[2015/2016] Introduction to software architecture

Ivano Malavolta

Introduction to

SOFTWARE ARCHITECTURE

Roadmap

Definitions and concepts

Architectural styles

Some contents of this part of lecture extracted from Henry Muccini’s lecture on software architecture at the University of L’Aquila (Italy)

Definitions and concepts

Outline

Definitions

Static descriptions

Dynamic descriptions

Why software architecture?

Context

© David Garlan, lecture @GSSI, a.y., 2013/2014

Refresh

Example: software architecture

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

L. Bass, P. Clements, R. Kazman, Software Architecture In Practise, Addison Wesley, 1998

System

subsystem Subsystem

component component component

Software Architecture definitionsPerry and Wolf, ’92 (aspects):–“Architecture is concerned with the selection of architectural elements, their interactions, and the constraints on those elements and their interactions necessary to provide a framework in which to satisfy the requirements and serve as a basis for the design.”–Elements are divided into processing elements, data elements and connection elements

Garlan and Shaw, ’93 (elements):– Architecture for a specific system may be captured as “a collection of computational components - or simply components - together with a description of the interactions between these components - the connectors -”

Sommerville, 7th edition, ’04 (process):– The design process for identifying the sub-systems making up a system and the framework for sub-system control and communication is architecturaldesign. The output of this design process is a description of the SA.

Develop systems “architecturally”

– Design at an architectural level of abstraction

– Build systems compositionally from parts

– Assure that the system will satisfy critical requirements before it is constructed

– Recognize and reuse standard architectures patterns & styles

– Reuse codified architectural design expertise à reduce costs through product-lines

If you think good architecture is expensive,

try bad architecture.

... Brian Foote and Joseph Yoder

In general terms…SA describes (in a more or less “formal” notation) how a system is structured into components and connectors…

– Components– Connectors

– Channels and Ports

… and how these components interact

– Scenarios

– State Diagrams–…

SA Structure (topology)

SA Dynamics (behavior)

Outline

Definitions

Static descriptions



ComponentsA component is a building block that is– A unit of computation or a data store, with an interface specifying the services it provides and requires– A unit of deployment

– A unit of reuse• e.g., client, server, database, filters, ...

C1S1S2S3

S’xS’Y

provided services

required services

Example

Components vs Objects

The level of abstraction is usually different

• Size– Objects tend to be small

– Components can be small (one object) or large (a library of objects or a complete application)

• An architectural component may be implemented by several objects

• Lifecycle – Objects are created and destroyed constantly

– Components are created and destroyed infrequently

ConnectorsA connector is a building block that enables interaction among components– Events

– Client/server middleware

–Messages and message buses– Shared variables

– Procedure calls (local or remote)– Pipes

Connectors may be implicit or explicit– Connectors sometimes are just channels

– Connectors sometimes have their own logic and complexity

Components and Connectors

A component is (or should be) independent of the context in which it is used to provide services

A connector is (or should be) dependent on the context in which it is used to connect components

Connectors sometimes are modeled as special kinds of components

Interfaces

An interface is the external connection of a component (or connector) that describes how to interact with it

Provided and required interfaces are important

Spectrum of interface specification– Loosely specified (events go in, events go out)– API style (list of functions)– Very highly specified (event protocols across the interface in

CSP)

Example

GUI

FeedServiceFeedService

CommonAction

NewsFeederAction

AdminAction

Factory

FeedDelegatePOJOs

NewsFeederDAO FeedDAO

NewsFeederDAO FeedDAO

FeedDelegate

Transfer Object

ValidatorService

NewsFeederDelegatePOJOs

NewsFeederDelegate

Browser(html

javascript)

