These concepts, as shown in Figure 2.1, are the role of the architect, the characteristics of the architecting tasks they perform, and the architecture that results.
Figure 2.1 Core Concepts Used in This Book
There is no shortage of definitions when it comes to architecture. Even some websites maintain collections of definitions (SEI 2009). The definition used in this book is that taken from IEEE 1471-2000, IEEE Recommended Practice for Architectural Description of Software-Intensive Systems (IEEE 1471 2000). This definition follows, with key characteristics highlighted:
- [Architecture is] The fundamental organization of a system embodied in its components, their relationships to each other, and to the environment, and the principles guiding its design and evolution. (IEEE 1471 2000)
This standard also defines the following terms related to this definition:
- [A system is] a collection of components organized to accomplish a specific function or set of functions. The term system encompasses individual applications, systems in the traditional sense, subsystems, systems of systems, product lines, product families, whole enterprises, and other aggregations of interest. A system exists to fulfill one or more missions in its environment. (IEEE 1471 2000)
- The environment, or context, determines the setting and circumstances of developmental, operational, political, and other influences upon that system. (IEEE 1471 2000)
- A mission is a use or operation for which a system is intended by one or more stakeholders to meet some set of objectives. (IEEE 1471 2000)
- [A system stakeholder is] an individual, team, or organization (or classes thereof) with interests in, or concerns relative to, a system. (IEEE 1471 2000)
As you can see, the term component is used in this definition. Most definitions of architecture do not define the term component, however, and IEEE 1471 is no exception, choosing to leave it deliberately vague because the term is intended to cover the many interpretations in the industry. A component may be logical or physical, technology-independent or technology-specific, large-grained or small-grained. For the purposes of this book, we use the definition of component from the Unified Modeling Language (UML) 2.2 specification:
- A component represents a modular part of a system that encapsulates its contents and whose manifestation is replaceable within its environment. A component defines its behavior in terms of provided and required interfaces. As such, a component serves as a type whose conformance is defined by these provided and required interfaces (encompassing both their static as well as dynamic semantics). One component may therefore be substituted by another only if the two are type conformant. (UML 2.2 2009)
It is worth considering some other definitions of architecture so that you can observe similarities among those definitions. Consider the following definitions, in which some of the key characteristics are highlighted:
- An architecture is the set of significant decisions about the organization of a software system, the selection of structural elements and their interfaces by which the system is composed, together with their behavior as specified in the collaborations among those elements, the composition of these elements into progressively larger subsystems, and the architectural style that guides this organization—these elements and their interfaces, their collaborations, and their composition. (Kruchten 2000)
- 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. (Bass 2003)
- The software architecture of a system or a collection of systems consists of all the important design decisions about the software structures and the interactions between those structures that comprise the systems. The design decisions support a desired set of qualities that the system should support to be successful. The design decisions provide a conceptual basis for system development, support, and maintenance. (McGovern 2004)
You can see that although the definitions are somewhat different, they have a large degree of commonality. Most definitions indicate that an architecture is concerned with both structure and behavior, is concerned with significant elements only, may conform to an architectural style, is influenced by its stakeholders and its environment, and embodies decisions based on rationale. All these themes and others are discussed in the following sections.
An Architecture Defines Structure
If you were to ask someone to describe architecture to you, nine times out of ten, that person would make some reference to something related to structure, quite often in relation to a building or some other civil-engineering structure, such as a bridge. Although other characteristics of these items exist, such as behavior, fitness for purpose, and even aesthetics, the structural characteristic is the most familiar and the most often mentioned.
It should not surprise you, then, that if you ask someone to describe the architecture of a software system that he or she is working on, you'll probably be shown a diagram that shows the structural aspects of the system, whether these aspects are architectural layers, components, or distribution nodes. Structure is indeed an essential characteristic of an architecture.
The structural aspects of an architecture manifest themselves in many ways, and most definitions of architecture are deliberately vague as a result. A structural element could be a subsystem, a process, a library, a database, a computational node, a legacy system, an off-the-shelf product, and so on.
Many definitions of architecture also acknowledge not only the structural elements themselves, but also the composition of structural elements, their relationships (and any connectors needed to support these relationships), their interfaces, and their partitioning. Again, each of these elements can be provided in a variety of ways. A connector, for example, could be a socket, be synchronous or asynchronous, be associated with a particular protocol, and so on.
