An architecture is the result of a set of business and technical decisions. There are many influences at work in its design, and the realization of these influences will change depending on the environment in which the architecture is required to perform. An architect designing a system for which the real-time deadlines are believed to be tight will make one set of design choices; the same architect, designing a similar system in which the deadlines can be easily satisfied, will make different choices. And the same architect, designing a non-real-time system, is likely to make quite different choices still. Even with the same requirements, hardware, support software, and human resources available, an architect designing a system today is likely to design a different system than might have been designed five years ago.
In any development effort, the requirements make explicit some?but only some?of the desired properties of the final system. Not all requirements are concerned directly with those properties; a development process or the use of a particular tool may be mandated by them. But the requirements specification only begins to tell the story. Failure to satisfy other constraints may render the system just as problematic as if it functioned poorly.
We begin building the ABC by identifying the influences to and from architectures.
Many people and organizations are interested in the construction of a software system. We call these stakeholders: The customer, the end users, the developers, the project manager, the maintainers, and even those who market the system are a few examples. Stakeholders have different concerns that they wish the system to guarantee or optimize, including things as diverse as providing a certain behavior at runtime, performing well on a particular piece of hardware, being easy to customize, achieving short time to market or low cost of development, gainfully employing programmers who have a particular specialty, or providing a broad range of functions. Figure 1.2 shows the architect receiving helpful stakeholder "suggestions."
Having an acceptable system involves properties such as performance, reliability, availability, platform compatibility, memory utilization, network usage, security, modifiability, usability, and interoperability with other systems as well as behavior. Indeed, we will see that these properties determine the overall design of the architecture. All of them, and others, affect how the delivered system is viewed by its eventual recipients, and so they find a voice in one or more of the system's stakeholders.
The underlying problem, of course, is that each stakeholder has different concerns and goals, some of which may be contradictory. Properties can be listed and discussed, of course, in an artifact such as a requirements document. But it is a rare requirements document that does a good job of capturing all of a system's quality requirements in testable detail. The reality is that the architect often has to fill in the blanks and mediate the conflicts.
In addition to the organizational goals expressed through requirements, an architecture is influenced by the structure or nature of the development organization. For example, if the organization has an abundance of idle programmers skilled in client-server communications, then a client-server architecture might be the approach supported by management. If not, it may well be rejected. Staff skills are one additional influence, but so are the development schedule and budget.
There are three classes of influence that come from the developing organization: immediate business, long-term business, and organizational structure.
An organization may have an immediate business investment in certain assets, such as existing architectures and the products based on them. The foundation of a development project may be that the proposed system is the next in a sequence of similar systems, and the cost estimates assume a high degree of asset re-use.
An organization may wish to make a long-term business investment in an infrastructure to pursue strategic goals and may view the proposed system as one means of financing and extending that infrastructure.
The organizational structure can shape the software architecture. In the case study in Chapter 8 (Flight Simulation: A Case Study in Architecture for Integrability), the development of some of the subsystems was subcontracted because the subcontractors provided specialized expertise. This was made possible by a division of functionality in the architecture that allowed isolation of the specialities.
If the architects for a system have had good results using a particular architectural approach, such as distributed objects or implicit invocation, chances are that they will try that same approach on a new development effort. Conversely, if their prior experience with this approach was disastrous, the architects may be reluctant to try it again. Architectural choices may also come from an architect's education and training, exposure to successful architectural patterns, or exposure to systems that have worked particularly poorly or particularly well. The architects may also wish to experiment with an architectural pattern or technique learned from a book (such as this one) or a course.
A special case of the architect's background and experience is reflected by the technical environment. The environment that is current when an architecture is designed will influence that architecture. It might include standard industry practices or software engineering techniques prevalent in the architect's professional community. It is a brave architect who, in today's environment, does not at least consider a Web-based, object-oriented, middleware-supported design for an information system.
Influences on an architecture come from a wide variety of sources. Some are only implied, while others are explicitly in conflict.
Almost never are the properties required by the business and organizational goals consciously understood, let alone fully articulated. Indeed, even customer requirements are seldom documented completely, which means that the inevitable conflict among different stakeholders' goals has not been resolved.
