10/27/2014. MTAT Software Engineering. Exams. Acknowledgements. Schedule of Lectures. A tale of two systems. Structure of Lecture 07

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1 MTAT Software Engineering Lecture 07: Architecture and Design Exams Date Time Place: Exam 1: Friday, 09-Jan-2015, 14:15-16:15, room Exam 2: Friday, 16-Jan-2015, 14:15-16:15, room Re-take Exam: Friday, 23-Jan-2015, 14:15-16:15, room Capacity limit of 90 students Fall 2014 Dietmar Pfahl You must register: First-come-first-serve (FIFO) principle Schedule of Lectures Acknowledgements Week 01: Introduction to SE Week 02: Requirements Engineering I Week 03: Requirements Engineering II Week 04: Analysis Week 05: Dev. Infrastructure I Week 06: Dev. Infrastructure II Week 07: ICS Day / ATI Päev 2014 Week 08: Architecture and Design Week 09: Refactoring Week 10: Verification and Validation Week 11: Software Quality Management Week 12: Agile/Lean Methods Week 13: Measurement & Process Improvement Week 14: Course wrap-up, review and exam preparation Week 15: no lecture Week 16: no lecture Textbooks/Slides: Ian Sommerville: Software Engineering, 9th edition, 2010 ( Hans van Vliet: Software Architecture, Free University of Amsterdam, Lecture 2008 Richard Taylor et al.: Software Architecture, University of California at Irvine, Lecture 2011 Alexander Serebrenik: Software architecture: Domain-Specific Software Architecture and Architectural Patterns, TU Eindhoven, Lecture 2013 George Fairbanks: Just Enough Software Architecture, 2012 (Video: Structure of Lecture 07 A tale of two systems What is it? Why bother? Terminology: Architect, Architecting, Architecture s and View Models Notation Patterns, Styles and DSSAs Analysis/Assessment Wrap-up 1

2 Let s design a system! The system is real Rackspace Rackspace: Architecture 1 Rackspace: Architecture 2 CSR: Customer Service Request CSR: Customer Service Request Rackspace: Architecture 3 Rackspace: Quality Attribute Trade-offs CSR: Customer Service Request ( 2

3 What if you don t think architecturally? Virtuosos and Roman Engineers Remember: All programs have an architecture... But not every architecture suits the program! Structure of Lecture 07 Terminology What is it? Why bother? Terminology: Architect, Architecting, Architecture s and View Models Notation Patterns, Styles and DSSAs Analysis/Assessment Wrap-up Architect Person Architecting Process Architecture Product The Role of the Architect Pre-Architecture Life-Cycle client, users requirements architect solutions developers stakeholders (few) functionality quality assess creates assess Characteristics: Iteration mainly on functional requirements agreement appearance, behaviour visualises architectural design prescribes construction, co-operation Few stakeholders involved No balancing of functional and quality requirements development 3

4 Adding Architecture: The Easy Way Architecture in the Life-Cycle stakeholders (many) stakeholders (few) functionality quality functionality quality architecture detailed design implementation agreement development Characteristics: Iteration on both functional and quality requirements Many stakeholders involved architecture agreement development Balancing of functional and quality requirements Architecture Drivers Importance Difficulty Rational Architecture Decisions Design Issues, Options and Decisions A designer is faced with a series of design issues These are sub-problems of the overall design problem. For example, a design issue can be the type and level of security. Security can be decomposed into authentication, authorization, and privacy. Each issue normally has several alternative solutions (or design options) The designer makes a design decision to resolve each issue. This process involves choosing the best option from among the alternatives. Attribute-Driven Design (ADD) ADD Example Iterations Top-level: usability separate UI 3-tier architecture Lower-level, within user interface: security authenticate users Lower-level, within data layer: availability active redundancy 4

