Unfortunately, some descriptions of software architecture fail to make this distinction e. A comprehensive examination of the scope and intellectual basis for software architecture can be found in Perry and Wolf [ ]. They present a model that defines a software architecture as a set of architectural elements that have a particular form , explicated by a set of rationale. Architectural elements include processing, data, and connecting elements. Form is defined by the properties of the elements and the relationships among the elements -- that is, the constraints on the elements.
The rationale provides the underlying basis for the architecture by capturing the motivation for the choice of architectural style, the choice of elements, and the form. My definitions for software architecture are an elaborated version of those within the Perry and Wolf [ ] model, except that I exclude rationale.
Although rationale is an important aspect of software architecture research and of architectural description in particular, including it within the definition of software architecture would imply that design documentation is part of the run-time system. The presence or absence of rationale can influence the evolution of an architecture, but, once constituted, the architecture is independent of its reasons for being.
Reflective systems [ 80 ] can use the characteristics of past performance to change future behavior, but in doing so they are replacing one lower-level architecture with another lower-level architecture, rather than encompassing rationale within those architectures. As an illustration, consider what happens to a building if its blueprints and design plans are burned. Does the building immediately collapse? No, since the properties by which the walls sustain the weight of the roof remain intact.
An architecture has, by design, a set of properties that allow it to meet or exceed the system requirements. Ignorance of those properties may lead to later changes which violate the architecture, just as the replacement of a load-bearing wall with a large window frame may violate the structural stability of a building. Thus, instead of rationale, our definition of software architecture includes architectural properties.
Rationale explicates those properties, and lack of rationale may result in gradual decay or degradation of the architecture over time, but the rationale itself is not part of the architecture. A key feature of the model in Perry and Wolf [ ] is the distinction of the various element types. Processing elements are those that perform transformations on data, data elements are those that contain the information that is used and transformed, and connecting elements are the glue that holds the different pieces of the architecture together.
I use the more prevalent terms of components and connectors to refer to processing and connecting elements, respectively. Garlan and Shaw [ 53 ] describe an architecture of a system as a collection of computational components together with a description of the interactions between these components--the connectors.
This model is expanded upon in Shaw et al. In addition to specifying the structure and topology of the system, the architecture shows the intended correspondence between the system requirements and elements of the constructed system. Further elaboration of this definition can be found in Shaw and Garlan [ ]. What is surprising about the Shaw et al.
In the process, software architecture as a whole is reduced to what is commonly found in most informal architecture diagrams: boxes components and lines connectors. Data elements, along with many of the dynamic aspects of real software architectures, are ignored. Such a model is incapable of adequately describing network-based software architectures, since the nature, location, and movement of data elements within the system is often the single most significant determinant of system behavior.
Components are the most easily recognized aspect of software architecture. Perry and Wolf's [ ] processing elements are defined as those components that supply the transformation on the data elements. Garlan and Shaw [ 53 ] describe components simply as the elements that perform computation. Our definition attempts to be more precise in making the distinction between components and the software within connectors.
A component is an abstract unit of software instructions and internal state that provides a transformation of data via its interface. Example transformations include loading into memory from secondary storage, performing some calculation, translating to a different format, encapsulation with other data, etc. The behavior of each component is part of the architecture insofar as that behavior can be observed or discerned from the point of view of another component [ 9 ].
In other words, a component is defined by its interface and the services it provides to other components, rather than by its implementation behind the interface. Parnas [ ] would define this as the set of assumptions that other architectural elements can make about the component.
Perry and Wolf [ ] describe connecting elements vaguely as the glue that holds the various pieces of the architecture together. A more precise definition is provided by Shaw and Clements [ ]: A connector is an abstract mechanism that mediates communication, coordination, or cooperation among components. Examples include shared representations, remote procedure calls, message-passing protocols, and data streams.
Perhaps the best way to think about connectors is to contrast them with components. Connectors enable communication between components by transferring data elements from one interface to another without changing the data. Internally, a connector may consist of a subsystem of components that transform the data for transfer, perform the transfer, and then reverse the transformation for delivery.
However, the external behavioral abstraction captured by the architecture ignores those details. In contrast, a component may, but not always will, transform data from the external perspective. As noted above, the presence of data elements is the most significant distinction between the model of software architecture defined by Perry and Wolf [ ] and the model used by much of the research labelled software architecture [ 1 , 5 , 9 , 53 , 56 , - , ].
Boasson [ 24 ] criticizes current software architecture research for its emphasis on component structures and architecture development tools, suggesting that more focus should be placed on data-centric architectural modeling. Similar comments are made by Jackson [ 67 ]. A datum is an element of information that is transferred from a component, or received by a component, via a connector. Examples include byte-sequences, messages, marshalled parameters, and serialized objects, but do not include information that is permanently resident or hidden within a component.