However, architects need to know and understand the nature, source, and priority of constraints on the project as early as possible. Therefore, they must identify and actively engage the stakeholders to solicit their needs and expectations. Without such engagement, the stakeholders will, at some point, demand that the architects explain why each proposed architecture is unacceptable, thus delaying the project and idling workers. Early engagement of stakeholders allows the architects to understand the constraints of the task, manage expectations, negotiate priorities, and make tradeoffs. Architecture reviews (covered in Part Three) and iterative prototyping are two means for achieving it.
It should be apparent that the architects need more than just technical skills. Explanations to one stakeholder or another will be required regarding the chosen priorities of different properties and why particular stakeholders are not having all of their expectations satisfied. For an effective architect, then, diplomacy, negotiation, and communication skills are essential.
The influences on the architect, and hence on the architecture, are shown in Figure 1.3. Architects are influenced by the requirements for the product as derived from its stakeholders, the structure and goals of the developing organization, the available technical environment, and their own background and experience.
The main message of this book is that the relationships among business goals, product requirements, architects' experience, architectures, and fielded systems form a cycle with feedback loops that a business can manage. A business manages this cycle to handle growth, to expand its enterprise area, and to take advantage of previous investments in architecture and system building. Figure 1.4 shows the feedback loops. Some of the feedback comes from the architecture itself, and some comes from the system built from it.
Here is how the cycle works:
The architecture affects the structure of the developing organization. An architecture prescribes a structure for a system; as we will see, it particularly prescribes the units of software that must be implemented (or otherwise obtained) and integrated to form the system. These units are the basis for the development project's structure. Teams are formed for individual software units; and the development, test, and integration activities all revolve around the units. Likewise, schedules and budgets allocate resources in chunks corresponding to the units. If a company becomes adept at building families of similar systems, it will tend to invest in each team by nurturing each area of expertise. Teams become embedded in the organization's structure. This is feedback from the architecture to the developing organization.
In the software product line case study in Chapter 15, separate groups were given responsibility for building and maintaining individual portions of the organization's architecture for a family of products. In any design undertaken by the organization at large, these groups have a strong voice in the system's decomposition, pressuring for the continued existence of the portions they control.
The architecture can affect the goals of the developing organization. A successful system built from it can enable a company to establish a foothold in a particular market area. The architecture can provide opportunities for the efficient production and deployment of similar systems, and the organization may adjust its goals to take advantage of its newfound expertise to plumb the market. This is feedback from the system to the developing organization and the systems it builds.
The architecture can affect customer requirements for the next system by giving the customer the opportunity to receive a system (based on the same architecture) in a more reliable, timely, and economical manner than if the subsequent system were to be built from scratch. The customer may be willing to relax some requirements to gain these economies. Shrink-wrapped software has clearly affected people's requirements by providing solutions that are not tailored to their precise needs but are instead inexpensive and (in the best of all possible worlds) of high quality. Product lines have the same effect on customers who cannot be so flexible with their requirements. In Chapter 15 (CelsuisTech, A Case Study in Product Line Development), we will show how a product line architecture caused customers to happily compromise their requirements because they could get high-quality software that fit their basic needs quickly, reliably, and at lower cost.
The process of system building will affect the architect's experience with subsequent systems by adding to the corporate experience base. A system that was successfully built around a tool bus or .NET or encapsulated finite-state machines will engender similar systems built the same way in the future. On the other hand, architectures that fail are less likely to be chosen for future projects.
A few systems will influence and actually change the software engineering culture, that is, the technical environment in which system builders operate and learn. The first relational databases, compiler generators, and table-driven operating systems had this effect in the 1960s and early 1970s; the first spreadsheets and windowing systems, in the 1980s. The World Wide Web is the example for the 1990s, as we will suggest in its case study in Chapter 13. J2EE may be the example for the first decade of the twenty-first century, as we will discuss in Chapter 16. When such pathfinder systems are constructed, subsequent systems are affected by their legacy.
These and other feedback mechanisms form what we call the ABC, illustrated in Figure 1.4, which depicts the influences of the culture and business of the development organization on the software architecture. That architecture is, in turn, a primary determinant of the properties of the developed system or systems. But the ABC is also based on a recognition that shrewd organizations can take advantage of the organizational and experiential effects of developing an architecture and can use those effects to position their business strategically for future projects.