Architecture Views and Viewpoints
Books, articles, research, and related publications on the different views of software architecture have been published. There are different schools of thought that prefer one architecture viewpoint over the other and, hence, practice and promote its adoption. In the spirit of this book’s theme, I do not devote a separate chapter to an exhaustive treatment of the different views of software architecture; rather, I present one that I have found to be practical and natural to follow and hence to use.
IBM (n.d.) defined a set of viewpoints called the IBM IT System Viewpoint Library. I have found it to be quite complete, with appropriate coverage of the various facets of a system’s architecture. The library consists of four basic viewpoints and six cross-cutting viewpoints. Figure 2.3 provides a pictorial representation.
Figure 2.3 Viewpoints in the IBM IT System Viewpoint Library (see “References”).
The four basic viewpoints of the IBM IT System Viewpoint Library are the following:
- Requirements—Models elements that capture all the requirements placed on the system, including business, technical, functional, and nonfunctional requirements. Use cases and use case models are the most common means of capturing the requirements viewpoint.
Solution—Models elements that define the solution satisfying the requirements and constraints; further organized into two categories:
- Functional—Focuses on the model elements that are structural in nature and with which the system is built by not only implementing the elements but also wiring the relationships between the elements (both static and dynamic). The functional architecture (the focus of Chapter 7, “The Functional Model”), broadly speaking, is the construct through which the details of this viewpoint are captured.
- Operational—Focuses on how the target system is built from the structural elements and how the functional view is deployed onto the IT environment (which consists of the network, hardware, compute power, servers, and so on). The operational model (the focus of Chapter 8, “The Operational Model”) is the most common architecture construct through which the details of this viewpoint are captured.
- Validation—Models elements that focus on assessing the ability of the system to deliver the intended functionality with the expected quality of service. Functional and nonfunctional test cases are often used as the validation criteria to attest to the system’s expected capabilities.
As shown in Figure 2.3, the four basic viewpoints are interrelated. The functional and operational viewpoints collectively realize (that is, implement and support) the requirements viewpoint; both the functional and operational viewpoints are validated for acceptance through the validation viewpoint. Note that the “solution” construct does not appear explicitly in Figure 2.3; for the sake of clarity, I have only shown the functional and operation constructs that collectively define the solution construct.
The library also contains six cross-cutting viewpoints, depicted in Figure 2.3 as concentric squares around the four basic viewpoints. The idea is to illustrate the point that the cross-cutting viewpoints influence one or more of the basic viewpoints.
The six cross-cutting viewpoints are as follows:
- Application—Focuses on meeting the system’s stated business requirements. The application architect plays the primary role in addressing this viewpoint.
- Technical—Focuses on the hardware, software, middleware (see Chapter 5, “The Architecture Overview,” for a definition), and packaged applications that collectively realize the application functionality and enable the application to run. The infrastructure and integration architects play the primary roles in addressing this viewpoint.
- Systems Management—Focuses on post-deployment management, maintenance, and operations of the system. The application maintenance and management teams play the primary roles in addressing this viewpoint.
- Availability—Focuses on addressing how the system will be made and kept available (for example, 99.5 percent uptime) per the agreed-upon service-level agreements. The infrastructure architect plays the primary role in addressing this viewpoint, with support from the application and the middleware architects.
- Performance—Focuses on addressing the performance of the system (for example, 400 milliseconds average latency between user request and the system response) per the agreed-upon service-level agreements. The application architect plays the primary role in addressing this viewpoint, with support from the middleware and infrastructure architects.
- Security—Focuses on addressing the security requirements of the system (for example, single sign-on, security of data transfer protocol, intrusion avoidance, among others). Some of the security requirements—for example, single sign-on—are addressed primarily by the application architect role, whereas other requirements such as data protocols (HTTPS, secure sockets) and intrusion avoidance are addressed primarily by the infrastructure architects.
There are many more details behind each of the basic and cross-cutting viewpoints. Each viewpoint has a set of elements that collectively characterize and define their responsibilities. Understanding them can provide key insights into how each viewpoint may be realized. Although there are many details behind each of the basic and cross-cutting viewpoints, the idea here is to acknowledge their existence and realize the fact that any system’s overall architecture has to typically address most, if not all, of the viewpoints. Awareness is key!
After having personally studied a handful of viewpoint frameworks, I feel that most, if not all, of them have a degree of commonality in their fundamental form. The reason is that each of the frameworks sets about to accomplish the same task of establishing a set of complementary perspectives from which the software architecture may be viewed, with the goal of covering the various facets of the architecture.
The choice of adopting a viewpoint framework, at least from the ones that are also quite established, hardened, and enduring, depends on your level of belief that it addresses your needs and your degree of comfort in its usability and adoption.