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Chapter 1: What is Software Architecture?

Chapter 1: What is Software Architecture?

What is Software Architecture?

Software application architecture is the process of defining a structured solution that meets all of the technical and operational requirements, while optimizing common quality attributes such as performance, security, and manageability. It involves a series of decisions based on a wide range of factors, and each of these decisions can have considerable impact on the quality, performance, maintainability, and overall success of the application.

Philippe Kruchten, Grady Booch, Kurt Bittner, and Rich Reitman derived and refined a definition of architecture based on work by Mary Shaw and David Garlan (Shaw and Garlan 1996). Their definition is:

“Software architecture encompasses the set of significant decisions about the organization of a software system including the selection of the structural elements and their interfaces by which the system is composed; behavior as specified in collaboration among those elements; composition of these structural and behavioral elements into larger subsystems; and an architectural style that guides this organization. Software architecture also involves functionality, usability, resilience, performance, reuse, comprehensibility, economic and technology constraints, tradeoffs and aesthetic concerns.”

In Patterns of Enterprise Application Architecture, Martin Fowler outlines some common recurring themes when explaining architecture. He identifies these themes as:

“The highest-level breakdown of a system into its parts; the decisions that are hard to change; there are multiple architectures in a system; what is architecturally significant can change over a system's lifetime; and, in the end, architecture boils down to whatever the important stuff is.”


In Software Architecture in Practice (2nd edition), Bass, Clements, and Kazman define architecture as follows:

“The software architecture of a program or computing system is the structure or structures of the system, which comprise software elements, the externally visible properties of those elements, and the relationships among them. Architecture is concerned with the public side of interfaces; private details of elements—details having to do solely with internal implementation—are not architectural.”


Why is Architecture Important?

Like any other complex structure, software must be built on a solid foundation. Failing to consider key scenarios, failing to design for common problems, or failing to appreciate the long term consequences of key decisions can put your application at risk. Modern tools and platforms help to simplify the task of building applications, but they do not replace the need to design your application carefully, based on your specific scenarios and requirements. The risks exposed by poor architecture include software that is unstable, is unable to support existing or future business requirements, or is difficult to deploy or manage in a production environment.

Systems should be designed with consideration for the user, the system (the IT infrastructure), and the business goals. For each of these areas, you should outline key scenarios and identify important quality attributes (for example, reliability or scalability) and key areas of satisfaction and dissatisfaction. Where possible, develop and consider metrics that measure success in each of these areas.


Figure 1

User, business, and system goals

Tradeoffs are likely, and a balance must often be found between competing requirements across these three areas. For example, the overall user experience of the solution is very often a function of the business and the IT infrastructure, and changes in one or the other can significantly affect the resulting user experience. Similarly, changes in the user experience requirements can have significant impact on the business and IT infrastructure requirements. Performance might be a major user and business goal, but the system administrator may not be able to invest in the hardware required to meet that goal 100 percent of the time. A balance point might be to meet the goal only 80 percent of the time.

Architecture focuses on how the major elements and components within an application are used by, or interact with, other major elements and components within the application. The selection of data structures and algorithms or the implementation details of individual components are design concerns. Architecture and design concerns very often overlap. Rather than use hard and fast rules to distinguish between architecture and design, it makes sense to combine these two areas. In some cases, decisions are clearly more architectural in nature. In other cases, the decisions are more about design, and how they help you to realize that architecture.

By following the processes described in this guide, and using the information it contains, you will be able to construct architectural solutions that address all of the relevant concerns, can be deployed on your chosen infrastructure, and provide results that meet the original aims and objectives.

Consider the following high level concerns when thinking about software architecture:

  • How will the users be using the application?
  • How will the application be deployed into production and managed?
  • What are the quality attribute requirements for the application, such as security, performance, concurrency, internationalization, and configuration?
  • How can the application be designed to be flexible and maintainable over time?
  • What are the architectural trends that might impact your application now or after it has been deployed?