5 Decision Space The space of possible designs that can be achieved by choosing different sets of alternatives. Issues that can be relevant in the decision process could be: - level of flexibility; - outsourcing/external acquisition of client technology (that mans need for separate presentation also relevant for the budget); - if using the Web/Internet; - performance;... Tree or Graph? Issues and options are not independent... A number of options become invalid due to a desired NFR (quality). For example, flexibility could be achieved through certain architectural patterns, like MVC which facilitates separation of concerns and layered architecture restricting client-server interactions. If we choose any of the two, we exclude the 'monolithic' sub-tree and we need a separate GUI layer. More than just IT Technical and non-technical issues and options are intertwined Architects deciding on the type of database versus Management deciding on new strategic partnership or Management deciding on budget Once again: Why is documenting design decisions important? Prevents repeating (expensive) past steps Explains why this is a good (better: suitable) architecture Emphasizes qualities and criticality for requirements/goals Provides context and background Design rationale example: Architecture in Construction of Buildings Software Architecture Architecture is conceptual. Architectural descriptions are concrete, but the architecture itself is inherently conceptual, and cannot be captured in any (set of) views nor in the code. Architecture is about fundamental things. Abstraction!!! Architecture exists in some context. We can only understand qualities in context. -> Views!!! 5

6 Software Architecture Definition (1) Software Architecture Definition (2) The architecture of a software system defines that system in terms of computational components and interactions among those components. 2 (from Shaw and Garlan, Software Architecture, Perspectives on an Emerging Discipline, Prentice-Hall, 1996) statement procedure module (design) pattern architecture The software architecture of a 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. 3 (from Bass, Clements, and Kazman, Software Architecture in Practice, SEI Series in Software Engineering. Addison-Wesley, 2003) Software Architecture Definition (3) 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 4 (from IEEE Standard on the Recommended Practice for Architectural Descriptions, 2000) Architecture Sloppy Definitions Architecture is high-level design Architecture is overall structure of the system Architecture is components and connectors Architecture is the structure, the behavior (properties), and the principles and guidelines governing its design and evolution over time Structure of Lecture 07 What is it? Why bother? Terminology: Architect, Architecting, Architecture s and View Models Notation Patterns, Styles and DSSAs Analysis/Assessment Wrap-up Analogy with Building Architecture Overall picture of building (client) Front view (client, beauty committee) Separate picture for water supply (plumber) Separate picture for electrical wiring (electrician) etc 6

7 IEEE Model for Architectural Descriptions Kruchten s 4+1 View Model Env ironment Mission Sy stem Stakeholder Architecture Architecture Description Rationale System stakeholder: an individual, team, or organization (or classes hereof) with interests in, or concerns relative to, a system. View: a representation of a whole system from the perspective of a related set of concerns. End-user Functionality Logical Scenarios Programmers Software management Implementation Concern Library View Model : A viewpoint establishes the purposes (concerns) and audience (stakeholders) for a view and the techniques or methods employed in constructing a view. Process Integrators Performance Scalability Deployment System engineers Topology Communications Kruchten s 4+1 View Model Kruchten s 4+1 View Model Supports the functional requirements, i.e., the services the system should provide to its end users. Typically, it shows the key abstractions (e.g., classes and associations amongst them). End-user Functionality Logical Process Integrators Performance Scalability Scenarios Programmers Software management Implementation Deployment System engineers Topology Communications Takes into account some nonfunctional requirements, such as performance and system availability. It addresses concurrency and distribution, system integrity, and faulttolerance. The process view also specifies which thread of control executes each operation of each class identified in the logical view. So the process view describes the mapping of functions to runtime elements. It concerns the dynamics of the system. A process is a group of tasks which form a logical unit. A process can be started, stopped, resumed, etc., and there is communication between processes. End-user Functionality Logical Process Integrators Performance Scalability Scenarios Programmers Software management Implementation Deployment System engineers Topology Communications Kruchten s 4+1 View Model Kruchten s 4+1 View Model End-user Functionality Logical Process Integrators Performance Scalability Scenarios Programmers Software management Implementation Deployment System engineers Topology Communications Focuses on the organization of the actual software modules in the software-development environment. The software is packaged in small chunks (program libraries or subsystems) that can be developed by one or more developers. End-user Functionality Logical Process Integrators Performance Scalability Scenarios Programmers Software management Implementation Deployment System engineers Topology Communications => Physical view: Defines how the various elements identified in the logical, process, and implementation views (networks, processes, tasks, and objects) must be mapped onto the various nodes. Takes into account the system's non-functional requirements such as system availability, reliability (faulttolerance), performance (throughput), and scalability. 7

