04 designing architectures

Designing Architectures Software Architecture Lecture 4

How Do You Design? Where do architectures come from? Method Efficient in familiar terrain Not always successful Predictable outcome (+ & - ) Quality of methods varies Creativity Fun! Fraught with peril May be unnecessary May yield the best

Objectives Creativity Enhance your skillset Provide new tools Method Focus on highly effective techniques Develop judgment: when to develop novel solutions, and when to follow established method

Engineering Design Process Feasibility stage: identifying a set of feasible concepts for the design as a whole Preliminary design stage: selection and development of the best concept. Detailed design stage: development of engineering descriptions of the concept. Planning stage: evaluating and altering the concept to suit the requirements of production, distribution, consumption and product retirement.

Potential Problems If the designer is unable to produce a set of feasible concepts, progress stops. As problems and products increase in size and complexity, the probability that any one individual can successfully perform the first steps decreases. The standard approach does not directly address the situation where system design is at stake, i.e. when relationship between a set of products is at issue.  As complexity increases or the experience of the designer is not sufficient, alternative approaches to the design process must be adopted.

Alternative Design Strategies Standard Linear model described above Cyclic Process can revert to an earlier stage Parallel Independent alternatives are explored in parallel Adaptive (“lay tracks as you go”) The next design strategy of the design activity is decided at the end of a given stage Incremental Each stage of development is treated as a task of incrementally improving the existing design

Identifying a Viable Strategy Use fundamental design tools: abstraction and modularity. But how? Inspiration, where inspiration is needed. Predictable techniques elsewhere. But where is creativity required? Applying own experience or experience of others.

The Tools of “Software Engineering 101” Abstraction Abstraction(1): look at details, and abstract “up” to concepts Abstraction(2): choose concepts, then add detailed substructure, and move “down” Example: design of a stack class Separation of concerns

A Few Definitions… from the OED Online Abstraction: “The act or process of separating in thought, of considering a thing independently of its associations; or a substance independently of its attributes; or an attribute or quality independently of the substance to which it belongs.” Reification: “The mental conversion of … [an] abstract concept into a thing.” Deduction: “The process of drawing a conclusion from a principle already known or assumed; spec. in Logic, inference by reasoning from generals to particulars ; opposed to INDUCTION.” Induction: “The process of inferring a general law or principle from the observation of particular instances (opposed to DEDUCTION, q.v.).”

Abstraction and the Simple Machines What concepts should be chosen at the outset of a design task? One technique: Search for a “simple machine” that serves as an abstraction of a potential system that will perform the required task For instance, what kind of simple machine makes a software system embedded in a fax machine? At core, it is basically just a little state machine. Simple machines provide a plausible first conception of how an application might be built. Every application domain has its common simple machines.

Simple Machines Domain Simple Machines Graphics Pixel arrays Transformation matrices Widgets Abstract depiction graphs Word processing Structured documents Layouts Process control Finite state machines Income Tax Software Hypertext Spreadsheets Form templates Web pages Hypertext Composite documents Scientific computing Matrices Mathematical functions Banking Spreadsheets Databases Transactions

Choosing the Level and Terms of Discourse Any attempt to use abstraction as a tool must choose a level of discourse, and once that is chosen, must choose the terms of discourse. Alternative 1 : initial level of discourse is one of the application as a whole (step-wise refinement). Alternative 2: work, initially, at a level lower than that of the whole application. Once several such sub-problems are solved they can be composed together to form an overall solution Alternative 3 : work, initially, at a level above that of the desired application. E.g. handling simple application input with a general parser.

Separation of Concerns Separation of concerns is the subdivision of a problem into (hopefully) independent parts. The difficulties arise when the issues are either actually or apparently intertwined. Separations of concerns frequently involves many tradeoffs Total independence of concepts may not be possible. Key example from software architecture: separation of components (computation) from connectors (communication)

The Grand Tool: Refined Experience Experience must be reflected upon and refined. The lessons from prior work include not only the lessons of successes, but also the lessons arising from failure. Learn from success and failure of other engineers Literature Conferences Experience can provide that initial feasible set of “alternative arrangements for the design as a whole”.

Patterns, Styles, and DSSAs 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 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 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. Architectural patterns are similar to DSSAs but applied “at a lower level” and within a much narrower scope.

State-Logic-Display: Three-Tiered Pattern Application Examples Business applications Multi-player games Web-based applications Software Architecture: Foundations, Theory, and Practice ; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John Wiley & Sons, Inc. Reprinted with permission.

Model-View-Controller (MVC) 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 Software Architecture: Foundations, Theory, and Practice ; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John Wiley & Sons, Inc. Reprinted with permission.

The Lunar Lander: A Long-Running Example 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

Architectural Styles 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 Many styles exist and we will discuss them in detail in the next lecture

Definitions of Architectural Style 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. Recurring organizational patterns & idioms Established, shared understanding of common design forms Mark of mature engineering field. Shaw & Garlan Abstraction of recurring composition & interaction characteristics in a set of architectures Taylor

Basic Properties of Styles A vocabulary of design elements Component and connector types; data elements e.g., pipes, filters, objects, servers A set of configuration rules Topological constraints that determine allowed compositions of elements e.g., a component may be connected to at most two other components A semantic interpretation Compositions of design elements have well-defined meanings Possible analyses of systems built in a style

Benefits of Using Styles Design reuse Well-understood solutions applied to new problems Code reuse Shared implementations of invariant aspects of a style Understandability of system organization A phrase such as “client-server” conveys a lot of information Interoperability Supported by style standardization Style-specific analyses Enabled by the constrained design space Visualizations Style-specific depictions matching engineers’ mental models

Style Analysis Dimensions What is the design vocabulary? Component and connector types What are the allowable structural patterns? What is the underlying computational model? What are the essential invariants of the style? What are common examples of its use? What are the (dis)advantages of using the style? What are the style’s specializations?

Some Common Styles Traditional, language-influenced styles 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 “ Derived” styles C2 CORBA

Main Program and Subroutines LL Software Architecture: Foundations, Theory, and Practice ; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John Wiley & Sons, Inc. Reprinted with permission.

04 designing architectures

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