From Requirements to Architecture Ana Moreira Software - - PDF document

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Software Architecture - Basics 1

From Requirements… to Architecture

Ana Moreira

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Goals

Understanding the importance of software architecture

what is software architecture

Discussing emerging issues in the transition from requirements to architecture

architectural patterns/styles importance of managing requirements requirements traceability

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What is…

Software Architecture ????

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"Magician Coder" View of Development

Detailed Design

Many stakeholders are not interested in such low-level choices

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Creating the illusion of simplicity

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Can be built by one person Requires Minimal modeling Simple process Simple tools

Architecting a dog house

Kruchten

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Architecting a house

Built most efficiently and timely by a team Requires Modeling Well-defined process Power tools

Kruchten

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Architecting a high rise

What about this? 452 meters high!

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Architecting Taj Mahal

Taj Mahal : Built by Mogol Shah Cihan (turkish origin) in memory of his beloved wife Mümtaz Mahal 1648

Took 22 years 20.000 workers

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Differences

Scale Process Cost Schedule Skills and development teams Materials and technologies Stakeholders Risks

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So, what is software architecture?

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Misconceptions about architecture

Architecture is just paper Architecture and design are the same things Architecture and infrastructure are the same things <my favorite technology> is the architecture A good architecture is the work of a single architect Architecture is simply structure Architecture can be represented in a single blueprint System architecture precedes software architecture Architecture cannot be measured or validated Architecture is a science Architecture is an art

Kruchten

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Architecture is just paper

A system’s architecture ultimately resides in executable code A system’s architecture may be visualized in models Every system has an architecture; some architectures are made manifest and visible, many others are not

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Architecture is design

All architecture is design, but not all design is architecture Architecture focuses on significant design decisions

decisions that are both structurally and behaviorally important as well as those that have a lasting impact

• n the performance, reliability, cost, and resilience of

the system

Architecture involves the how and the why, not just the what

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Architecture is infrastructure

Infrastructure is an integral and important part of architecture, but there is more to architecture than just infrastructure Significant patterns will manifest themselves at many different levels and dimensions of a system Too narrow a view of architecture will lead to a very pretty infrastructure, but the wrong infrastructure for the problem at hand

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<my favorite technology> is the architecture

A given technology only serves to implement some dimension of an architecture

The network is the architecture The database is the architecture The transaction server is the architecture J2EE is the architecture

Architecture is more than just a list of products Technology shapes an architecture, but a resilient architecture should never be bound to all of the technologies that form it

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Architecture is the work of a single architect

Conceptual integrity is essential, but the complexity

• f most interesting systems leads development to

be a team sport Fred Brooks (1975), but then Fred Brooks (1995) The architecture team is

Not a committee Not a problem clearinghouse Not an ivory tower

The architecture team

Needs a clear leader Requires a mix of specialties Manifests itself at many levels in the system

Coplien & Harris, Organizational Patterns

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Architecture is structure

Architecture does involve structure, decomposition, and interfaces Architecture also involves behavior A system’s architecture is always projected to a given context

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Architecture is flat

Architecture is flat only in trivial systems Multiple stakeholders with multiple concerns lead to multiple views with multiple blueprints Using a single blueprint to represent all or most of system’s architecture leads to a semantic muddle The 4+1 view model has proven to be both necessary and sufficient for most interesting systems

Kruchten, “The 4+1 Model View”

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System architecture comes first

Software has a longer life than hardware Complex systems require well-informed hardware/software tradoffs, which cannot be made in a strict sequence Forcing a hardware-first process typically leaves to stove pipe systems

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Architecture can’t be measured

The very purpose of a blueprint is to provide a tangible artifact that can be used to visualize, specify, construct, document - and reason about - a system A system’s architecture can be used to

Mitigate technical risks through the release of a continuous stream of executables Improve learning and understanding and communicate important decisions Accelerate testing and attack integration risks Set expectations Break in the development environment and the team

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Architecture is a science

There exists only a modest body of knowledge about software architecture Scientific and analytical methods are lacking; those that do exist are hard to apply There is no perfect design; architecture involves the management of extreme ambiguity and contradiction Experience counts: the best architects are grown, not born

Petroski, Small Things Considered

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So, what is architecture (in general)?

Describes (is) structure consisting of components which interact among them forming the foundation of the whole....

Computer Architecture Car Architecture Building Architecture

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What is Architecture?