Figure 2.2 shows an example of some structural elements. This figure shows a UML component diagram containing some structural elements in an order processing system. You see three components, named Order Entry, Customer Management, and Account Management. The Order Entry component is shown as depending on the Customer Management component and also on the Account Management component, indicated by a UML dependency.
Figure 2.2 UML Component Diagram Showing Structural Elements
An Architecture Defines Behavior
As well as defining structural elements, an architecture defines the interactions among these structural elements. These interactions provide the desired system behavior.
Figure 2.3 is a UML sequence diagram showing several interactions that, together, allow the system to support the creation of an order in an order processing system. The figure shows five interactions. First, a Sales Clerk actor creates an order by using an instance of the Order Entry component. The Order Entry instance gets customer details by using an instance of the Customer Management component. Then the Order Entry instance uses an instance of the Account Management component to create the order, populate the order with order items, and place the order.
Figure 2.3 UML Sequence Diagram Showing Behavioral Elements
It should be noted that Figure 2.3 is consistent with Figure 2.2 in that you can derive the dependencies shown in Figure 2.2 from the interactions defined in Figure 2.3. An instance of Order Entry, for example, depends on an instance of Customer Management during its execution, as shown by the interactions in Figure 2.3. This dependency is reflected in a dependency relationship between the corresponding Order Entry and Customer Management components, as shown in Figure 2.2.
An Architecture Focuses on Significant Elements
Although an architecture defines structure and behavior, it is not concerned with defining all the structure and all the behavior. It is concerned only with those elements that are deemed to be significant. Significant elements are those that have a long and lasting effect, such as the major structural elements, those elements associated with essential behavior, and those elements that address significant qualities such as reliability and scalability. In general, the architecture is not concerned with the fine-grained details of these elements. Architectural significance can also be phrased as economical significance, because the primary drivers for considering certain elements over others are the cost of creation and the cost of change.
Because an architecture focuses on significant elements only, it provides a particular focus of the system under consideration—the focus that is most relevant to the architect. In this sense, an architecture is an abstraction of the system that helps an architect manage complexity.
It is also worth noting that the set of significant elements is not static and may change over time. As a consequence of requirements being refined, risks identified, executable software built, and lessons learned, the set of significant elements may change. The relative stability of the architecture in the face of change, however, is to some extent the sign of a good architecture, the sign of a well-executed architecting process, and the sign of a good architect. Continual revision of the architecture due to relatively minor changes is not a good sign. If the architecture is relatively stable, however, the converse is true.
An Architecture Balances Stakeholder Needs
An architecture is created ultimately to address a set of system stakeholder needs, but often, meeting all the expressed needs is not possible. A stakeholder may ask for some functionality within a specified time frame, for example, but the two needs—functionality and time frame—are mutually exclusive. Either the scope can be reduced to meet the schedule, or all the functionality can be provided within an extended time frame. Similarly, different stakeholders may express conflicting needs, and again, an appropriate balance must be achieved. Making trade-offs, therefore, is an essential aspect of the architecting process, and negotiation is an essential characteristic of the architect.
Just to give you an idea of the task at hand, consider the following needs of a set of stakeholders:
- The needs of the end user are associated with intuitive and correct behavior, performance, reliability, usability, availability, and security.
- The needs of the system administrator are associated with intuitive behavior, administration, and tools to aid monitoring.
- The needs of the marketer are associated with competitive features, time to market, positioning with other products, and cost.
- The needs of the customer are associated with cost, stability, and schedule.
- The needs of the developer are associated with clear requirements and a simple, consistent design approach.
- The needs of the project manager are associated with predictability in the tracking of the project, schedule, productive use of resources, and budget.
- The needs of the maintainer are associated with a comprehensible, consistent, and documented design approach, as well as the ease with which modifications can be made.
Another challenge for the architect, as you can see from this list, is that the stakeholders are not concerned only that the system provide the required functionality. Many of the concerns that are listed are non-functional in nature (in that they do not contribute to the functionality of the system). Such concerns represent system qualities or constraints. Non-functional requirements are quite often the most significant requirements as far as an architect is concerned; they are discussed in detail in Chapter 7, "Defining the Requirements."
An Architecture Embodies Decisions Based on Rationale
An important aspect of an architecture is not just the end result—the architecture itself—but also the rationale that explains why it is the way it is. Thus, as described in Chapter 4, "Documenting a Software Architecture," important considerations are the documenting of the decisions that led to this architecture and the rationale for these decisions.