The Goals of Architecture

Application architecture seeks to build a bridge between business requirements and technical requirements by understanding use cases, and then finding ways to implement those use cases in the software. The goal of architecture is to identify the requirements that affect the structure of the application. Good architecture reduces the business risks associated with building a technical solution. A good design is sufficiently flexible to be able to handle the natural drift that will occur over time in hardware and software technology, as well as in user scenarios and requirements. An architect must consider the overall effect of design decisions, the inherent tradeoffs between quality attributes (such as performance and security), and the tradeoffs required to address user, system, and business requirements.

Keep in mind that the architecture should:

  • Expose the structure of the system but hide the implementation details.
  • Realize all of the use cases and scenarios.
  • Try to address the requirements of various stakeholders.
  • Handle both functional and quality requirements.

The Architectural Landscape

It is important to understand the key forces that are shaping architectural decisions today, and which will change how architectural decisions are made in the future. These key forces are driven by user demand, as well as by business demand for faster results, better support for varying work styles and workflows, and improved adaptability of software design.

Consider the following key trends:

  • User empowerment. A design that supports user empowerment is flexible, configurable, and focused on the user experience. Design your application with appropriate levels of user personalization and options in mind. Allow the user to define how they interact with your application instead of dictating to them, but do not overload them with unnecessary options and settings that can lead to confusion. Understand the key scenarios and make them as simple as possible; make it easy to find information and use the application.
  • Market maturity. Take advantage of market maturity by taking advantage of existing platform and technology options. Build on higher level application frameworks where it makes sense, so that you can focus on what is uniquely valuable in your application rather than recreating something that already exists and can be reused. Use patterns that provide rich sources of proven solutions for common problems.
  • Flexible design. Increasingly, flexible designs take advantage of loose coupling to allow reuse and to improve maintainability. Pluggable designs allow you to provide post-deployment extensibility. You can also take advantage of service orientation techniques such as SOA to provide interoperability with other systems.
  • Future trends. When building your architecture, understand the future trends that might affect your design after deployment. For example, consider trends in rich UI and media, composition models such as mashups, increasing network bandwidth and availability, increasing use of mobile devices, continued improvement in hardware performance, interest in community and personal publishing models, the rise of cloud-based computing, and remote operation.

Key Architecture Principles

Consider the following key principles when designing your architecture:

  • Build to change instead of building to last. Consider how the application may need to change over time to address new requirements and challenges, and build in the flexibility to support this.
  • Model to analyze and reduce risk. Use design tools, modeling systems such as Unified Modeling Language (UML), and visualizations where appropriate to help you capture requirements and architectural and design decisions, and to analyze their impact. However, do not formalize the model to the extent that it suppresses the capability to iterate and adapt the design easily.
  • Use models and visualizations as a communication and collaboration tool. Efficient communication of the design, the decisions you make, and ongoing changes to the design, is critical to good architecture. Use models, views, and other visualizations of the architecture to communicate and share your design efficiently with all the stakeholders, and to enable rapid communication of changes to the design.
  • Identify key engineering decisions. Use the information in this guide to understand the key engineering decisions and the areas where mistakes are most often made. Invest in getting these key decisions right the first time so that the design is more flexible and less likely to be broken by changes.

Consider using an incremental and iterative approach to refining your architecture. Start with a baseline architecture to get the big picture right, and then evolve candidate architectures as you iteratively test and improve your architecture. Do not try to get it all right the first time—design just as much as you can in order to start testing the design against requirements and assumptions. Iteratively add details to the design over multiple passes to make sure that you get the big decisions right first, and then focus on the details. A common pitfall is to dive into the details too quickly and get the big decisions wrong by making incorrect assumptions, or by failing to evaluate your architecture effectively. When testing your architecture, consider the following questions:

  • What assumptions have I made in this architecture?
  • What explicit or implied requirements is this architecture meeting?
  • What are the key risks with this architectural approach?
  • What countermeasures are in place to mitigate key risks?
  • In what ways is this architecture an improvement over the baseline or the last candidate architecture?


