Download system - Department of Computer and Information Science

Information system architectures and architecting
A practical tour
Einar Landre
Cell Network AS
[email protected]
Topics covered
Introduction
•
Definitions and terminology
History of information systems and their software architectures
•
Client / Server
•
Web
•
Components
•
Services
•
Future trends and requirements
From domain model to code – A practical tour
•
Levels of design
•
System decomposition
•
Services
Academic foundation
•
Design by contract, Open-Closed, Liskov, Dependency inversion, Package stability
References
Introduction
Ancient architecture
Facts Cheops (Khufu)
height = 280 cubits
perimeter = 1760 cubits
perimeter = 2*p*height
1:43.200 scale model of earth
Kings chamber 3-4-5 triangle
cubit = 52.35 cm
Architecting a dog’s house
Can be built by one person
Requires
Minimal modeling
Simple process
Simple tools
Architecting a house
Built most efficiently and timely by a team
Requires
Modeling
Well-defined process
Power tools
Forces in software
Cost
Capacity
Functionality
Compatibility
Fail safe
Availability
Performance
Technology churn
Fault tolerance
Throughput
Resilience
The challenge over the next 20 years will not be speed or cost or performance;
it will be a question of complexity.
Bill Raduchel, Chief Strategy Officer, Sun Microsystems
Our enemy is complexity, and it’s our goal to kill it.
Jan Baan
Defining architecting and architecture
Architecting, the planning and building of structures, is as old as human societies
and as modern as the exploration of the solar system.
Architecting, the art and science of building systems.
Eberhardt Rechtin, The art of systems architecting
Architecture – The set of design decisions about any system (or smaller component)
that keeps its implementors and maintainers from exercising needles creativity.
A (software) systems architecture consists of:
•
The structure of it’s parts (including design-time, test-time, runtime hardware
and software parts).
•
The nature and relevant external visible properties of those parts (modules
with interfaces, hardware units, objects).
•
The relationships and constraints between them.
Software architecture
Defines how the software is built
•
Acts as the knowledge base of the software
•
Foundation for improvement
•
Foundation for change
•
Foundation for new features
Characteristics of a good architecture
•
Built from recognizable patterns and archetypes
•
Facilitates change and extension
•
Supports the open closed design principle
•
Easy to understand
•
Supports the driving requirements
•
Clear separation of concern
•
Balanced distribution of responsibility
•
Balances economic and technical constraints
Architecting versus Engineering
Architecting, deals largely with unmeasurables using non quantitative tools and
guidelines based on practical lessons learned (heuristic)
•
Software design patterns
•
Best practices
Engineering, deals almost entirely with measurables using analytical tools derived
from mathematics and the hard sciences
•
Proven reliability of a system
•
Formal validation and verification of correctness
•
Response time requirements
Architecture depends on purpose
Architectures are tightly connected to their purpose, and to some extent they are
only understood through their purpose
•
The pyramids?
To be successful a architecture must meet two requirements:
•
Acceptable cost
•
Acceptable time
Some architectures has been stable for 100 years
•
Automobiles
•
Airplanes
•
Ships
•
Railroad systems
Others close to thousand
•
Cathedrals
Software as critical system component
Software – the centerpiece of complex system design
•
Airplanes
•
Ships – (The frigate project, probably the largest IT project in the country)
•
Healthcare
•
Business (banking, retail, public services, traditional industry)
Classical systems engineering is based on Decomposition & Integration
•
The system hierarchy
•
Software become a sub-system of its processor unit
Software architectures are layered
•
Library units call another library unit
Software and hardware hierarcys become disconnected
•
The engine control software is a subsystem of the engine.
•
The user interface is a subsystem of the dash board.
•
The software architecture is layered (user interaction and engine control)
•
Understanding this is critical when architecting software intensive systems
The situation illustrated
Car Engineer View
Car
Dashboard
Engine
Software Engineer View
User Interface
Communication
Controller
Engine
Engine
Control
Controller
View
Both views are correct, but their purpose and target group differs
System and software architecture dependencies
Purpose
System
System - response to a need/problem
•
Bank self service
•
Mobile communication system
•
Naval communication systems
•
Energy supply system
System architecture (software intensive)
The structures and parts of a system
System Architecture
Software
Architecture
•
Defines software environment
•
Naval communication system
•
Satellites, phones, antennas,….
