[2017/2018] Introduction to Software Architecture

Ivano Malavolta
Introduction to
SOFTWARE
ARCHITECTURE
VRIJE
UNIVERSITEIT
AMSTERDAM

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

Software Architecture definitions
Perry 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 architectural
design. 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
– Reason on critical requirements before implementing them
– Reuse codified architectural design expertise à patterns & styles
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)

Components
A 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, ...
C1
S1
S2
S3
S’x
S’Y
provided
services
required
services

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

Connectors
A 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)

Example
GUI
FeedService
FeedService
Common
Action
NewsFeeder
Action
Admin
Action
Factory
FeedDelegate
POJOs
NewsFeeder
DAO
FeedDAO
NewsFeederDAO FeedDAO
FeedDelegate
Transfer
Object
ValidatorService
NewsFeeder
DelegatePOJOs
NewsFeederDelegate
Browser
(html
javascript)
Web
Services
DATABASE
Trasformation
Validation
BusinessFactory
Validation
Service
BusinessExtensionIn
BusinessExtensionOut
PresentationExtensionOut
PresentationExtensionIn

SA dynamics
The SA dynamics is expressed in terms of:
- 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 Server
Delivery Order
Process
SA Static Description
An example : e-commerce system

SA Dynamic Description :
Browse Catalogue Sequence Diagram
CustomerInterface
Registered Customer
CustomerProcess CatalogServer
Catalog DB
Involved
BrowseCatalog
BrowseCatalog
ReadStatus
Catalog Page
Output Page
Catalog Info

CustomerInterface
Registered Customer
CustomerProcess CartServer
PlaceOrderReq
PlaceOrder
ReadStatus
Cart DB
Involved
pageOrder
OutputPage
Order DB
Involved
OrderServer
EmptyCart
Cart DB
Involved
CustomerServer
ReadInfo
Customer
DB Involved
DeliveryOrderProcess
createNewOrder
OrderInfo
newOrder
CartInfo
CustomerInfo
OrderInfo
Place Order Sequence Diagram (success)

Place Order Sequence Diagram (empty cart)
CustomerInterface
Registered Customer
CustomerProcess CartServer
PlaceOrderReq
PlaceOrder
ReadStatus
Cart DB
Involved
errorPage
OutputPage
emptyCart

Advantages of explicit architecture
System analysis
– Analysis of system features before they are 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 qualities
Performance
– 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 style
and is influential in the control of a quality attribute response
Tactics complement and refine styles that make up the
architecture
Design decision Quality attribute
promotes
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
• Peer to peer
• Pipes and filters

Model-View-Controller style
Context: 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 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 style
Context
– 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 example 1
topics nodes

Publish-subscribe example 2
© Len Bass, Paul Clements, Rick Kazman,

Publish-Subscribe Solution
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

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
Application
Presentation
Session
Transport
Network
Data Link
Physical
Application
Presentation
Session
Transport
Network
Data Link
Physical
Network
Data Link
Physical
Network
Data Link
Physical

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 allows 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
Context
Various computational components need to share and manipulate
large amounts of data. This data does not belong solely to any one
of those components.
Problem
How can systems store and manipulate persistent data that is
accessed by multiple independent components?
Solution
In 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

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

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

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 Solution
Data 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

Relationships between tactics and
styles
Styles 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 quality
• e.g., evolvability, performance, security, 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 '14
Proceedings of the Conference on The Future of Software
Engineering, ACM Press, 2014.
1. Perry, D. E.; Wolf, A. L. (1992). "Foundations for the study of software
architecture". 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.

Contact
Ivano Malavolta |
Assistant professor
Vrije Universiteit Amsterdam
iivanoo
i.malavolta@vu.nl
www.ivanomalavolta.com

[2017/2018] Introduction to Software Architecture

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