Web Services

DATABASE

TrasformationValidation

BusinessFactory

ValidationService

BusinessExtensionIn

BusinessExtensionOut

PresentationExtensionOut

PresentationExtensionIn

Outline

Definitions

Static descriptions



SA dynamics

The SA dynamics is expressed in terms of component interactions via connectors

- Labeled Transition Systems

- Automata- UML StateCharts, Sequence Diagrams, Activity Diagrams

- State Diagrams

- Message Sequence Charts

- …

Customer Interface

Customer Process

Web Server

Customer Server

Order Server

Cart Server

Catalog ServerDelivery OrderProcess

SA Static Description

An example : e-commerce system

SA Dynamic Description : Browse Catalogue Sequence Diagram

CustomerInterface

Registered Customer

CustomerProcess CatalogServer

Catalog DBInvolved

BrowseCatalogBrowseCatalog

ReadStatus

Catalog Page

Output Page

Catalog Info


CustomerInterface

Registered Customer

CustomerProcess CartServer

PlaceOrderReqPlaceOrder

ReadStatus

Cart DBInvolved

pageOrderOutputPage

Order DBInvolved

OrderServer

EmptyCart

Cart DBInvolved

CustomerServer

ReadInfoCustomerDB Involved

DeliveryOrderProcess

createNewOrder

OrderInfo

newOrder

CartInfo

CustomerInfo

OrderInfo

SA Dynamic Description : Place Order Sequence Diagram (success)


SA Dynamic Description : Place Order Sequence Diagram (empty cart)

CustomerInterface

Registered Customer

CustomerProcess CartServer

PlaceOrderReqPlaceOrder

ReadStatus

Cart DBInvolved

errorPageOutputPage

emptyCart


Outline

Definitions

Static descriptions



Advantages of explicit architecture

System analysis– Analysis of the system before it has been built– Costs saving and risks mitigation

Large-scale reuse– The architecture (or part of it) may be reusable across a range

of systems– Design decisions reuse à saves design costs + less risks

Stakeholders communication– Architecture may be used as a focus of discussion by system

stakeholders– Early design decisions reasoning, when it is still relatively easy

to adapt

Architecture and software qualitiesPerformance

– Localise critical operations and minimise communications

Security– Use a layered architecture with critical assets in the inner layers

Safety– Localise safety-critical features in a small number of sub-systems

Availability– Include redundant components and mechanisms for fault tolerance

Maintainability– Use fine-grain, replaceable components

These are all examples of TACTICS

Tactics

A tactic is a design decision that refines a high level styleand is influential in the control of a quality attribute response

Tactics complement and refine styles that make up the architecture

Design decision Quality attributepromotes tactic

Example: tactics for availability

© David Garlan, lecture @GSSI, a.y., 2013/2014

Tactics may originate conflicts

For example:

• Using large-grain components improves performance but reduces maintainability

• Introducing redundant data improves availability but makes security more difficult

• Localising safety-related features may mean more communication so degraded performance

© Len Bass, Paul Clements, Rick Kazman, Software Architecture in Practice, 3rd edition

Architectural styles

(aka patterns)

Outline

What is an architectural style?

Styles catalogue

What is an architectural style?

An architectural style establishes a relationship between:

• Context

– A recurring situation in the world that gives rise to a problem

• Problem

– The problem, appropriately generalized, that arises in the context

• Solution:– a set of element types

• e.g., data repositories, processes, and objects– a set of interaction mechanisms or connectors

• e.g., method calls, events, or message bus– a topological layout of the components– a set of semantic constraints

Common styles catalogue

• MVC• Publish-subscribe• Layered• Shared-data• Client-server• Peer to peer• Pipes and filters

Model-View-Controller styleContext: User interface software is typically the most frequently modified portion of an interactive application. Users often wish to look at data from different perspectives, such as a bar graph or a pie chart. These representations should both reflect the current state of the data.

Problem: How can user interface functionality be kept separate from application functionality and yet still be responsive to user input, or to changes in the underlying application’s data? And how can multiple views of the user interface be created, maintained, and coordinated when the underlying application data changes?

Solution: The model-view-controller (MVC) style separates application functionality into three kinds of components:

– A model, which contains the application’s data– A view, which displays some portion of the underlying data and

interacts with the user– A controller, which mediates between the model and the view and

manages the notifications of state changes

MVC Example

MVC Solution - 1

The MVC pattern breaks system functionality into three components: a model, a view, and a controller that mediates between the model and the view

• Elements: – The model is a representation of the application data or state, and

it contains (or provides an interface to) application logic– The view is a user interface component that either produces a

representation of the model for the user or allows for some form of user input, or both

– The controller manages the interaction between the model and the view, translating user actions into changes to the model or changes to the view

MVC Solution - 2

Relations: The notifies relation connects instances of model, view, and controller, notifying elements of relevant state changes

Constraints: – There must be at least one instance of each of model, view, and

controller– The model component should not interact directly with the

controller

Weaknesses:– The complexity may not be worth it for simple user interfaces– The model, view, and controller abstractions may not be good fits

for some user interface toolkits

Publish-Subscribe styleContext

– There are a number of independent producers and consumers of data that must interact. The precise number and nature of the data producers and consumers are not predetermined or fixed, nor is the data that they share.

