Basic Concepts Acknowledgement: slides from: Software Architecture: Foundations, Theory, and Practice; R. Taylor, N. Medvidovic, E.Dashofy
“Principal” implies a degree of importance that grants a design decision “architectural status” • Implies that not all design decisions are architectural (i.e. they do not necessarily impact a system’s architecture) • How one defines “principal” will depend on what the stakeholders define as the system goals Software Architecture Definition(source: Taylor et al., “Software Architecture: Foundations, Theory, and Practice) “[Software architecture is] the set of principal design decisions governing a system” • Design decisions encompass every facet of the system under development • Structure, Behavior, Interaction, Non-functional properties
Temporal Aspects 3 Design decisions are made and unmade over a system’s lifetime Architecture has a temporal aspect At any given point in time the system has only one architecture A system’s architecture will change over time
Prescriptive vs. Descriptive Architecture 4 • A system’s prescriptive architecture captures the design decisions made prior to the system’s construction • It is the as-conceived or as-intended architecture • A system’s descriptive architecture describes how the system has been built • It is the as-implemented or as-realized architecture
As-Designed vs. As-Implemented 5 Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John Wiley & Sons, Inc. Reprinted with permission.
Architectural Evolution 6 • When a system evolves, ideally its prescriptive architecture is modified first • In practice, the system – and thus its descriptive architecture – is often directly modified • This happens because of • Developer sloppiness • Perception of short deadlines which prevent thinking through and documenting • Lack of documented prescriptive architecture • Need or desire for code optimizations • Inadequate techniques or tool support
Architectural Degradation 7 • Architectural drift is introduction of principal design decisions into a system’s descriptive architecture that • are not included in, encompassed by, or implied by the prescriptive architecture • but which do not violate any of the prescriptive architecture’s design decisions • Architectural erosion is the introduction of architectural design decisions into a system’s descriptive architecture that violate its prescriptive architecture
Architectural Recovery 8 • If architectural degradation is allowed to occur, one will be forced to recover the system’s architecture sooner or later • Architectural recovery is the process of determining a software system’s architecture from its implementation-level artifacts • Implementation-level artifacts can be • Source code • Executable files • Java .class files
Implementation-Level View of an Application Complex and virtuallyincomprehensible! 9
Deployment 10 • A software system cannot fulfill its purpose until it is deployed • Executable modules are physically placed on the hardware devices on which they are supposed to run • The deployment view of an architecture can be critical in assessing whether the system will be able to satisfy its requirements • Possible assessment dimensions • Available memory • Power consumption • Required network bandwidth
Software Architecture’s Elements Components Connectors 12 • A software system’s architecture should be a composition and interplay of different elements • Processing • Data (also referred as information or state) • Interaction
Components 13 Elements that encapsulate processing and data in a system’s architecture are referred to as software components Definition: A software component is an architectural entity that • encapsulates a subset of the system’s functionality and/or data • restricts access to that subset via an explicitly defined interface • has explicitly defined dependencies on its required execution context • Components typically provide application-specific services
Connectors 14 In complex systems interaction may become more important and challenging than the functionality of the individual components Definition: A software connector is an architectural building block tasked with effecting and regulating interactions among components • In many software systems connectors are usually simple procedure calls or shared data accesses • Much more sophisticated and complex connectors are possible! • Connectors typically provide application-independent interaction facilities
Examples of Connectors 15 • Procedure call connectors • Shared data access connectors • E.g. shared memory • Distribution connectors • Remote procedure call connector • Message passing connectors • Streaming connectors • Wrapper/adaptor connectors
[programming] Design PatternsArchitectural StylesArchitectural PatternsDomain Specific Software Architectures 16 Certain design choices regularly result in solutions with superior properties Compared to other possible alternatives, these solutions are more elegant, effective, efficient, dependable, evolvable, scalable, and so on
17 Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John Wiley & Sons, Inc. Reprinted with permission.
Domain-Specific Software Architectures 18 • A DSSA is an assemblage of software components • specialized for a particular type of task (domain), • generalized for effective use across that domain, and • composed in a standardized structure (topology) effective for building successful applications. • Since DSSAs are specialized for a particular domain they are only of value if one exists for the domain wherein the engineer is tasked with building a new application. • DSSAs are the pre-eminent means for maximal reuse of knowledge and prior development and hence for developing a new architectural design.
Architectural Patterns 19 An architectural pattern is a set of architectural design decisions that are applicable to a recurring design problem, and parameterized to account for different software development contexts in which that problem appears.
State-Logic-Display: Three-Tiered Pattern 20 • Application Examples • Business applications • Multi-player games • Web-based applications
Model-View-Controller (MVC) 21 Objective: Separation between information, presentation and user interaction. When a model object value changes, a notification is sent to the view and to the controller. So that the view can update itself and the controller can modify the view if its logic so requires. When handling input from the user the windowing system sends the user event to the controller; If a change is required, the controller updates the model object.
Sense-Compute-Control Objective: Structuring embedded control applications 22
The Lunar Lander 23 • A simple computer game that first appeared in the 1960’s • Simple concept: • You (the pilot) control the descent rate of the Apollo-era Lunar Lander • Throttle setting controls descent engine • Limited fuel • Initial altitude and speed preset • If you land with a descent rate of < 5 fps: you win (whether there’s fuel left or not) • “Advanced” version: joystick controls attitude & horizontal motion
Sense-Compute-Control LL 24 Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John Wiley & Sons, Inc. Reprinted with permission.
Definitions of Architectural Style 26 • Definition. An architectural style is a named collection of architectural design decisions that • are applicable in a given development context • constrain architectural design decisions that are specific to a particular system within that context • elicit beneficial qualities in each resulting system. • A primary way of characterizing lessons from experience in software system design • Reflect less domain specificity than architectural patterns • Useful in determining everything from subroutine structure to top-level application structure
Some Common Styles • Traditional, language-influenced • Main program and subroutines • Object-oriented • Layered • Virtual machines • Client-server • Data-flow styles • Batch sequential • Pipe and filter • Shared memory • Blackboard • Rule based • Interpreter • Interpreter • Mobile code • Implicit invocation • Event-based • Publish-subscribe • Peer-to-peer 27