. Effectively managing the complexity will not only reduce the effort needed for design but can also reduce the scope of introducing errors during design.

Following are the principles of Software Design

1) Problem Partitioning

For small problem, we can handle the entire problem at once but for the significant problem, divide the problems and conquer the problem it means to divide the problem into smaller pieces so that each piece can be captured separately.

For software design, the goal is to divide the problem into manageable pieces.

2) Abstraction

An abstraction is a tool that enables a designer to consider a component at an abstract level without bothering about the internal details of the implementation. Abstraction can be used for existing element as well as the component being designed.

3) Modularity

Modularity specifies to the division of software into separate modules which are differently named and addressed and are integrated later on in to obtain the completely functional software. It is the only property that allows a program to be intellectually manageable. Single large programs are difficult to understand and read due to a large number of reference variables, control paths, global variables, etc.

4) Strategy of Design

A good system design strategy is to organize the program modules in such a method that are easy to develop and latter too, change. Structured design methods help developers to deal with the size and complexity of programs. Analysts generate instructions for the developers about how code should be composed and how pieces of code should fit together to form a program.

5) Modular Design

Modular design reduces the design complexity and results in easier and faster implementation by allowing parallel development of various parts of a system.