In this chapter, you will learn about the key design principles and guidelines for software architecture. Software architecture is often described as the organization or structure of a system, where the system represents a collection of components that accomplish a specific function or set of functions. In other words, architecture is focused on organizing components to support specific functionality. This organization of functionality is often referred to as grouping components into “areas of concern.” Figure 1 illustrates common application architecture with components grouped by different areas of concern.

Figure 1

Common application architecture

In addition to the grouping of components, other areas of concern focus on interaction between the components and how different components work together. The guidelines in this chapter examine different areas of concern that you should consider when designing the architecture of your application.

Key Design Principles

When getting started with your design, keep in mind the key principles that will help you to create an architecture that adheres to proven principles, minimizes costs and maintenance requirements, and promotes usability and extendibility. The key principles are:

  • Separation of concerns. Divide your application into distinct features with as little overlap in functionality as possible. The important factor is minimization of interaction points to achieve high cohesion and low coupling. However, separating functionality at the wrong boundaries can result in high coupling and complexity between features even though the contained functionality within a feature does not significantly overlap.
  • Single Responsibility principle. Each component or module should be responsible for only a specific feature or functionality, or aggregation of cohesive functionality.
  • Principle of Least Knowledge (also known as the Law of Demeter or LoD). A component or object should not know about internal details of other components or objects.
  • Don’t repeat yourself (DRY). You should only need to specify intent in one place. For example, in terms of application design, specific functionality should be implemented in only one component; the functionality should not be duplicated in any other component.
  • Minimize upfront design. Only design what is necessary. In some cases, you may require upfront comprehensive design and testing if the cost of development or a failure in the design is very high. In other cases, especially for agile development, you can avoid big design upfront (BDUF). If your application requirements are unclear, or if there is a possibility of the design evolving over time, avoid making a large design effort prematurely. This principle is sometimes known as YAGNI ("You ain’t gonna need it").

When designing an application or system, the goal of a software architect is to minimize the complexity by separating the design into different areas of concern. For example, the user interface (UI), business processing, and data access all represent different areas of concern. Within each area, the components you design should focus on that specific area and should not mix code from other areas of concern. For example, UI processing components should not include code that directly accesses a data source, but instead should use either business components or data access components to retrieve data.

However, you must also make a cost/value determination on the investment you make for an application. In some cases, you may need to simplify the structure to allow, for example, UI data binding to a result set. In general, try to consider the functional boundaries from a business viewpoint as well. The following high level guidelines will help you to consider the wide range of factors that can affect the ease of designing, implementing, deploying, testing, and maintaining your application.

Design Practices

  • Keep design patterns consistent within each layer. Within a logical layer, where possible, the design of components should be consistent for a particular operation. For example, if you choose to use the Table Data Gateway pattern to create an object that acts as a gateway to tables or views in a database, you should not include another pattern such as Repository, which uses a different paradigm for accessing data and initializing business entities. However, you may need to use different patterns for tasks in a layer that have a large variation in requirements, such as an application that contains business transaction and reporting functionality.
  • Do not duplicate functionality within an application. There should be only one component providing a specific functionality—this functionality should not be duplicated in any other component. This makes your components cohesive and makes it easier to optimize the components if a specific feature or functionality changes. Duplication of functionality within an application can make it difficult to implement changes, decrease clarity, and introduce potential inconsistencies.
  • Prefer composition to inheritance. Wherever possible, use composition over inheritance when reusing functionality because inheritance increases the dependency between parent and child classes, thereby limiting the reuse of child classes. This also reduces the inheritance hierarchies, which can become very difficult to deal with.
  • Establish a coding style and naming convention for development. Check to see if the organization has established coding style and naming standards. If not, you should establish common standards. This provides a consistent model that makes it easier for team members to review code they did not write, which leads to better maintainability.
  • Maintain system quality using automated QA techniques during development. Use unit testing and other automated Quality Analysis techniques, such as dependency analysis and static code analysis, during development. Define clear behavioral and performance metrics for components and sub-systems, and use automated QA tools during the build process to ensure that local design or implementation decisions do not adversely affect the overall system quality.
  • Consider the operation of your application. Determine what metrics and operational data are required by the IT infrastructure to ensure the efficient deployment and operation of your application. Designing your application’s components and sub-systems with a clear understanding of their individual operational requirements will significantly ease overall deployment and operation. Use automated QA tools during development to ensure that the correct operational data is provided by your application’s components and sub-systems.