Software architecture
The structures and parts of software
•
Includes design time, test times,
language constraints and interfaces
Summary
Today I am more convinced than ever. Conceptual
integrity is central to product quality. Having a system
architect is the most important step toward conceptual
integrity.
Fredrick P. Brooks, JR
The mythical man month after twenty years
History of information systems and their software architectures
In 1974 IBM released its Systems Network Architecture (SNA)
3270
Terminal(1)
3270
Terminal(32)
3274
3705
Terminal
Front-End
Controller
Controller
Phone
Lines
3270
Terminal(1)
Terminal
3270
Terminal(32)
V
T
3705
Front-End
3274
MVS
A
M
CICS
TSO
RJE
Controller
Controller
Before SNA terminals was physically attached to programs
SNA enabled effective use of thousands of terminals (users)
Application areas involved:
• 3270 terminal (synchronous terminal and printer)
• Transaction Processing, Time sharing and Batch
The almighty god in a SNA network was VTAM (Virtual Telecommunications Access Method)
Software architectures still monolitic (user interface, data and algorithms in one chunk)
1974 was also the year Kerf & Kahn released the TCP/IP specification
D
A
T
A
Client / Server – The architecture of the 1980ties
Client
User Interface
Server
protocol
&
Business Logic
Files
&
Databases
Originally used to scale mini computer networks
• Client machine(s) responsible for user interaction and business logic
• Server machine(s) responsible for data and common services as print
Applied at both at system and software levels
• Boosted by the BSD Unix release embedding the TCP/IP protocol stack in 1981
• Unix workstations (SUN) and later PC the dominant users of the architecture
Identified problems:
• Tight coupling of client and server made changes hard
• Distribution of software to many clients
• Lack of scalability in the large
• Sensitive to network latency
• Unreliable outside local area network environment
• Client and Server share state
Internet and Web oriented architectures (1994 – today)
Client
Browser
HTTP Transport
Internet
Server
Web Server
Browser installed on any type of computer with graphical user interface attached to Internet
• http://www.cellnetwork.no - The Unified Resource Locator (URL) was born
Web server provided textual content formatted in HTML
Java launches and become famous for its ability to download code (the applet)
Web servers evolve to handle dynamic content
• Common Gateway Interface (CGI) and Perl
• Programs are impossible to maintain
Sun launches the servlet concept, enabling server side dynamic HTML management
The need to simplify user interface programming results in tag libraries
• Sun – Java Server Pages (JSP), MS got ASP and Open Source got PHP
New server side technologies has emerged including J2EE and MS .NET
Component architectures (1990 – today)
Computer
Component
Component
Computer
TCP/IP
Networ
k
Component
Component
Convergence of distributed object models (CORBA) and Transaction Processing Monitors
• Enterprise Java Beans (EJB)
• Distributed Component Object Model (DCOM) from Microsoft
• Move software towards assembly of “pluggable-parts”
Based on the concept of hiding implementation from specification
• Object Oriented
• EJB uses the Java interface construct combined with Remote Method Invocation
• Network transparent
Identified problems
• Solutions become more rigid than first anticipated (not as easy to plug)
• More TP monitor than distributed objects
• Sensitive to network latency
The N-tier web architecture – practical use of components
Client
Browser
HTTP Transport
Server - side
Internet
Web
Application
Database
Server
Server
Server
(EJB)
The server side is dominated by the N-tier architecture
• Web, Application and Database servers are large software components
• They can reside on one or more physical computers
• The architecture provides scalability and redundancy
• Based on the same principles as IBM applied in 1974
• Designed to handle thousands of interactive users
Identified problems:
• More rigid than first anticipated
• More TP monitor than distributed object model
• Sensitive to network latency
Beyond components – Network to Network Services
Network to Network
XML
Network
Network
system
system
Systems in different networks can communicate
• Also known as web services
• Supports synchronous and asynchronous communication
Supported by mechanisms such as
• UDDI (Universal Description, Discovery & Integration)
• SOAP (Simple Object Access Protocol – XML)
• Systems within network built on N-tier technology
Typical use:
• Place an order at a supplier system
Problems:
• Scalability
• Management
Challenge - Systems become more and more distributed
Deutsche’s fallacies of networking becomes an issue:
1.