This information is relevant to many stakeholders, especially those who must maintain the system. This information is often valuable to the architects themselves when they need to revisit the rationale behind the decisions that were made, so that they don't end up having to retrace their steps unnecessarily. This information is used when the architecture is reviewed and the architect needs to justify the decisions that have been made, for example.
Also, some of these decisions may have been imposed on the architect and, in this sense, represent constraints on the solution. A companywide policy to use particular technologies and products may exist, for example, and this policy needs to be accommodated in the solution.
An Architecture May Conform to an Architectural Style
Most architectures are derived from systems that have a similar set of concerns. This similarity can be described as an architectural style, a term borrowed from the styles used in building architectures. You can think of an architectural style as being a particular kind of pattern, albeit an often complex and composite pattern (several patterns applied together).
- Every well-structured software-intensive system is full of patterns. (Booch 2009)
An architectural style represents a codification of experience, and it is good practice for architects to look for opportunities to reuse such experience. Examples of architectural styles include a distributed style, a pipes-and-filters style, a data-centered style, a rule-based style, service-oriented architecture, and so on. Architectural styles are discussed further in Chapter 5, "Reusable Architecture Assets." A given system may exhibit more than one architectural style.
- [An architectural style] defines a family of systems in terms of a pattern of structural organization. More specifically, an architectural style defines a vocabulary of components and connector types, and a set of constraints on how they can be combined. (Shaw 1996)
The application of an architectural style (and reusable assets in general) makes the life of an architect somewhat easier because such assets are already proved, thereby reducing risk and, of course, effort. A style is normally documented in terms of the rationale for using it (so there is less thinking to be done) and in terms of its key structures and behaviors (so there is less architecture documentation to be produced because you can simply refer to the style instead). Architecture assets are discussed in detail in Chapter 5, "Reusable Architecture Assets."
An Architecture Is Influenced by Its Environment
A system resides in an environment, and this environment influences the architecture. This is sometimes referred to as architecture in context. In essence, the environment determines the boundaries within which the system must operate, which then influence the architecture. The environmental factors that influence the architecture include the business mission that the architecture will support, the system stakeholders, internal technical constraints (such as the requirement to conform to organizational standards), and external technical constraints (such as the need to use a specified technology, interface to an external system, or conform to external regulatory standards).
Conversely, as eloquently described in Software Architecture in Practice, 2nd ed. (Bass 2003), the architecture may also influence its environment. The creation of an architecture may contribute reusable assets to the owning organization, for example, thereby making such assets available to the next development effort. Another example is the selection of a software package that is used within the architecture, such as a customer relationship management (CRM) system, which subsequently requires users to change the processes they follow to accommodate the way that the package works.
An Architecture Influences Development Team Structure
An architecture defines coherent groupings of related elements that address a given set of concerns. An architecture for an order processing system, for example, may have defined groupings of elements for order entry, account management, customer management, fulfillment, integrations with external systems, persistence, and security.
Each of these groupings may require different skill sets. Therefore, it makes perfect sense to align the software development team structures with the architecture after it has been defined. Often, however, the architecture is influenced by the initial team structure and not vice versa. This pitfall is best avoided, because the result typically is a less-than-ideal architecture. Conway's Law states, "If you have four groups working on a compiler, you'll get a four-pass compiler." In practice, architects often unintentionally create architectures that reflect the organization creating the architecture.
Although organizing work in line with the architecture can be beneficial, this somewhat idealized view is not always practical. For purely pragmatic reasons, the current team structure and the skills available (both in the current team and the maintenance teams) represent a very real constraint on what is possible, and the architect must take this constraint into account. The geographic distribution of the team is another constraint that needs to be accommodated:
- The architectural partitioning should reflect the geographic partitioning, and vice versa. Architectural responsibilities should be assigned so decisions can be made (geographically) locally. (Coplien 2005)
An Architecture Is Present in Every System
It is also worth noting that every system has an architecture, even if this architecture is not formally documented or if the system is extremely simple and, say, consists of a single element. Documenting the architecture usually has considerable value; this topic is discussed in detail in Chapter 4, "Documenting a Software Architecture."
- Ultimately, every software-intensive system has an architecture, be it intentional or accidental. Every such architecture serves to hold back the forces upon that system in a manner that is functional, economical and elegant. (Booch 2009)
If an architecture is not documented, it is difficult (if not impossible) to assess the architecture and prove that it meets the stated requirements in terms of development-time qualities such as flexibility, accommodation of best practices, and so on. A lack of documentation can also make it extremely difficult to maintain the system over time.
