2.5 Design Concepts: The Building Blocks for Creating Structures
Design is not random, but rather is planned, intentional, rational, and directed. The process of design may seem daunting at first. When facing the “blank page” at the beginning of any design activity, the space of possibilities might seem impossibly huge and complex. However, there is some help here. The software architecture community has created and evolved, over the course of decades, a body of generally accepted design principles that can guide us to create high-quality designs with predictable outcomes.
For example, some well-documented design principles are oriented toward the achievement of specific quality attributes:
To help achieve high modifiability, aim for good modularity, which means high cohesion and low coupling.
To help achieve high availability, avoid having any single point of failure.
To help achieve scalability, avoid having any hard-coded limits for critical resources.
To help achieve security, limit the points of access to critical resources.
To help achieve testability, externalize state.
. . . and so forth.
In each case, these principles have been evolved over decades of dealing with those quality attributes in practice. In addition, we have evolved reusable realizations of these abstract approaches in design and, eventually, in code. We call these reusable realizations design concepts, and they are the building blocks from which the structures that make up the architecture are created. Different types of design concepts exist, and here we discuss some of the most commonly used, including reference architectures, deployment patterns, architectural patterns, tactics, and externally developed components (such as frameworks). While the first four are conceptual in nature, the last one is concrete.
2.5.1 Reference Architectures
Reference architectures are blueprints that provide an overall logical structure for particular types of applications. A reference architecture is a reference model mapped onto one or more architectural patterns. It has been proven in business and technical contexts, and typically comes with a set of supporting artifacts that eases its use.
An example of a reference architecture for the development of web applications is shown in Figure 2.3 on the next page. This reference architecture establishes the main layers for this type of application—presentation, business, and data—as well as the types of elements that occur within the layers and the responsibilities of these elements, such as UI components, business components, data access components, service agents, and so on. Also, this reference architecture introduces cross-cutting concerns, such as security and communication, that need to be addressed. As this example shows, when you select a reference architecture for your application, you also adopt a set of issues that you need to address during design. You may not have an explicit requirement related to communications or security, but the fact that these elements are part of the reference architecture require you to make design decisions about them.
Figure 2.3 Example reference architecture for the development of web applications from the Microsoft Application Architecture Guide (Key: UML)
Reference architectures may be confused with architectural styles, but these two concepts are different. Architectural styles (such as “Pipe and Filter” and “Client Server”) define types of components and connectors in a specified topology that are useful for structuring an application either logically or physically. Such styles are technology and domain agnostic. Reference architectures, in contrast, provide a structure for applications in specific domains, and they may embody different styles. Also, while architectural styles tend to be popular in academia, reference architectures seem to be preferred by practitioners—which is also why we favor them in our list of design concepts.
While there are many reference architectures, we are not aware of any catalog that contains an extensive list of them.
2.5.2 Architectural Design Patterns
Design patterns are conceptual solutions to recurring design problems that exist in a defined context. While design patterns originally focused on decisions at the object scale, including instantiation, structuring, and behavior, today there are catalogs with patterns that address decisions at varying levels of granularity. In addition, there are specific patterns to address quality attributes such as security or integration.
While some people argue for the differentiation between what they consider to be architectural patterns and the more fine-grained design patterns, we believe there is no principled difference that can be solely attributed to scale. We consider a pattern to be architectural when its use directly and substantially influences the satisfaction of some of the architectural drivers (see Section 2.2).
Figure 2.4 shows an example architectural pattern that is useful for structuring the system, the Layers pattern. When you choose a pattern such as this one, you must decide how many layers you will need for your system. Figure 2.5 shows a pattern to support concurrency, which is useful to increase performance. This pattern, too, needs to be instantiated—that is, it needs to be adapted to the specific problem and design context. Instantiation is discussed in Chapter 3.
Figure 2.4 The Layers pattern for structuring an application from Pattern-Oriented Software Architecture
Figure 2.5 The Half-Sync/Half-Async pattern to support concurrency from Pattern-Oriented Software Architecture (Source: Softserve)
Although reference architectures may be considered as a type of pattern, we prefer to consider them separately because of the important role they play in structuring an application and because they are more directly connected to technology stacks. Also, a reference architecture typically incorporates other patterns and often constrains these patterns. For example, the reference architecture for web applications shown in Figure 2.3 incorporates the Layers pattern but also establishes how many layers need to be used. This reference architecture also incorporates other patterns such as an Application Facade and Data Access Components.
2.5.3 Deployment Patterns
Another type of pattern that we prefer to consider separately is deployment patterns. These patterns provide models on how to physically structure the system to deploy it. Some deployment patterns, such as the one shown in Figure 2.6, are useful to establish an initial physical structure of the system in terms of tiers (physical nodes). More specialized deployment patterns, such as the Load-Balanced Cluster in Figure 2.7, are used to satisfy quality attributes such as availability, performance, and security.
Figure 2.6 Four-tier deployment pattern from the Microsoft Application Architecture Guide (Key: UML)
Figure 2.7 Load-Balanced Cluster deployment pattern for performance from the Microsoft Application Architecture Guide (Key: UML)
In general, an initial structure for the system is obtained by mapping the logical elements that are obtained from reference architectures (and other patterns) into the physical elements defined by deployment patterns.
Architects can use collections of fundamental design techniques to achieve a response for particular quality attributes. We call these architectural design primitives tactics. Tactics, like design patterns, are techniques that architects have been using for years. We do not invent tactics, but simply capture what architects actually have done in practice, over the decades, to manage quality attribute response goals.