The network is reliable
2.
The latency is zero
3.
Bandwidth is infinite
4.
The network is secure
5.
The topology doesn’t change
6.
There is one administrator
7.
Transport cost is zero
8.
There is one administrator
These issues are not handled by classical architectures such as:
•
N-tier
•
Client / Server
Distributed architectures – Participant to Participant
Network
protocol
Participant
Network
Participant
Participant can be anything from a super computer, printer, mobile phone, PDA or car
•Participants may be limited with respect to power supply, memory and cpu capacity
• Participants will be switched on and off
• A participant must advertise its services, and be able to find other participants services
Existing architectures does not support this:
• They fail on Deutsche’s fallacies
• Dynamic lookup of services
Sun Jini network technology provides a solution:
• Dynamic distribution of networked services is built into the language run-time environment
• www.jini.org
• rio.jini.org
• java.sun.com/jini
JavaSpaces – an example of a distributed object store
A JavaSpace is defined by a Java interface:
write(Entry tmpl, Transaction txn, Long lease)
read(Entry tmpl, Transaction txn, Long timeout)
readIfExist
take
takeIfExist(Entry tmpl, Transaction txn, Long timeout)
notify(Entry tmpl
snapshot(Entry e)
An entry is a Java object implementing the Entry interface
Class PersonEntry implements Entry, PersonBean {
Public String name; // Space requires public
Public String address:
Public void setName(String name)
Public String getName()
JavaSpace is based on Linda Tuple spaces developed at
Yale (Gelerntner)
Example of a space based web architecture
The Servlet receives HTTP requests and
process these requests.
Web Container
Business objects are stored as JavaBeans in
a JavaSpace, and the servlet will read and
write bean objects to and from the space
Servlet
Behind the space specialized agents listens
for specific types of requests in the space and
produces valid response objects.
read
write
take
The effect of this architecture is total
decoupling of client side from server side.
JavaSpace
JavaBean
The space can be located anywhere and
neither the servlet nor the agents need to
worry about that.
Void setX(i:X)
X getX()
DB
Agent
DB
Agent
DB
Agent
This architecture is an example of a
alternative to client/server and N-tier, though
the blueprint conforms to an N-tier solution.
Architectural evolution in terms of generations
N-tier
Participant to Participant
Network to Network
N-tier
Client / Server
•
Distribute applications and services across
systems
•
Requires a tightly controlled network
•
An extension of the client/server model
•
CORBA, EJB and DCOM
Network to Network
•
Systems in different networks can
communicate
•
Systems itself built with N-tier technology
•
Web services, XML, UDDI, SOAP
Participant to participant
•
A participant in one network can identify and
communicate with a participant located in
another network
•
Jini network technology
Summary
Web is similar to IBM’s terminal world of 1974
•
Systems Network Architecture
Client/Server and N-tier components requires stable and controlled networks
•
Deutsche’s fallacies
•
Understanding round-trip delay and latency is required
•
Components more rigid than first anticipated
New architectures required for next generation of distributed collaborative systems
•
Jini Network technology provides a solution
Architectures are critical in today’s software systems
•
The more complex systems success depends on architecture at both system
and software levels.
From domain model to code
A practical tour based on Java
The design process – Building a working system
Decompose system into modules
•
Maximize cohesion
•
Minimize coupling
Determine relations between modules
•
Inheritance
•
Composition
•
Identify where flexibility is desirable and where it is not
Determine the form of inter module communication
•
Remote Procedure Calls
•
Messaging
Specify module interfaces
•
Should be well defined
•
Facilitate independent testing
•
Improve group communication
Characteristics of bad design and their cause
Rigid
•
hard to change because every change affect the whole system
Fragile
•
when making a change, unexpected parts of the system fails
Immobile
•
hard to reuse in other applications because of tight couplings
The main cause of bad design is direct mapping of the domain model
•
Violating documented design principles
•
Object oriented languages makes this worse
•
What about components?