An Architecture Has a Particular Scope
Many kinds of architecture exist, the best known being the architecture associated with buildings and other civil-engineering structures. Even in the field of software engineering, you often come across different forms of architecture. In addition to the concept of software architecture, for example, you may encounter concepts such as enterprise architecture, system architecture, information architecture, hardware architecture, application architecture, infrastructure architecture, and data architecture. You also hear other terms mentioned. Each of these terms defines a specific scope of the architecting activities.
Unfortunately, the industry has come to no agreement on the meanings of all these terms or their relationships to one another, resulting in different meanings for the same term (homonyms) and two or more terms that mean the same thing (synonyms). You can infer the scope of some of these terms, as used in this book, from Figure 2.4, in which we focus on the architecture of software-intensive systems. As you consider this figure and the discussion that follows it, you will almost certainly find elements that you disagree with or that you use differently within your organization. Consider this example to be an acknowledgment of (and one interpretation of) several possible scopes of architecting activities.
Figure 2.4 Different Architecting Scopes
Inspiration for the elements shown in Figure 2.4 came from various sources. IEEE Std 12207-1995, the IEEE Standard for Information Technology—Software Life Cycle Processes, defines a system as follows:
- [A system is] an integrated composite that consists of one or more of the processes, hardware, software, facilities and people, that provides a capability to satisfy a stated need or objective. (IEEE 12207 1995)
A configuration of the Rational Unified Process for Systems Engineering (RUP SE), also known as Model-Driven Systems Development (MDSD), contains a similar definition:
- [A system is] a set of resources that provide services that are used by an enterprise to carry out a business purpose or mission. System components typically consist of hardware, software, data, and workers. (Cantor 2003)
The various elements shown in Figure 2.4 are
- Software. This element is the main focus of this book and considers items such as components, relationships between components, and interactions between components.
- Hardware. This element considers items such as CPUs, memory, hard disks, peripheral devices such as printers, and the elements used to connect these elements.
- Information. This element considers the information used within the system.
- Workers. This element considers the people-related aspects of a system, such as business processes, organizational structures, roles and responsibilities, and core competencies of the organization.
- System. As described in the preceding definitions, a system comprises software, hardware, information, and workers.
- Enterprise. This element is similar to a system in that it, too, considers elements such as hardware, software, information, and workers. An enterprise, however may span multiple systems and place constraints on the systems that are part of the enterprise. An enterprise also has a stronger link to the business than a system does, in that an enterprise focuses on the attainment of the business objectives and is concerned with items such as business strategy, business agility, and organizational efficiency. Further, an enterprise may cross company boundaries.
Because we focus on software-intensive systems in this book, it is worth making some additional observations. In particular, we should note that a software-intensive system is a system. Therefore, you should understand the relationship between the software and those elements with which it must coexist:
- Software and workers. Although workers are not considered to be part of the systems considered by this book, a relationship exists in terms of the functionality that the system must provide to support any people who interact with the system. Ensuring that this functionality is provided is, in this book, the responsibility of the application architect.
- Software and hardware. A particular aspect of the environment that must always be considered in software-intensive systems is the hardware on which the software executes. The resulting system, therefore, is a combination of software and hardware, and this combination allows properties such as reliability and performance to be achieved. Software cannot achieve these properties in isolation from the hardware on which it executes. Ensuring the appropriate consideration of hardware is, in this book, the responsibility of the infrastructure architect.
- Software and information. Software elements may produce and consume persistent information during their execution. Ensuring the appropriate consideration of information is, in this book, the responsibility of the data architect.
For each specific type of architecture, a corresponding type of architect exists (software architect, hardware architect, and so on), as well as a corresponding type of architecting (software architecting, hardware architecting, and so on).
Now that you've gotten through these definitions, you may have many unanswered questions. What is the difference between an enterprise architecture and a system architecture? Is an enterprise a system? Is an information architecture the same as the data architecture found in some data-intensive software applications? Unfortunately, no set of agreed-on answers to these questions exists. For now, be aware that these different terms exist but that the industry has no consistent definition of these terms and how they relate to one another. We recommend, therefore, that you select the terms that are relevant to your organization and define them appropriately. Then you will achieve some consistency, at least, and reduce the potential for miscommunication.