8 4 + 1: Scenario Structure of Lecture 07 Consists of a small subset of important scenarios (e.g., use cases) to show that the elements of the four views work together seamlessly. This view is redundant with the other ones (hence the "+1"), but it plays two critical roles: Acts as a driver to help designers discover architectural elements during the architecture design; Validates and illustrates the architecture design, both on paper and as the starting point for the tests of an architectural prototype. End-user Functionality Logical Process Integrators Performance Scalability Programmers Software management Implementation Scenarios Deployment System engineers Topology Communications What is it? Why bother? Terminology: Architect, Architecting, Architecture s and View Models Notation Patterns, Styles and DSSAs Analysis/Assessment Wrap-up Standard Notation: UML Architecture presentations in practice By and large two flavors: Powerpoint slides for managers, users, consultants, etc UML diagrams, for technicians A small sample Conceptual View Customer, Users More technical view Developers (same system as on previous slide) 8

9 Runtime View Deployment View High-level overview of the architecture (Logical view Implementation view) A University Course Catalogue System Application layer This Application layer has all the boundary classes that represent the application screens that the user sees. Business Services layer The Business Services process layer has all the controller classes that represent the use case managers that drive the application behavior. This layer represents the client-to-mid-tier border. Middleware layer The Middleware layer supports access to Relational DBMS and OODBMS. Runtime Component Deployment View Component clientars User machine Search App. Server 1, App Server 2 (WebLogic 7.0) ReserveAndBuy App. Server 1, App Server 2 (WebLogic 7.0) Authenticate App. Server 1, App Server 2 (WebLogic 7.0) ArsStartup App. Server 1, App Server 2 (WebLogic 7.0) LocalRestart App. Server 1, App Server 2 (WebLogic 7.0) ARS Database DB Server (MS SQL Server) CreditCard Database DB Server (MS SQL Server) CreditCard Naming server & Replication mgr God Naming server & Replication mgr Base Reuse package The Base Reuse package includes classes to support list functions and patterns. (see doc on course wiki) Deployment view of the architecture. Process view of the architecture. Shows the tasks (processes and threads) involved in the system's execution, their interactions and configurations. Processes exist to support student registration, professor functions, registration closing, and access to the external Billing System and Course Catalog System. Shows the various physical nodes for the most typical platform configurations. Also shows the allocation of tasks (from the Process View) to the physical nodes. So, Different representations For different people For different purposes These representations are both descriptive and prescriptive Structure of Lecture 07 What is it? Why bother? Terminology: Architect, Architecting, Architecture s and View Models Notation Patterns, Styles and DSSAs Analysis/Assessment Wrap-up 9

10 Learning from Others: Patterns, Styles, and DSSAs How to solve a problem? Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; 2008 John Wiley & Sons, Inc. Reprinted with permission. Examples of Domains Domain-Specific Software Architectures Compilers for programming languages Consumer electronics Electronic commerce system/web stores Video game Business applications Basic/Standard/ Pro 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. DSSAs are the pre-eminent means for maximal reuse of knowledge and prior development. We can subdivide, too: Avionics systems -> Boeing Jets -> Boeing Domain-Specific Software Architectures Reference Architecture Example A DSSA comprises: A reference architecture, which describes a general computational framework for a significant domain of applications A component library, which contains reusable chunks of domain expertise, and An application configuration method for selecting and configuring components within the architecture to meet particular application requirements Examples: ADAGE for avionics, AIS for adaptive intelligent systems, and MetaH for missile guidance, navigation, and control systems Structural view of Lunar Lander DSSA Invariant with explicit points of variation Satellite relay Sensors 10