Application Layers

  • Separate the areas of concern. Break your application into distinct features that overlap in functionality as little as possible. The main benefit of this approach is that a feature or functionality can be optimized independently of other features or functionality. In addition, if one feature fails, it will not cause other features to fail as well, and they can run independently of one another. This approach also helps to make the application easier to understand and design, and facilitates management of complex interdependent systems.
  • Be explicit about how layers communicate with each other. Allowing every layer in an application to communicate with or have dependencies upon all of the other layers will result in a solution that is more challenging to understand and manage. Make explicit decisions about the dependencies between layers and the data flow between them.
  • Use abstraction to implement loose coupling between layers. This can be accomplished by defining interface components such as a façade with well known inputs and outputs that translate requests into a format understood by components within the layer. In addition, you can also use Interface types or abstract base classes to define a common interface or shared abstraction (dependency inversion) that must be implemented by interface components.
  • Do not mix different types of components in the same logical layer. Start by identifying different areas of concern, and then group components associated with each area of concern into logical layers. For example, the UI layer should not contain business processing components, but instead should contain components used to handle user input and process user requests.
  • Keep the data format consistent within a layer or component. Mixing data formats will make the application more difficult to implement, extend, and maintain. Every time you need to convert data from one format to another, you are required to implement translation code to perform the operation and incur a processing overhead.

Key Design Considerations

This guide describes the major decisions that you must make, and which help to ensure that you consider all of the important factors as you begin and then iteratively develop your architecture design. The major decisions, briefly described in the following sections, are:

  • Determine the Application Type
  • Determine the Deployment Strategy
  • Determine the Appropriate Technologies
  • Determine the Quality Attributes
  • Determine the Crosscutting Concerns

For a more detailed description of the design process, see Chapter 4 "A Technique for Architecture and Design."


  • Overview
  • Summary of Key Architectural Styles
  • Client/Server Architectural Style
  • Component-Based Architectural Style
  • Domain Driven Design Architectural Style
  • Layered Architectural Style
  • Message-bus Architectural Style
  • N-Tier / 3-Tier Architectural Style
  • Object-Oriented Architectural Style
  • Service-Oriented Architectural Style
  • Additional Resources


This chapter describes and discusses high level patterns and principles commonly used for applications today. These are often referred to as the architectural styles, and include patterns such as client/server, layered architecture, component-based architecture, message bus architecture, and service-oriented architecture (SOA). For each style, you will find an overview, key principles, major benefits, and information that will help you choose the appropriate architectural styles for your application. It is important to understand that the styles describe different aspects of applications. For example, some architectural styles describe deployment patterns, some describe structure and design issues, and others describe communication factors. Therefore, a typical application will usually use a combination of more than one of the styles described in this chapter.

Layered Architectural Style

Layered architecture focuses on the grouping of related functionality within an application into distinct layers that are stacked vertically on top of each other. Functionality within each layer is related by a common role or responsibility. Communication between layers is explicit and loosely coupled. Layering your application appropriately helps to support a strong separation of concerns that, in turn, supports flexibility and maintainability.

The layered architectural style has been described as an inverted pyramid of reuse where each layer aggregates the responsibilities and abstractions of the layer directly beneath it. With strict layering, components in one layer can interact only with components in the same layer or with components from the layer directly below it. More relaxed layering allows components in a layer to interact with components in the same layer or with components in any lower layer.