Design in practice – Levels
Architectural (system) design:
Scope: Subsystems, Processors, Tasks, Packages, safety & reliability
Patterns: Micro kernel, Rendezvous, Broker, Proxy
Define terminology
Mechanistic design:
Scope: Class collaboration
Patterns: Design Patterns (GOF) and Core J2EE patterns 
Detail design:
Scope: Class, Data and O-R mapping
Phases of design, scope and deliveries
Source: Doing hard time, Douglas 1999
Design phase
Scope
What is specified
Architectural
System wide
Number and type of processors
Processor wide
Packages of objects running on each processor
Inter-process communication
Concurrency model, and inter-thread communication
strategies
Software layering and vertical slices
Error handling policies
Mechanistic
Inter-object
Instances of design patterns of multiple collaborating
objects
Containers and design-level classes and objects
Medium-level error handling policies
Detailed
Intra-object
Algorithmic detail within class
Details of data members (types, ranges)
Details of functional members (arguments)
Architectural design – Processors (physical)
Web Server
EJB Container
Web Server
Web Server
EJB Container
Database Cluster
Processor boundary = network boundary
Think of the software layers
Architectural design - Tasks
Definition
•
Separate function that must occur or appear to occur concurrently
Task types:
•
Event driven
•
Clock driven
•
Priority and Critical
•
Task coordinator
Implementation:
•
Java Threads
•
Agents
•
Message driven beans
•
Standalone processes
•
EJB session beans
Architectural design – Packages
Packages is a grouping mechanism of functionality
•
UML has a representation, the same has Ada , C++ and Java
A poor package structure in Java will haunt the system in its lifetime
•
Separate specification from implementation
•
Use separate source threes
Package structure defines the architecture
Specifications:
•
no.cellnetwork.marketplace.business.MarketServiceFactory
•
no.cellnetwork.marketplace.business.UserAccountService
Implementation:
•
no.cellnetwork.marketplace.business.MarketServiceFactoryImpl
•
no.cellnetwork.marketplace.business.UserAccountServiceImpl
Architectural design – Packages and sub-systems
Defence system
Group functionality into logical packages
Ground
Segment
Required to manage complexity
Airborn
Platform
Identify interfaces and package dependencies
Abstract versus concrete packages
Communication
System
Subsea
Segment
Commercial system
Reporting &
Statistics
User
Managemet
Common
Messaging
Account
Management
Trade Engine
Architectural design – Packages and Layers
User Interface Layer
User Interaction
Layer
•
Responsible for all user interactions
•
Realized by portal frameworks and to some
extent Swing components.
•
Includes Web services and XML interfaces
for communication
Business Service Layer
Business
Services Layer
•
Responsible for domain specific functions
•
Realized by JavaBeans,Session Beans, Jini
Services and Servlets and other ordinary
classes
Data & Integration Layer
Data & Integration
Layer
•
Responsible for data access and access to
other systems
•
Implemented in databases (SQL), Entity
Beans and Data Access Objects
•
Asynchronous messaging a part of this layer
Architecture – Illustrated
User Interaction Layer
User Interaction Layer
•Web, Rich client (swing) and Mobile
Business Service Layer
Business
Business
Business
Service
Service
Service
•
Tag libraries a issue
•
Usability a issue
•
Information architecture a issue
Business Service Layer
•
Defined by interfaces and interfaces only.
•
Interfaces should be network ready. Eg.
Throws RemoteException.
•
Data & Integration Layer
Access Service
Access Service
Message Service
Agent
Data
Data & Integration Layer
•
Defined by interfaces, message standards
and database tables.
•
Agents are self contained processes with a
well defined purpose
•
Agents can also implement domain specific
business rules
•
Message service can be JMS, Corba,
JavaSpaces
•
Data can be local databases or external
legacy systems. Communication managed
by agents
Agent
Data
Data
Implemented as EJB, Servlet,JavaBean’s
Mechanistic design
Mechanistic design is concerned with adding and organizing
classes to support a particular implementation strategy
Bruce Powel Douglass
Goal:
Transform the analysis model into a effective working design
•
Maximize cohesion
•
Minimize couplings
Tools:
•
Separate specifications from implementation
•
Design patterns (GOF book)
•
Inheritance and composition
•
What about EJB’s?