11 Reference Architecture Reference Architecture Reference Architecture MURA: Microsoft Upstream Reference Architecture DSSAs also include Extreme Case of DSSA... Product Line Architecture 11

12 Product Line Architecture Why? How do Product Lines come to be? Example: Lunar Lander Game Example: Lunar Lander Game Product Lines in the Lunar Lander Game Product Lines: Components, Features, Products 12

13 Better Representation: Variability Model DSSAs vs. Product Lines Architectural Patterns State-Logic-Display (a.k.a. Three-Tier Pattern) State-Logic-Display (a.k.a. Three-Tier Pattern) State-Logic-Display in Web Development 13

14 Tiers and Layers Model-View-Controller (MVC) Note: The middle tier might be multitiered (resulting in an n-tier architecture) Model-View-Controller (MVC) Do you recall...? Boundary = View Entity = Model Control = Controller Two Flavors of MVC: Passive Model Two Flavors of MVC: Active Model The passive model is employed when one controller manipulates the model exclusively. The controller modifies the model and then informs the view that the model has changed and should be refreshed. The model is completely independent of the view and the controller, i.e. there is no means for the model to report changes in its state. The HTTP protocol is an example of this. There is no simple way in the browser to get asynchronous updates from the server. The browser displays the view and responds to user input, but it does not detect changes in the data on the server. Only when the user explicitly requests a refresh is the server interrogated for changes. Solution: Observer Design Pattern! 14

15 Two Flavors of MVC: Active Model Observer Pattern: Benefits and Liabilities of MVC The observer pattern is a software design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods. It is mainly used to implement distributed event handling systems. Benefits Figure shows the structure of the active MVC using Observer and how the observer isolates the model from referencing views directly. Liabilities Java code example: Patterns, Styles, and DSSAs Patterns Styles Patterns Pipes and filters Data abstraction and object-oriented organization Layered systems Repositories Event-based, implicit invocation... and many more Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; 2008 John Wiley & Sons, Inc. Reprinted with permission. Architectural patterns <> Architectural styles <> Design patterns. While architectural styles define the components and connectors possible within the system (more asking what? ), architectural patterns define the implementation strategies of those same components and connectors (a bit more on how? ). Example: Model-View-Controller (MVC) pattern At the same time, a good architecture will make use of design patterns (on a more fine-granular level) Example: Observer pattern used to implement (active) MVC pattern Different architectural styles will find different architectural and design patterns more or less helpful. Design Patterns The Observer Pattern A design pattern is a way of reusing abstract knowledge about a problem and its solution. A pattern is a description of the problem and the essence of its solution. It should be sufficiently abstract to be reused in different settings. Pattern descriptions usually make use of object-oriented characteristics such as inheritance and polymorphism. ELEMENTS: Name A meaningful pattern identifier. Problem description. Solution description (might have an example) Not a concrete design but a template for a design solution that can be instantiated in different ways. Benefits and Consequences The results and trade-offs of applying the pattern. Name: Observer Problem description Situations where multiple displays of state are needed. Solution description Separates the display of object state from the object itself. See UML description. Consequences Optimisations to enhance display performance are difficult. 15