The layers of an application may reside on the same physical computer (the same tier) or may be distributed over separate computers (n-tier), and the components in each layer communicate with components in other layers through well-defined interfaces. For example, a typical Web application design consists of a presentation layer (functionality related to the UI), a business layer (business rules processing), and a data layer (functionality related to data access, often almost entirely implemented using high-level data access frameworks). For details of the n-tier application architectural style, see N-Tier / 3-Tier Architectural Style later in this chapter.

Common principles for designs that use the layered architectural style include:

  • Abstraction. Layered architecture abstracts the view of the system as whole while providing enough detail to understand the roles and responsibilities of individual layers and the relationship between them.
  • Encapsulation. No assumptions need to be made about data types, methods and properties, or implementation during design, as these features are not exposed at layer boundaries.
  • Clearly defined functional layers. The separation between functionality in each layer is clear. Upper layers such as the presentation layer send commands to lower layers, such as the business and data layers, and may react to events in these layers, allowing data to flow both up and down between the layers.
  • High cohesion. Well-defined responsibility boundaries for each layer, and ensuring that each layer contains functionality directly related to the tasks of that layer, will help to maximize cohesion within the layer.
  • Reusable. Lower layers have no dependencies on higher layers, potentially allowing them to be reusable in other scenarios.
  • Loose coupling. Communication between layers is based on abstraction and events to provide loose coupling between layers.

Examples of layered applications include line-of-business (LOB) applications such as accounting and customer-management systems; enterprise Web-based applications and Web sites, and enterprise desktop or smart clients with centralized application servers for business logic.

A number of design patterns support the layered architectural style. For example, Separated Presentationpatterns encompass a range of patterns that the handling of the user's interactions from the UI, the presentation and business logic, and the application data with which the user works. Separated Presentation allows graphical designers to create a UI while developers generate the code to drive it. Dividing the functionality into separate roles in this way provides increased opportunities to test the behavior of individual roles. The following are the key principles of the Separated Presentation patterns:

  • Separation of concerns. Separated Presentation patterns divide UI processing concerns into distinct roles; for example, MVC has three roles: the Model, the View, and the Controller. The Model represents data (perhaps a domain model that includes business rules); the View represents the UI; and the Controller handles requests, manipulates the model, and performs other operations.
  • Event-based notification. The Observer pattern is commonly used to provide notifications to the View when data managed by the Model changes.
  • Delegated event handling. The controller handles events triggered from the UI controls in the View.

Other examples of Separated Presentation patterns are the Passive View pattern and the Supervising Presenter (or Supervising Controller) pattern.

The main benefits of the layered architectural style, and the use of a Separated Presentation pattern, are:

  • Abstraction. Layers allow changes to be made at the abstract level. You can increase or decrease the level of abstraction you use in each layer of the hierarchical stack.
  • Isolation. Allows you to isolate technology upgrades to individual layers in order to reduce risk and minimize impact on the overall system.
  • Manageability. Separation of core concerns helps to identify dependencies, and organizes the code into more manageable sections.
  • Performance. Distributing the layers over multiple physical tiers can improve scalability, fault tolerance, and performance.
  • Reusability. Roles promote reusability. For example, in MVC, the Controller can often be reused with other compatible Views in order to provide a role specific or a user-customized view on to the same data and functionality.
  • Testability. Increased testability arises from having well-defined layer interfaces, as well as the ability to switch between different implementations of the layer interfaces. Separated Presentation patterns allow you to build mock objects that mimic the behavior of concrete objects such as the Model, Controller, or View during testing.

Consider the layered architectural style if you have existing layers that are suitable for reuse in other applications, you already have applications that expose suitable business processes through service interfaces, or your application is complex and the high-level design demands separation so that teams can focus on different areas of functionality. The layered architectural style is also appropriate if your application must support different client types and different devices, or you want to implement complex and/or configurable business rules and processes.

Consider a Separated Presentation pattern if you want improved testability and simplified maintenance of UI functionality, or you want to separate the task of designing the UI from the development of the logic code that drives it. These patterns are also appropriate when your UI view does not contain any request processing code, and does not implement any business logic.


Chapter 1: What is Software Architecture?

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