Problem– How can we create integration mechanisms that support the

ability to transmit messages among the producers and consumers so they are unaware of each other’s identity, or potentially even their existence?

Solution– Components interact via announced messages, or events.

Components subscribe to a set of events. – Publisher components place events on the bus by announcing

them; the connector then delivers those events to the subscriber components that have registered an interest in those events.

Publish-Subscribe Solution – 1

Elements: – Any component with at least one publish or subscribe port– The publish-subscribe connector, which will have announce and

listen roles for components that wish to publish and subscribe to events

Relations:– The attachment relation associates components with the publish-

subscribe connector by prescribing which components announce events and which components are registered to receive events

Weaknesses: – Typically increases latency and has a negative effect on scalability

and predictability of message delivery time– Less control over ordering of messages– Delivery of messages is not guaranteed

Publish-subscribe example 1

© Len Bass, Paul Clements, Rick Kazman,

Publish-subscribe example 2

topics nodes

Layered Style (Virtual Machine Example)

Java Virtual Machine

Processor

Operating

System

Java

Virtual Machine

Java

Application

(Virtual Machine Style)

The Layered System Style

A layered system is organized hierarchically, each layer providing service to the layer above and below

• Components– Programs or subprograms deployed in a layer

• Connectors– Protocols

• Procedure calls or system calls

• Stylistic invariants– Each layer provides a service only to the immediate layer “above”

(at the next higher level of abstraction) and uses the service only of the immediate layer “below” (at the next lower level of abstraction)

Layered System Example: OSI Protocol Stack

ApplicationPresentation

SessionTransportNetworkData LinkPhysical

ApplicationPresentation

SessionTransportNetworkData LinkPhysical

NetworkData LinkPhysical

NetworkData LinkPhysical

Layered System Advantages and Disadvantages

Advantages– Decomposability: Effective separation of concerns and different

level of abstractions– Maintainability: Changes that do not affect layer interfaces are easy

to make– Adaptability/Portability: Can replace inner layers as long as

interfaces remain the same – Understandability: Strict set of dependencies allow you to ignore

outer layers

Disadvantages– Not all systems are easily structured in a layered fashion– Performance degrades with too many layers– Can be difficult to cleanly assign functionality to the “right” layer

Shared-Data style

ContextVarious computational components need to share and manipulate large amounts of data. This data does not belong solely to any one of those components.

ProblemHow can systems store and manipulate persistent data that is accessed by multiple independent components?

SolutionIn the shared-data pattern, interaction is dominated by the exchange of persistent data between multiple data accessors and at least one shared-data store. Exchange may be initiated by the accessors or the data store. The connector type is data reading and writing.

Shared Data Solution

Elements:

– Shared-data store• Concerns include types of data stored, data

performance-oriented properties, data distribution, and number of accessors permitted

– Data accessor component– Data reading and writing connector

Shared Data Example

Advantages and disadvantages

Advantages– Simplicity: Only one connector (the blackboard) that everyone

uses

– Evolvability: New types of components can be added easily

Disadvantages– Blackboard becomes a bottleneck with too many clients

Client-server

• One component is a server offering a service• The other components are clients using the service

• Server implementation is transparent but can be centralized or distributed, single-threaded or multi-threaded– Single interface point with physically distributed implementation– Dynamic, transparent selection from among multiple interface

points

Client/Server Style example

3-tier client-server systems

3-tier client-server systems are a common class of distributed business systems

• First tier: Client (user interface) tier– Presentation logic

• Second (middle, “business logic”) tier: Servers acting as “business objects”, encapsulating abstract, integrated models of multiple, disparate data sources– Computation

• Third (back-end, database) tier: Legacy business applications providing data services– Database

Weaknesses: Substantial up-front cost and complexity

3-Tier Client-server systems (example)