Practical design step one – decomposing the domain model
Identified services and data objects
ContractService
BidBean
•
findAll
•
getPrice
•
findBySeller
•
setPrice
•
findByBuyer
OfferService
CarMarketBean
•
make
•
setPrice
•
Find
•
getPrice
BidService
•
make
•
accept
•
getPrice
•
find
•
getBuyer
RequestService
•
make
•
find
Contract
Marketplace services and factory specification
Service specification
Specification consists of:
•
Specification is composed of package and interface
•
The service throws RemoteException and is implicit networked
enabled
•
Its up to the implementer to decide on distribution or not
Sample code
package no.cellnetwork.business.marketplace;
import Java.rmi.RemoteException;
public interface RequestService {
public Collection find(...) throws RemoteException;
public void make(..) throws RemoteException;
}
RequestService – EJB design
Service implementation – EJB example
Specify EJB specific interfaces
package no.cellnetwork.business.marketplace;
import javax.ejb.EJBObject;
public interface BidServiceRemote extends EJBObject, BidService{}
public interface BidServiceHome extends EJBHome {
public
BidServiceRemote create() throws RemoteException,,;
}
Implementing the bean
package no.cellnetwork.business.marketplace;
public class BidServiceBean implements SessionBean, BidService {
public Collection find(){}
public void make() {}
public void accept() {}
}
Implementing the factory
public RequestService createRequestService() {
RequestServiceRemote remote = null;
InitialContex ctx = new InitialContext();
try {
Object ref = ctx.lookup("RequestService");
RequestServiceHome home =
(RequestServiceHome)PortableRemoteObject.narrow(
ref,RequestServiceHome.class);
remote = home.create();
} catch (Exception e) {
//
throw new MarketException("Could not create
RequestService");
}
return (RequestService)remote;
}
Detail design – the last step before code
Scope:
•
Classes and type safe attributes
•
Representing complex data structures
•
Database design and OR mapping
•
Object oriented databases and Java Data Objects
Making attributes type safe
Ada provides this:
•
Type Missile_Speed_Type is float 0.0..6000.0;
•
Type Missile_Range_Type is float 0.0..4000.0;
•
Missile_Speed : Missile_Speed_Type;
•
Missile_Range : Missile_Range_Type;
•
Some_Float : Float;
•
Some_Float := Missile_Range + Missile_Speed; -- Stopped by compiler !!
Java requires class encapsulation:
•
Lack of operator overloading an issue:
•
Class Speed_Type …..
•
Class Range_Type ……
Mapping objects to relational databases
Database on 3’d normal form is good for objects too
•
No redundancy - performance an issue, use your brain
•
No internal dependency - unique rows
Database should be designed to support the object model
•
Relations a result of business methods in objects
•
Complex queries best done manually (Torque is a tool but performance an issue)
•
Stored procedure speeds performance
What about entity beans
•
Think of it as a persistent object
•
Spann one table, though EJB 2.0 supports foreign key
•
Small result sets
Consider to use a Data Access Service
•
Returns valueObjects (JavaBean’s)
•
Encapsulates your SQL
Using the DataAccessService
Composite data structures (GOF 104)
Key success factors
Architecture
•
Services ( interface’s)
•
Layers (packages)
•
Separate specification from implementation (package+interface = true)
Understanding of OO design principles
•
More than inheritance
•
Patterns a good tool
•
Understand the network boundary (bandwidth & latency)
A good process addressing the right problem at the right time
•
Hacking is banned – Model your system and evolve it carefully
•
Starting with the database is banned – Database derived from object model
•
Think in terms of design levels - Stay at the right abstraction level
Academic foundation
Design challenges
Bad design is the result of violating well documented design principles:
•
Maximize cohesion
•
Minimize coupling
Academic foundation:
•
Design by contract
•
The Open / Closed principle
•
Liskov’s substitution principle
•
The dependency inversion principle
Design by contract – the assertion mechanism
Pre-conditions
• Specify properties that must hold whenever an operation is called
•
Client responsible for checking
Post-conditions
• Describe properties that the operation guarantees when completed
•
Class responsible for ensuring
Invariants
• Global properties of class that must be preserved at all times
•
Class responsible for ensuring consistency
Exception
arises when pre-conditions satisfies but one or more post-conditions fail
Inheritance & Design by contract
Clas s
Parents invariant rule
•
Class invariants of parent are retained in the
subclass
Assertion redefinition rule
•
Pre-conditions may only be weakened in the
subclass
•
Post-conditions may only be strengthened in
the subclass
Subclas s
The open – closed principle
Client is closed
Client
Server
Software entities (classes,
modules, components) should
be open for extension but closed
for modification
Closed Client
•
Client is open
Client
AbstractServer
Open Client
•
ServerOne
ServerTwo
The client is closed because, in order
to use another server, its code must
be changed to mention the new
server.