Tactics are design decisions that influence the control of a quality attribute response. For example, if you want to design a system to have low latency or high throughput, you could make a set of design decisions that would mediate the arrival of events (requests for service), resulting in responses that are produced within some time constraints, as shown in Figure 2.8.
Figure 2.8 Tactics mediate events and responses.
Tactics are both simpler and more primitive than patterns. They focus on the control of a single quality attribute response (although they may, of course, trade off this response with other quality attribute goals). Patterns, in contrast, typically focus on resolving and balancing multiple forces—that is, multiple quality attribute goals. By way of analogy, we can say that a tactic is an atom, whereas a pattern is a molecule.
Tactics provide a top-down way of thinking about design. A tactics categorization begins with a set of design objectives related to the achievement of a quality attribute, and presents the architect with a set of options from which to choose. These options then need to be further instantiated through some combination of patterns, frameworks, and code.
For example, in Figure 2.9, the design objectives for performance are “Control Resource Demand” and “Manage Resources”. An architect who wants to create a system with “good” performance needs to choose one or more of these options. That is, the architect needs to decide if controlling resource demand is feasible, and if managing resources is feasible. In some systems, the events arriving at the system can be managed, prioritized, or limited in some way. If this is not possible, then the architect can manage resources only as part of an attempt to generate responses within acceptable time constraints. Within the “Manage Resources” category, an architect might choose to increase resources, introduce concurrency, maintain multiple copies of computations, maintain multiple copies of data, and so forth. These tactics then need to be instantiated. As an example, an architect might choose the Half-Sync/Half-Async pattern (see Figure 2.5) as a way of introducing (and managing) concurrency, or the Load-Balanced Cluster deployment pattern (see Figure 2.7) to maintain multiple copies of computations. As we will see in Chapter 3, the choice, combination, and tailoring of tactics and patterns are some of the key steps of the ADD process. There are existing tactics categorizations for the quality attributes of availability, interoperability, modifiability, performance, security, testability, and usability.
Figure 2.9 Performance tactics from Software Architecture in Practice
2.5.5 Externally Developed Components
Patterns and tactics are abstract in nature. However, when you are designing a software architecture, you need to make these design concepts concrete and closer to the actual implementation. There are two ways to achieve this: You can code the elements obtained from tactics and patterns or you can associate technologies with one or more of these elements in the architecture. This “buy versus build” choice is one of the most important decisions you will make as an architect.
We consider technologies to be externally developed components, because they are not created as part of the development project. Several types of externally developed components exist:
Technology families. A technology family represents a group of specific technologies with common functional purposes. It can serve as a placeholder until a specific product or framework is selected. An example is a relational database management system (RDBMS) or an object-oriented to relational mapper (ORM). Figure 2.10 shows different technology families in the Big Data domain (in regular text).
Figure 2.10 A technology family tree for the Big Data application domain
Products. A product (or software package) refers to a self-contained functional piece of software that can be integrated into the system that is being designed and that requires only minor configuration or coding. An example is a relational database management system, such as Oracle or Microsoft SQL Server. Figure 2.10 shows different products in the Big Data domain (in italics).
Application frameworks. An application framework (or just framework) is a reusable software element, constructed out of patterns and tactics, that provides generic functionality addressing recurring domain and quality attribute concerns across a broad range of applications. Frameworks, when carefully chosen and properly implemented, increase the productivity of programmers. They do so by enabling programmers to focus on business logic and end-user value, rather than underlying technologies and their implementations. As opposed to products, framework functions are generally invoked from the application code or are “injected” using some type of aspect-oriented approach. Frameworks usually require extensive configuration, typically through XML files or other approaches such as annotations in Java. A framework example is Hibernate, which is used to perform object-oriented to relational mapping in Java. Several types of frameworks are available: Full-stack frameworks, such as Spring, are usually associated with reference architectures and address general concerns across the different elements of the reference architecture, while non-full-stack frameworks, such as JSF, address specific functional or quality attribute concerns.
Platforms. A platform provides a complete infrastructure upon which to build and execute applications. Examples of platforms include Java, .Net, or and Google Cloud.
The selection of externally developed components, which is a key aspect of the design process, can be a challenging task because of their extensive number. Here are a few criteria you should consider when selecting externally developed components:
Problem that it addresses. Is it something specific, such as a framework for object-oriented to relational mapping or something more generic, such as a platform?
Cost. What is the cost of the license and, if it is free, what is the cost of support and education?
Type of license. Does it have a license that is compatible with the project goals?
Support. Is it well supported? Is there extensive documentation about the technology? Is there an extensive user or developer community that you can turn to for advice?
Learning curve. How hard is it to learn this technology? Have others in your organization already mastered it? Are there courses available?
Maturity. Is it a technology that has just appeared on the market, which may be exciting but still relatively unstable or unsupported?
Popularity. Is it a relatively widespread technology? Are there positive testimonials or adoption by mature organizations? Will it be easy to hire people who have deep knowledge of it? Is there an active developer community or user group?
Compatibility and ease of integration. Is it compatible with other technologies used in the project? Can it be integrated easily in the project?
Support for critical quality attributes. Does it limit attributes such as performance? Is it secure and robust?
Size. Will the use of the technology have a negative impact on the size of the application under development?
Unfortunately, the answers to these questions are not always easy to find and the selection of a particular technology may require you do some research or, eventually, to create prototypes that will help you in the selection process. These criteria will have a significant effect on your total cost of ownership.