Key

Webbrowser

Sig

nOnF

ilter

*.do

*.screen

MainServlet

TemplateServlet

ScreenJSP

index.jsp

Sign OnNotifier

mappings.xml

screendefinitions.xml

sign-on-config.xml

OrderFacade

EJB tier Back endWeb tierClient tier

CatalogFacade OPC

AdventureCatalog

DB

UserMgmtFacade

OpcOrderTrackingService

OpcPurchaseOrderService

Client-sideapplication

JavaEEfilter

Statelesssessionbean

Java EEapplication

Contextlistener

Datastore

FileServlet

ContainerWeb servicesendpoint

SOAPcall

FileI/O

Javacall

HTTP/HTTPS

JDBC

Peer-to-peer style

Context: need to cooperate and collaborate to provide a service to a distributed community of users

Problem: How can a set of “equal” distributed computational entities be connected to each other via a common protocol so that they can organize and share their services with high availability and scalability?

Solution: components directly interact as peers. All peers are “equal” and no peer or group of peers can be critical for the health of the system. Peer-to-peer communication is typically a request/reply interaction without the asymmetry found in the client-server pattern

Example: Napster

Example: Gnutella

Example: Skype

Advantages and Disadvantages

Advantages– Interoperability A natural high-level architectural style for

heterogeneous distributed systems– Scalability: Powerful enough server tiers can accommodate many

clients

– Distributability: Components communicate over a network

Disadvantages– Visibility, Maintainability: Difficult to analyze and debug

• Distributed state

• Potential for deadlock, starvation, race conditions, service outages

– Require sophisticated interoperability mechanisms• Data marshalling and unmarshalling• Proxies and stubs for RPC

• Legacy wrappers

Pipe and Filter Pattern

Context: Many systems are required to transform streams of discrete data items, from input to output. Many types of transformations occur repeatedly in practice, and so it is desirable to create these as independent, reusable parts

Problem: Such systems need to be divided into reusable, loosely coupled components with simple, generic interaction mechanisms. The components, being generic and loosely coupled, are easily reused. The components, being independent, can execute in parallel

Solution: The pattern of interaction in the pipe-and-filter pattern is characterized by successive transformations of streams of data. Data arrives at a filter’s input port(s), is transformed, and then is passed via its output port(s) through a pipe to the next filter. A single filter can consume data from, or produce data to, one or more ports

Pipe and Filter SolutionData is transformed from a system’s external inputs to its external outputs through a series of transformations performed by its filters connected by pipes

Elements: – Filter, which is a component that transforms data read on its input port(s) to data

written on its output port(s)– Pipe, which is a connector that conveys data from a filter’s output port(s) to another

filter’s input port(s). A pipe preserves the sequence of data items, and it does not alter the data passing through

Relations: The attachment relation associates the output of filters with the input of pipes and vice versa

Constraints:– Pipes connect filter output ports to filter input ports– Connected filters must agree on the type of data being passed along the

connecting pipe

Pipe-and-filter example

Many similarities between patterns and styles– Goal: packaged engineering experience– Formulation: organization and interaction among “key”

components

Differences:

Architectural styles vs design patterns

Architectural styles Design patterns

few many

large-scale system organization

localized, small-scaledesign solutions

Relationships between tactics and stylesStyles are built from tactics

à if a style is a molecule, a tactic is an atom

MVC, for example utilizes the tactics:– Increase semantic coherence

– Encapsulation– Use an intermediary

– Use run-time binding

What this lecture means to you?

Software architecture is the main instrument for reasoning about

– high level of system design

– system-level properties and qualities• e.g., modularity, evolvability, etc.

– large-scale reuse

Architectural style: reusable pattern – for solving recurrent problems– for obtaining qualities “out-of-the-box”

Suggested readings

1. David Garlan. “Software architecture: a travelogue.” ICSE '14Proceedings of the Conference on The Future of SoftwareEngineering, ACM Press, 2014.

1. Perry, D. E.; Wolf, A. L. (1992). "Foundations for the study of softwarearchitecture". ACM SIGSOFT Software Engineering Notes 17 (4):40.doi:10.1145/141874.141884.

2. Garlan & Shaw (1994). "An Introduction to Software Architecture".Retrieved 2012-09-13.

References

ContactIvano Malavolta |

Post-doc researcherGran Sasso Science Institute

iivanoo

[email protected]

www.ivanomalavolta.com

[2015/2016] Introduction to software architecture

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