Architecture (Webster’s): 1: the art or science of building; specifically: the art or practice of designing and building structures and especially habitable ones 2 a>:formation or construction as or as if as the result of conscious act <the architecture of the garden> b: a unifying or coherent form or structure <the novel lacks architecture> 3: architectural product or work 4: a method or style of building 5: the manner in which the components of a computer or computer system are organized and integrated

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Architecture defined

Perry and Wolf, 1992

A set of architectural (or design) elements that have a particular form

Boehm et al., 1995

A software system architecture comprises:

A collection of software and system components, connections, and

constraints

A collection of system stakeholders' need statements A rationale which demonstrates that the components, connections, and constraints

define a system that, if implemented, would satisfy the collection of system stakeholders' need statements

Clements et al., 1997

The software architecture of a program or computing system is the structure or structures of the system, which comprise software components, the externally visible properties of those components, and the relationships among them

http://www.sei.edu/architecture/definitions.html

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Architecture defined

Formal Definition

IEEE 1471-2000

Software architecture is the fundamental

• rganization of a system, embodied in its

components, their relationships to each other and the environment, and the principles governing its design and evolution

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Architecture defined

Software architecture encompasses the set of significant decisions about the organization of a software system

Selection of the structural elements and their interfaces by which a system is composed Behavior as specified in collaborations among those elements Composition of these structural and behavioral elements into larger subsystems Architectural style that guides this organization

Booch, Kruchten, Reitman, Bittner, and Shaw

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Architecture defined

What is software architecture?

“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, L; Clements, P.; Kazman,

• R. Software Architecture in Practice. 2nd Edition, Boston, MA: Addison-

Wesley, 2003.]

Requirements artefacts are software architects “best friends”.

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Common elements 1/2

Architecture defines major components Architecture defines component relationships (structures) and interactions Architecture omits content information about components that does not pertain to their interactions Behavior of components is a part of architecture insofar as it can be discerned from the point of view of another component

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Common elements 2/2

Every system has an architecture (even a system composed of one component) Architecture defines the rationale behind the components and the structure Architecture definitions do not define what a component is Architecture is not a single structure – no single structure is the architecture

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Basic Architectural Abstractions

Component – locus of computation Interface – locus of contract; e.g. provided and required services Connector – locus of interaction

Component Interface Connector Interface

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Software Architecture: Essentials

Components

What are the elements? Examples: filters, databases, layers, clients, servers…

Connections

How do components communicate? Examples: procedure calls, messages, pipes, event broadcast

Topology

How are the components and connections organized topologically?

Constraints (including constraints on change)

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Influences on the Software Architecture Software architecture is influenced by several dimensions:

Technical environment Business Social … architect’s experience

On the other hand, software architecture also has an impact on the above doimensions

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Influences on the Software Architecture

[Bass, L; Clements, P.; Kazman, R. Software Architecture in Practice. 2nd Edition, Boston, MA: Addison-Wesley, 2003.]

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Stakeholders

A stakeholder refers to anyone/anything that has a direct or indirect interest on the system to be built.

Clients Final users Developers Project managers Consultants …

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Stakeholders’ influence on software architecture

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Importance of Stakeholders’s influences

Each stakeholder has her/his own system goals and expectations. Some may be conflicting… Software architecture should consider those and be able to prioritise conflicting goals.

Many of these goals refer to software qualities (quality attributes/non-functional erquirements)

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Forces in Software

Technology churn Scalability Performance Capacity Fail safe/Fault tolerance Reliability/Availability Security Functionality Cost/Schedule Compatibility Resilience

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Architecture vs. Design

non-functional requirements (“ilities”) functional requirements (domains) Important: this is a general guideline – sometimes the borders are blurred

Architecture: where non-functional decisions are cast & functional reqs. are partitioned Design: where functional requirements are accomplished

architecture architecture design design

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Architecture is Early

Architecture represents the set of earliest design decisions

Hardest to change Most critical to get right

Architecture is the first design artifact where a system’s quality attributes are addressed

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Architecture Drives

Architecture serves as the blueprint for the system but also the project:

Team structure Documentation organization Work breakdown structure Scheduling, planning, budgeting Unit testing, integration

Architecture establishes the communication and coordination mechanisms among components

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In Summary: Architectural Design

An early stage of the system design process. Represents the link between specification and design processes. Often carried out in parallel with some specification activities. It involves identifying major system components and their communications.

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Advantages of explicit architecture

Stakeholder communication

Architecture may be used as a focus of discussion by system stakeholders.

System analysis

Means that analysis of whether the system can meet its non-functional requirements is possible.

Large-scale reuse

The architecture may be reusable across a range of systems.

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Architectural Design

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Components Metamodel: syntax

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Components Metamodel: semantics

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Typical architectural design decisions

What are the main system components and respective interfaces? How will the system be distributed? What approach will be used to structure the system? How will the system be decomposed into modules? What communication strategy should be used? What architectural styles are appropriate? How should the architecture be documented? How will the architectural design be evaluated? …