16 Three Types of Patterns Benefits of Design Patterns Creational patterns: Deal with initializing and configuring classes and objects Structural patterns: Deal with decoupling interface and implementation of classes and objects Composition of classes or objects Behavioral patterns: Deal with dynamic interactions among societies of classes and objects How they distribute responsibility Observer Design patterns enable large-scale reuse of software architectures and also help document systems Patterns explicitly capture expert knowledge and design tradeoffs and make it more widely available Patterns help improve developer communication Pattern names form a common vocabulary Structure of Lecture 07 Architecture Evaluation/Analysis What is it? Why bother? Terminology: Architect, Architecting, Architecture s and View Models Notation Patterns, Styles and DSSAs Analysis/Assessment Wrap-up Assess whether architecture allows system to meet certain quality goals e.g. regarding maintainability, modifiability, reliability, performance,... Note: the architecture is assessed, while we hope the results will hold for a system yet to be built Software architecture Properties implementation properties? System Qualities Analysis Techniques and Scenarios Exercise: Analyse of Alarm Clock Architectures Questioning techniques: how does the system react to various situations; often make use of scenarios Measuring techniques: rely on quantitative measures; architecture metrics, simulation, etc. Different types of scenarios, e.g. usecases, likely changes, stress situations, risks, far-into-the-future scenarios Which stakeholders to ask for scenarios? When do you have enough scenarios? Consider standard alarm clocks that you have seen. For this exercise, consider each of the following to be representative of an architectural style of alarm clocks: An LED alarm clock for a bedroom, A LCD travel alarm, An analog alarm clock (there are several varieties; choose one). 16

17 List of Analysis Methods Scenario-based Architecture Analysis Method (SAAM) SAAM (Scenario-based Architecture Analysis Method) SAAMER (Software Architecture Analysis Method for Evolution and Reusability) ATAM (The Architecture Trade-Off Analysis Method) SBAR (Scenario-Based Architecture Reengineering)... and many more... Describe architecture(s) Based on desired system qualities, develop scenarios for kinds of activities the system must support kinds of changes anticipated Classify scenarios in direct & indirect direct -- execution of scenario requires no change of architecture indirect execution of scenario requires change of architecture Evaluate indirect scenarios: list changes and estimate cost Reveal scenario interaction Overall evaluation Scenario-based Architecture Analysis Method (SAAM) List of scenarios U1 and M1 interact because they require changes to the same components Scenario-based Architecture Analysis Method (SAAM) Overview: SAAM inputs and activities Examples: User U1 wants to have added another feature Maintainer M1 wants to port the system to platform XYZ Maintainer M2 wants to change the look-and-feel of the GUI Goal: By analysing the extend and difficulty (cost) of required changes (indirect scenarios) as well as the extent of scenario interactions, SAAM allows an insight to the future product capabilities, if the given architecture is chosen. Conditions for Successful Architecture Analysis/Evaluation Clear goals and defined quality requirements for the architecture Controlled scope focus on most important goals and quality requirements Cost-effectiveness Key personnel availability e.g. for scenario dev. Competent evaluation team ideally independent from developers/owners of architecture Managed expectations Structure of Lecture 07 What is it? Why bother? Terminology: Architect, Architecting, Architecture s and View Models Notation Patterns, Styles and DSSAs Analysis/Assessment Wrap-up 17

18 Why is Architecture Important? Further Reading Architecture is the vehicle for stakeholder communication Architecture manifests the earliest set of design decisions Constraints on implementation Dictates organizational structure Inhibits or enables quality attributes Architecture is a transferable abstraction of a system Product lines share a common architecture Allows for template-based development Basis for training Len Bass et al, Sofware Architecture in Practice, 2008 (3 rd edition). Jan Bosch, Design & Use of Software Architectures, Frank Buschmann et al., Pattern-Oriented Software Architecture: A System of Patterns, Part II: George Fairbanks: Just Enough Software Architecture, Erich Gamma et al., Design Patterns: Elements of Reusable Object-Oriented Software, C. Hofmeister et al., Applied Software Architecture, Philippe B. Kruchten, The 4+1 view model of architecture, IEEE Software, 12(6):42-50, November Mary Shaw and David Garlan, Software Architecture; Perspectives of an Emerging Discipline, Richard Taylor et al.: Software Architecture, University of California at Irvine, Lecture Ian Sommerville: Software Engineering, 9th edition, ( Hans van Vliet: Software Architecture, Free University of Amsterdam, Lecture Next Lecture Date/Time: Friday, 31-Oct, 14:15-16:00 Topic: Refactoring (and TDD) For you to do: Finish and submit Lab Task 4 18