From the architectural perspective, a "file" is a transformation that a file system component might make from a "file name" datum received on its interface to a sequence of bytes recorded within an internally hidden storage system. Components can also generate data, as in the case of a software encapsulation of a clock or sensor. The nature of the data elements within a network-based application architecture will often determine whether or not a given architectural style is appropriate.
This is particularly evident in the comparison of mobile code design paradigms [ 50 ], where the choice must be made between interacting with a component directly or transforming the component into a data element, transferring it across a network, and then transforming it back to a component that can be interacted with locally. It is impossible to evaluate such an architecture without considering data elements at the architectural level. Abowd et al.
Various style-specific concrete notations may be used to represent these visually, facilitate the description of legal computations and interactions, and constrain the set of desirable systems. Strictly speaking, one might think of a configuration as being equivalent to a set of specific constraints on component interaction.
For example, Perry and Wolf [ ] include topology in their definition of architectural form relationships. However, separating the active topology from more general constraints allows an architect to more easily distinguish the active configuration from the potential domain of all legitimate configurations. Additional rationale for distinguishing configurations within architectural description languages is presented in Medvidovic and Taylor [ 86 ]. The set of architectural properties of a software architecture includes all properties that derive from the selection and arrangement of components, connectors, and data within the system.
Examples include both the functional properties achieved by the system and non-functional properties, such as relative ease of evolution, reusability of components, efficiency, and dynamic extensibility, often referred to as quality attributes [ 9 ]. Properties are induced by the set of constraints within an architecture.
Constraints are often motivated by the application of a software engineering principle [ 58 ] to an aspect of the architectural elements. For example, the uniform pipe-and-filter style obtains the qualities of reusability of components and configurability of the application by applying generality to its component interfaces -- constraining the components to a single interface type.
Hence, the architectural constraint is "uniform component interface," motivated by the generality principle, in order to obtain two desirable qualities that will become the architectural properties of reusable and configurable components when that style is instantiated within an architecture. The goal of architectural design is to create an architecture with a set of architectural properties that form a superset of the system requirements.
The relative importance of the various architectural properties depends on the nature of the intended system. Section 2. Since an architecture embodies both functional and non-functional properties, it can be difficult to directly compare architectures for different types of systems, or for even the same type of system set in different environments. Styles are a mechanism for categorizing architectures and for defining their common characteristics [ 38 ]. Each style provides an abstraction for the interactions of components, capturing the essence of a pattern of interaction by ignoring the incidental details of the rest of the architecture [ ].
Perry and Wolf [ ] define architectural style as an abstraction of element types and formal aspects from various specific architectures, perhaps concentrating on only certain aspects of an architecture. An architectural style encapsulates important decisions about the architectural elements and emphasizes important constraints on the elements and their relationships. This definition allows for styles that focus only on the connectors of an architecture, or on specific aspects of the component interfaces.
In contrast, Garlan and Shaw [ 53 ], Garlan et al. Specifically, an architectural style determines the vocabulary of components and connectors that can be used in instances of that style, together with a set of constraints on how they can be combined [ 53 ]. This restricted view of architectural styles is a direct result of their definition of software architecture -- thinking of architecture as a formal description, rather than as a running system, leads to abstractions based only in the shared patterns of box and line diagrams.
New architectures can be defined as instances of specific styles [ 38 ]. Since architectural styles may address different aspects of software architecture, a given architecture may be composed of multiple styles. Likewise, a hybrid style can be formed by combining multiple basic styles into a single coordinated style. Some architectural styles are often portrayed as "silver bullet" solutions for all forms of software. However, a good designer should select a style that matches the needs of the particular problem being solved [ ].
Choosing the right architectural style for a network-based application requires an understanding of the problem domain [ 67 ] and thereby the communication needs of the application, an awareness of the variety of architectural styles and the particular concerns they address, and the ability to anticipate the sensitivity of each interaction style to the characteristics of network-based communication [ ].
Unfortunately, using the term style to refer to a coordinated set of constraints often leads to confusion. This usage differs substantially from the etymology of style, which would emphasize personalization of the design process. Loerke [ 76 ] devotes a chapter to denigrating the notion that personal stylistic concerns have any place in the work of a professional architect.
Instead, he describes styles as the critics' view of past architecture, where the available choice of materials, the community culture, or the ego of the local ruler were responsible for the architectural style, not the designer. In other words, Loerke views the real source of style in traditional building architecture to be the set of constraints applied to the design, and attaining or copying a specific style should be the least of the designer's goals.
Since referring to a named set of constraints as a style makes it easier to communicate the characteristics of common constraints, we use architectural styles as a method of abstraction, rather than as an indicator of personalized design. In parallel with the software engineering research in architectural styles, the object-oriented programming community has been exploring the use of design patterns and pattern languages to describe recurring abstractions in object-based software development.
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