The client is open because it uses
services published for an abstract
class. In order to introduce change to
the server, the designer need only to
add new derived server classes. The
Client class remains unaltered.
Liskov substitution principle (Polymorphism)
Functions that use base class interfaces must not depend on nor be
confused by any derivatives of those interfaces
•
This rule is a logical consequence of the open-closed principle
More formally:
•
Consider a function F that uses type T.
•
Given S a subtype of T, F should be able to use objects of type S
without knowing it.
Breaking it requires code like this:
void F(T input) {
if (input instanceoff S) { …….
Barbara Liskov’s work is featured in Jim
Coplien’s book Advanced C++ Programming
Styles and Idioms
The dependency inversion principle
Abstractions should not depend on details. Details should depend on abstractions.
Inverted dependency with abstract layers
•
Each layer derives from an abstract class. Lower layers used by higher layer
through lower layer’s abstract interface. So – Layer’s depends on abstract
classes
Policy
PolicyLayer
Mechanism
<<abstract>>
MechanismInterface
MechanismLayer
<<abstract>>
UtilityInterface
Utility
UtilityLayer
Package Stability
The dependencies between packages in a design should be in the direction of the
stability of the packages. A package should only depend upon packages that are
more stable than that it is.
Robert Martin’s Package Stability Metrics
•
•
•
Ca - Afferent Couplings: The number of classes outside this package that
depend upon classes within this package.
Ce – Efferent Couplings: The number of classes inside this package that
depends upon classes outside this package.
I – Instability: (Ce / (Ca+Ce)): This metric has a range [0,1]. I=0 indicates a
maximally stable package. I=1 indicates a maximally instable package.
Not all packages should be stable
I=1, instable
I=0, Stable
I=1, instable
•
If all packages in a system where
maximally stable, the system would
be unchangeable.
•
We want to design our package
structure so that some packages
are instable and some are stable.
•
The ideal configuration for a system
with three packages has the
changeable packages on top. They
depend upon stable packages at
the bottom.
The stable abstraction principle
Packages that are maximally stable should be maximally abstract. Instable
packages should be concrete. The abstraction of a package should be in proportion
to its stability.
Abstraction versus stability
Abstraction (A) = Abstract classes / total classes
Instability = Ce / (Ca + Ce)
1
A=1, I=1: Abstract and no dependants
Instability
1
A=0, I=0 Stable and concrete
It should be noted that many packages do fall within (0,0) zone. An example would be a database schema. Database
schemas are notorously volatile and are highly dependent upon. This is one of the reasons that the interface between OO
applications and databases is so difficult.
References
The art of systems architecting, 2nd edition, 2002, Maier, Rechtin, ISBN: 0-8493-0440-7
Objects, Components and Frameworks with UML, D’Souza, Wills, 1999, ISBN 0-201-31012-0
Pattern oriented software architectures, Patterns for concurrent and networked objects, 2000,
Schmidt et al, ISBN: 0-471-60695-2
Object oriented software engineering, Jacobson, 1992, ISBN: 0-201-54435-0
The Jini specification, 2nd edition, Waldo et al, ISBN: 0-201-72617-3
Doing hard time, Douglas, 1999, ISBN: 0-201-49837-5
Design patterns, 1995, Gamma et al, ISBN: 0-201-63361-2
Core J2EE Patterns, 2001, Crupi et al, ISBN 0-130-64884-1
Developing enterprise java applications with J2EE and UML, Ahmed, Umrysh,2002,
ISBN 0-201-73829-5
www.sei.cmu.edu
www.bredemeyer.com